Dark energy with polytropic equation of state
Utpal Mukhopadhyay; Saibal Ray
2005-10-19
Equation of state parameter plays a significant role for guessing the real nature of dark energy. In the present paper polytropic equation of state $p=\\omega\\rho^n$ is chosen for some of the kinematical $\\Lambda$-models viz., $\\Lambda \\sim (\\dot a/a)^2$, $\\Lambda \\sim \\ddot a/a$ and $\\Lambda \\sim \\rho$. Although in dust cases ($\\omega=0$) closed form solutions show no dependency on the polytropic index $n$, but in non-dust situations some new possibilities are opened up including phantom energy with supernegative ($\\omegastate parameter.
Generalized equation of state for dark energy
Barboza, E. M. Jr.; Alcaniz, J. S. [Observatorio Nacional, 20921-400, Rio de Janeiro - RJ (Brazil); Zhu, Z.-H. [Department of Astronomy, Beijing Normal University, Beijing 100875 (China); Silva, R. [Departamento de Fisica, Universidade Federal do Rio Grande do Norte, 59072-970 Natal - RN (Brazil); Departamento de Fisica, Universidade do Estado do Rio Grande do Norte, 59610-210, Mossoro - RN (Brazil)
2009-08-15
A generalized parametrization w{sub {beta}}(z) for the dark energy equation of state is proposed and some of its cosmological consequences are investigated. We show that in the limit of the characteristic dimensionless parameter {beta}{yields}+1, 0 and -1 some well-known equation of state parametrizations are fully recovered whereas for other values of {beta} the proposed parametrization admits a wider and new range of cosmological solutions. We also discuss possible constraints on the w{sub {beta}}(z) parameters from current observational data.
The dark energy equation of state
A. A. Usmani; P. P. Ghosh; Utpal Mukhopadhyay; P. C. Ray; Saibal Ray
2008-03-11
We perform a study of cosmic evolution with an equation of state parameter $\\omega(t)=\\omega_0+\\omega_1(t\\dot H/H)$ by selecting a phenomenological $\\Lambda$ model of the form, $\\dot\\Lambda\\sim H^3$. This simple proposition explains both linearly expanding and inflationary Universes with a single set of equations. We notice that the inflation leads to a scaling in the equation of state parameter, $\\omega(t)$, and hence in equation of state. In this approach, one of its two parameters have been pin pointed and the other have been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.
Dark energy models with variable equation of state parameter
Anil Kumar Yadav; Farook Rahaman; Saibal Ray
2010-09-24
The dark energy models with variable equation of state parameter $\\omega$ is investigated by using law of variation of Hubble's parameter that yields the constant value of deceleration parameter. The equation of state parameter $\\omega$ is found to be time dependent and its existing range for this model is consistent with the recent observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy clustering statistics. The physical significance of the dark energy models has also been discussed.
A new equation of state for dark energy
Dragan Slavkov Hajdukovic
2009-11-04
In the contemporary Cosmology, dark energy is modeled as a perfect fluid, having a very simple equation of state: pressure is proportional to dark energy density. As an alternative, I propose a more complex equation of state, with pressure being function of three variables: dark energy density, matter density and the size of the Universe. One consequence of the new equation is that, in the late-time Universe, cosmological scale factor is linear function of time; while the standard cosmology predicts an exponential function.The new equation of state allows attributing a temperature to the physical vacuum, a temperature proportional to the acceleration of the expansion of the Universe. The vacuum temperature decreases with the expansion of the Universe, approaching (but never reaching) the absolute zero.
Scaling cosmology with variable dark-energy equation of state
Castro, David R.; Velten, Hermano; Zimdahl, Winfried E-mail: velten@physik.uni-bielefeld.de
2012-06-01
Interactions between dark matter and dark energy which result in a power-law behavior (with respect to the cosmic scale factor) of the ratio between the energy densities of the dark components (thus generalizing the ?CDM model) have been considered as an attempt to alleviate the cosmic coincidence problem phenomenologically. We generalize this approach by allowing for a variable equation of state for the dark energy within the CPL-parametrization. Based on analytic solutions for the Hubble rate and using the Constitution and Union2 SNIa sets, we present a statistical analysis and classify different interacting and non-interacting models according to the Akaike (AIC) and the Bayesian (BIC) information criteria. We do not find noticeable evidence for an alleviation of the coincidence problem with the mentioned type of interaction.
NASA Astrophysics Data System (ADS)
Kumar, Suresh; Xu, Lixin
2014-10-01
In this paper, we study a cosmological model in general relativity within the framework of spatially flat Friedmann-Robertson-Walker space-time filled with ordinary matter (baryonic), radiation, dark matter and dark energy, where the latter two components are described by Chevallier-Polarski-Linder equation of state parameters. We utilize the observational data sets from SNLS3, BAO and Planck + WMAP9 + WiggleZ measurements of matter power spectrum to constrain the model parameters. We find that the current observational data offer tight constraints on the equation of state parameter of dark matter. We consider the perturbations and study the behavior of dark matter by observing its effects on CMB and matter power spectra. We find that the current observational data favor the cold dark matter scenario with the cosmological constant type dark energy at the present epoch.
Nonparametric reconstruction of the dark energy equation of state
Heitmann, Katrin [Los Alamos National Laboratory; Holsclaw, Tracy [Los Alamos National Laboratory; Alam, Ujjaini [Los Alamos National Laboratory; Habib, Salman [Los Alamos National Laboratory; Higdon, David [Los Alamos National Laboratory; Sanso, Bruno [UC SANTA CRUZ; Lee, Herbie [UC SANTA CRUZ
2009-01-01
The major aim of ongoing and upcoming cosmological surveys is to unravel the nature of dark energy. In the absence of a compelling theory to test, a natural approach is to first attempt to characterize the nature of dark energy in detail, the hope being that this will lead to clues about the underlying fundamental theory. A major target in this characterization is the determination of the dynamical properties of the dark energy equation of state w. The discovery of a time variation in w(z) could then lead to insights about the dynamical origin of dark energy. This approach requires a robust and bias-free method for reconstructing w(z) from data, which does not rely on restrictive expansion schemes or assumed functional forms for w(z). We present a new non parametric reconstruction method for the dark energy equation of state based on Gaussian Process models. This method reliably captures nontrivial behavior of w(z) and provides controlled error bounds. We demollstrate the power of the method on different sets of simulated supernova data. The GP model approach is very easily extended to include diverse cosmological probes.
Dark Energy as a Modification of the Friedmann Equation
Gia Dvali; Michael S. Turner
2003-01-25
Dark energy could actually be the manifestation of a modification to the Friedmann equation arising from new physics (e.g., extra dimensions). Writing the correction as $(1-\\Omega_M)H^\\alpha /H_0^{\\alpha -2}$, we explore the phenomenology and detectability of such. We show that: (i) $\\alpha$ must be $\\la 1$; (ii) such a correction behaves like dark energy with equation-of-state $w_{\\rm eff} = -1 + {\\alpha \\over 2}$ in the recent past ($10^4> z\\gg 1$) and $w=-1$ in the distant future and can mimic $w<-1$ without violating the weak-energy condition; (iii) $w_{\\rm eff}$ changes, $dz/dw|_{z\\sim 0.5} \\sim {\\cal O}(0.2)$, which is likely detectable; and (iv) a future supernova experiment like SNAP that can determine $w$ with precision $\\sigma_w$, could determine $\\alpha$ to precision $\\sigma_\\alpha \\approx 2 \\sigma_w$.
The state of the dark energy equation of state
Alessandro Melchiorri; Laura Mersini; Carolina J. Ödman; Mark Trodden
2003-01-01
By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with the Hubble parameter measurement from the Hubble space telescope and luminosity measurements of type Ia supernovae, we demonstrate the bounds on the dark energy equation of state wQ to be -1.45
The Unified Equation of State for Dark Matter and Dark Energy
Wei Wang; Yuan-xing Gui; Suhong Zhang; Guanghai Guo; Ying Shao
2005-04-05
We assume that dark matter and dark energy satisfy the unified equation of state: $p=B(z)\\rho$, with $p=p_{dE}$, $\\rho=\\rho_{dm}+\\rho_{dE}$, where the pressure of dark matter $p_{dm}=0$ has been taken into account. A special function $B=-\\frac{A}{(1+z)^{\\alpha}}$ is presented, which can well describe the evolution of the universe. In this model, the universe will end up with a Big Rip. By further simple analysis, we know other choices of the function $B$ can also describe the universe but lead to a different doomsday.
A new equation of state for dark energy model
Feng, Lei [Department of Physics, Nanjing University, Nanjing 210093 (China); Lu, Tan, E-mail: fenglei@chenwang.nju.edu.cn, E-mail: t.lu@pmo.ac.cn [Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 (China)
2011-11-01
A new parameterization for the dark energy equation of state(EoS) is proposed and some of its cosmological consequences are also investigated. This new parameterization is the modification of Efstathiou' dark energy EoS parameterization. w(z) is a well behaved function for z >> 1 and has same behavior in z at low redshifts with Efstathiou' parameterization. In this parameterization there are two free parameter w{sub 0} and w{sub a}. We discuss the constraints on this model's parameters from current observational data. The best fit values of the cosmological parameters with 1? confidence-level regions are: ?{sub m} = 0.2735{sup +0.0171}{sub ?0.0163}, w{sub 0} = ?1.0537{sup +0.1432}{sub ?0.1511} and w{sub a} = 0.2738{sup +0.8018}{sub ?0.8288}.
Equation of state of dark energy in f (R ) gravity
NASA Astrophysics Data System (ADS)
Takahashi, Kazufumi; Yokoyama, Jun'ichi
2015-04-01
f (R ) gravity is one of the simplest generalizations of general relativity, which may explain the accelerated cosmic expansion without introducing a cosmological constant. Transformed into the Einstein frame, a new scalar degree of freedom appears and it couples with matter fields. In order for f (R ) theories to pass the local tests of general relativity, it has been known that the chameleon mechanism with a so-called thin-shell solution must operate. If the thin-shell constraint is applied to a cosmological situation, it has been claimed that the equation-of-state parameter of dark energy w must be extremely close to -1 . We argue this is due to the incorrect use of the Poisson equation, which is valid only in the static case. By solving the correct Klein-Gordon equation perturbatively, we show that a thin-shell solution exists even if w deviates appreciably from -1 .
Reconstruction of the dark energy equation of state
Vázquez, J. Alberto; Bridges, M.; Lasenby, A.N.; Hobson, M.P. E-mail: mb435@mrao.cam.ac.uk E-mail: a.n.lasenby@mrao.cam.ac.uk
2012-09-01
One of the main challenges of modern cosmology is to investigate the nature of dark energy in our Universe. The properties of such a component are normally summarised as a perfect fluid with a (potentially) time-dependent equation-of-state parameter w(z). We investigate the evolution of this parameter with redshift by performing a Bayesian analysis of current cosmological observations. We model the temporal evolution as piecewise linear in redshift between 'nodes', whose w-values and redshifts are allowed to vary. The optimal number of nodes is chosen by the Bayesian evidence. In this way, we can both determine the complexity supported by current data and locate any features present in w(z). We compare this node-based reconstruction with some previously well-studied parameterisations: the Chevallier-Polarski-Linder (CPL), the Jassal-Bagla-Padmanabhan (JBP) and the Felice-Nesseris-Tsujikawa (FNT). By comparing the Bayesian evidence for all of these models we find an indication towards possible time-dependence in the dark energy equation-of-state. It is also worth noting that the CPL and JBP models are strongly disfavoured, whilst the FNT is just significantly disfavoured, when compared to a simple cosmological constant w = ?1. We find that our node-based reconstruction model is slightly disfavoured with respect to the ?CDM model.
Reconstruction of the dark energy equation of state
NASA Astrophysics Data System (ADS)
Vázquez, J. Alberto; Bridges, M.; Hobson, M. P.; Lasenby, A. N.
2012-09-01
One of the main challenges of modern cosmology is to investigate the nature of dark energy in our Universe. The properties of such a component are normally summarised as a perfect fluid with a (potentially) time-dependent equation-of-state parameter w(z). We investigate the evolution of this parameter with redshift by performing a Bayesian analysis of current cosmological observations. We model the temporal evolution as piecewise linear in redshift between 'nodes', whose w-values and redshifts are allowed to vary. The optimal number of nodes is chosen by the Bayesian evidence. In this way, we can both determine the complexity supported by current data and locate any features present in w(z). We compare this node-based reconstruction with some previously well-studied parameterisations: the Chevallier-Polarski-Linder (CPL), the Jassal-Bagla-Padmanabhan (JBP) and the Felice-Nesseris-Tsujikawa (FNT). By comparing the Bayesian evidence for all of these models we find an indication towards possible time-dependence in the dark energy equation-of-state. It is also worth noting that the CPL and JBP models are strongly disfavoured, whilst the FNT is just significantly disfavoured, when compared to a simple cosmological constant w = -1. We find that our node-based reconstruction model is slightly disfavoured with respect to the ?CDM model.
Layzer-Irvine equation: New perspectives and the role of interacting dark energy
NASA Astrophysics Data System (ADS)
Avelino, P. P.; Barreira, A.
2012-03-01
We derive the Layzer-Irvine equation in the presence of a homogeneous (or quasihomogeneous) dark energy component with an arbitrary equation of state. We extend the Layzer-Irvine equation to homogeneous and isotropic universes with an arbitrary number of dimensions and obtain the corresponding virial relation for sufficiently relaxed objects. We find analogous equations describing the dynamics of cosmic string loops and other p-branes of arbitrary dimensionality, discussing the corresponding relativistic and nonrelativistic limits. Finally, we generalize the Layzer-Irvine equation to account for a nonminimal interaction between dark matter and dark energy, discussing its practical use as a signature of such an interaction.
Transition of the dark energy equation of state in an interacting holographic dark energy model
Bin Wang; Yungui Gong; Elcio Abdalla
2005-01-01
A model of holographic dark energy with an interaction with matter fields has been investigated. Choosing the future event horizon as an IR cutoff, we have shown that the ratio of energy densities can vary with time. With the interaction between the two different constituents of the universe, we observed the evolution of the universe, from early deceleration to late
Investigating Possible Inflection in the Dark Energy Equation of State
Hu, Yazhou; Li, Xiao-Dong; Zhang, Zhenhui
2014-01-01
In this paper we investigate the index of the dark energy equation of state $w$ by using the latest observational data. To explore possible non-linear evolutions in $w$, we adopt a second order $parabolic$ parametrization $w(z)=w_0+w_a(a_0-a)^2$, which has the advantage of describing the possible $inflection$ point in $w$. We use a combined dataset of the SNLS3 supernovae sample, the cosmic microwave background measurements from WMAP9 and Planck, the Hubble parameter measurement from HST, and the baryon acoustic oscillations measurements which includes the most recent results from BOSS DR11 and "improved" WiggleZ. We find that a crossing of $w=-1$ at $z\\sim 0.9-1.5$ is favored at about $1\\ \\sigma$ confidence level (CL), with $w>-1$ at high redshift and $w<-1$ at lower redshift. More interestingly, we find an inflection of $w$ at $z\\sim 0.25-0.4$ is mildly favored at $1\\ \\sigma$ CL, with a phantom behavior at the extreme point $w(z\\sim0.25)<-1$ favored at about $2\\ \\sigma$ CL. Our reconstructed $w$ is in...
Dark-energy equation of state: how far can we go from ??
Hrvoje Stefancic
2006-09-28
The equation of state of dark energy is investigated to determine how much it may deviate from the equation of state of the cosmological constant (CC). Two aspects of the problem are studied: the "expansion" around the vacuum equation of state and the problem of the crossing of the cosmological constant boundary.
Inference for the dark energy equation of state using Type IA supernova data
Christopher Genovese; Peter Freeman; Larry Wasserman; Robert Nichol; Christopher Miller
2009-05-18
The surprising discovery of an accelerating universe led cosmologists to posit the existence of "dark energy"--a mysterious energy field that permeates the universe. Understanding dark energy has become the central problem of modern cosmology. After describing the scientific background in depth, we formulate the task as a nonlinear inverse problem that expresses the comoving distance function in terms of the dark-energy equation of state. We present two classes of methods for making sharp statistical inferences about the equation of state from observations of Type Ia Supernovae (SNe). First, we derive a technique for testing hypotheses about the equation of state that requires no assumptions about its form and can distinguish among competing theories. Second, we present a framework for computing parametric and nonparametric estimators of the equation of state, with an associated assessment of uncertainty. Using our approach, we evaluate the strength of statistical evidence for various competing models of dark energy. Consistent with current studies, we find that with the available Type Ia SNe data, it is not possible to distinguish statistically among popular dark-energy models, and that, in particular, there is no support in the data for rejecting a cosmological constant. With much more supernova data likely to be available in coming years (e.g., from the DOE/NASA Joint Dark Energy Mission), we address the more interesting question of whether future data sets will have sufficient resolution to distinguish among competing theories.
Equation of state of the trans-Planckian dark energy and the coincidence problem
NASA Astrophysics Data System (ADS)
Bastero-Gil, Mar; Mersini, Laura
2003-05-01
Observational evidence suggests that our Universe is presently dominated by a dark energy component and is undergoing accelerated expansion. We recently introduced a model, motivated by string theory for short-distance physics, for explaining dark energy without appealing to any fine tuning. The idea of trans-Planckian dark energy (TDE) was based on the freeze-out mechanism of the ultralow frequency modes, ?(k), of very short distances, by the expansion of the background universe, ?(k)?H. In this paper we address the issue of the stress-energy tensor for nonlinear short-distance physics and explain the need to modify Einstein equations in this regime. From the modified Einstein equations we then derive the equation of state for the TDE model, which has the distinctive feature of being continually time dependent. The explanation of the coincidence puzzle relies entirely on the intrinsic time evolution of the TDE equation of state.
Perceiving the equation of state of Dark Energy while living in a Cold Spot
Wessel Valkenburg
2012-01-31
The Cold Spot could be an adiabatic perturbation on the surface of last scattering, in which case it is an over-density with comoving radius of the order of 1 Gpc. We assess the effect that living in a similar structure, without knowing it, has on our perception of the equation of state of Dark Energy. We find that structures of dimensions such that they could cause the Cold Spot on the CMB, affect the perceived equation of state of Dark Energy possibly up to ten percent.
Perceiving the equation of state of Dark Energy while living in a Cold Spot
Valkenburg, Wessel, E-mail: w.valkenburg@thphys.uni-heidelberg.de [Institut für Theoretische Teilchenphysik und Kosmologie, RWTH Aachen University, D-52056 Aachen (Germany)
2012-01-01
The Cold Spot could be an adiabatic perturbation on the surface of last scattering, in which case it is an over-density with comoving radius of the order of 1 Gpc. We assess the effect that living in a similar structure, without knowing it, has on our perception of the equation of state of Dark Energy. We find that structures of dimensions such that they could cause the Cold Spot on the CMB, affect the perceived equation of state of Dark Energy possibly up to ten percent.
The dark energy cosmic clock: a new way to parametrise the equation of state
Tarrant, Ewan R.M.; Copeland, Edmund J.; Padilla, Antonio; Skordis, Constantinos, E-mail: ppxet@nottingham.ac.uk, E-mail: ed.copeland@nottingham.ac.uk, E-mail: antonio.padilla@nottingham.ac.uk, E-mail: skordis@nottingham.ac.uk [School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD (United Kingdom)
2013-12-01
We propose a new parametrisation of the dark energy equation of state, which uses the dark energy density, ?{sub e} as a cosmic clock. We expand the equation of state in a series of orthogonal polynomials, with ?{sub e} as the expansion parameter and determine the expansion coefficients by fitting to SNIa and H(z) data. Assuming that ?{sub e} is a monotonic function of time, we show that our parametrisation performs better than the popular Chevallier-Polarski-Linder (CPL) and Gerke and Efstathiou (GE) parametrisations, and we demonstrate that it is robust to the choice of prior. Expanding in orthogonal polynomials allows us to relate models of dark energy directly to our parametrisation, which we illustrate by placing constraints on the expansion coefficients extracted from two popular quintessence models. Finally, we comment on how this parametrisation could be modified to accommodate high redshift data, where any non-monotonicity of ?{sub e} would need to be accounted for.
The dark energy cosmic clock: a new way to parametrise the equation of state
NASA Astrophysics Data System (ADS)
Tarrant, Ewan R. M.; Copeland, Edmund J.; Padilla, Antonio; Skordis, Constantinos
2013-12-01
We propose a new parametrisation of the dark energy equation of state, which uses the dark energy density, ?e as a cosmic clock. We expand the equation of state in a series of orthogonal polynomials, with ?e as the expansion parameter and determine the expansion coefficients by fitting to SNIa and H(z) data. Assuming that ?e is a monotonic function of time, we show that our parametrisation performs better than the popular Chevallier-Polarski-Linder (CPL) and Gerke and Efstathiou (GE) parametrisations, and we demonstrate that it is robust to the choice of prior. Expanding in orthogonal polynomials allows us to relate models of dark energy directly to our parametrisation, which we illustrate by placing constraints on the expansion coefficients extracted from two popular quintessence models. Finally, we comment on how this parametrisation could be modified to accommodate high redshift data, where any non-monotonicity of ?e would need to be accounted for.
Dark energy from cosmological fluids obeying a Shan-Chen non-ideal equation of state
Donato Bini; Andrea Geralico; Daniele Gregoris; Sauro Succi
2014-08-23
We consider a Friedmann-Robertson-Walker universe with a fluid source obeying a non-ideal equation of state with "asymptotic freedom," namely ideal gas behavior (pressure changes directly proportional to density changes) both at low and high density regimes, following a fluid dynamical model due to Shan and Chen. It is shown that, starting from an ordinary energy density component, such fluids naturally evolve towards a universe with a substantial "dark energy" component at the present time, with no need of invoking any cosmological constant. Moreover, we introduce a quantitative indicator of darkness abundance, which provides a consistent picture of the actual matter-energy content of the universe.
Observational constraints on dark energy with a fast varying equation of state
NASA Astrophysics Data System (ADS)
De Felice, Antonio; Nesseris, Savvas; Tsujikawa, Shinji
2012-05-01
We place observational constraints on models with the late-time cosmic acceleration based on a number of parametrizations allowing fast transitions for the equation of state of dark energy. In addition to the model of Linder and Huterer where the dark energy equation of state w monotonically grows or decreases in time, we propose two new parametrizations in which w has an extremum. We carry out the likelihood analysis with the three parametrizations by using the observational data of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. Although the transient cosmic acceleration models with fast transitions can give rise to the total chi square smaller than that in the ?-Cold-Dark-Matter (?CDM) model, these models are not favored over ?CDM when one uses the Akaike information criterion which penalizes the extra degrees of freedom present in the parametrizations.
Can the dark energy equation-of-state parameter w be less than -1?
Sean M. Carroll; Mark Hoffman; Mark Trodden
2003-01-01
Models of dark energy are conveniently characterized by the equation-of-state parameter w=p\\/rho, where rho is the energy density and p is the pressure. Imposing the dominant energy condition, which guarantees stability of the theory, implies that w=>-1. Nevertheless, it is conceivable that a well-defined model could (perhaps temporarily) have w<-1 , and indeed such models have been proposed. We study
Entropy Growth and the Dark Energy Equation of State
Wilfried Buchmuller; Joerg Jaeckel
2006-10-27
We revisit the conjecture of a generalized second law of thermodynamics which states that the combined entropy of matter and horizons must grow. In an expanding universe a generalized second law restricts the equation of state. In particular, it conflicts with long phases of a phantom, w<-1, equation of state.
Dynamical Mutation of Dark Energy
L. R. Abramo; R. C. Batista; L. Liberato; R. Rosenfeld
2008-01-03
We discuss the intriguing possibility that dark energy may change its equation of state in situations where large dark energy fluctuations are present. We show indications of this dynamical mutation in some generic models of dark energy.
Effect of dark energy sound speed and equation of state on CDM power spectrum
Rizwan Ul Haq Ansari; Sanil Unnikrishnan
2014-07-15
We study the influence of equation of state $w$ and effective sound speed $c_e$ of the dark energy perturbations on the cold dark matter(CDM) power spectrum.We consider different cases of the equation of state and the effective sound speed, the cold dark matter power spectrum is found to be generically suppressed in these cases as compared to the $\\Lambda$CDM model. The suppression at different length scales depends on the value of $w$ and $c_e$, and the effect of different $w$ is profoundly seen at all length scales. The influence of sound speed is significantly seen only at the intermediate length scales and is negligible at scales very much larger and smaller than the Hubble scale.
Constraining a scalar field dark energy with variable equation of state for matter
A. Sil; S. Som
2014-12-01
The red-shift $z_{eq}$, marking the end of radiation era and the beginning of matter-dominated era, can play an important role to reconstruct dark-energy models. A variable equation of state for matter that can bring a smooth transition from radiation to matter-dominated era in a single model is proposed to estimate $z_{eq}$ in dark energy models and hence its viability. Two one-parameter models with minimally coupled scalar fields playing the role of dark energy are chosen to demonstrate this point. It is found that for desired late time behavior of the models, the estimated value of $z_{eq}$ is highly sensitive on the value of the parameter in each of these models.
Fables of reconstruction: controlling bias in the dark energy equation of state
Crittenden, Robert G.; Zhao, Gong-Bo; Samushia, Lado [Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 3FX (United Kingdom); Pogosian, Levon [Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6 (Canada); Zhang, Xinmin, E-mail: Robert.Crittenden@port.ac.uk, E-mail: gong-bo.zhao@port.ac.uk, E-mail: levon@sfu.ca, E-mail: lado.samushia@port.ac.uk, E-mail: xmzhang@ihep.ac.cn [Theoretical Physics Division, Institute of High Energy Physics, Chinese Academy of Science, P.O.Box 918-4, Beijing 100049 (China)
2012-02-01
We develop an efficient, non-parametric Bayesian method for reconstructing the time evolution of the dark energy equation of state w(z) from observational data. Of particular importance is the choice of prior, which must be chosen carefully to minimise variance and bias in the reconstruction. Using a principal component analysis, we show how a correlated prior can be used to create a smooth reconstruction and also avoid bias in the mean behaviour of w(z). We test our method using Wiener reconstructions based on Fisher matrix projections, and also against more realistic MCMC analyses of simulated data sets for Planck and a future space-based dark energy mission. While the accuracy of our reconstruction depends on the smoothness of the assumed w(z), the relative error for typical dark energy models is ?<10% out to redshift z = 1.5.
Dark Energy Models With Variable Equation Of State Parameter
Utpal Mukhopadhyay; Saibal Ray; Farook Rahaman
2010-01-05
Two variable $\\Lambda$ models, viz. $\\Lambda \\sim (\\dot a/a)^2$ and $\\Lambda \\sim \\rho$ have been studied under the assumption that the equation of state parameter $\\omega$ is a function of time. The selected $\\Lambda$ models are found to be equivalent both in four and five dimensions. The possibility of signature flip of the deceleration parameter is also shown.
Constraining the equation of state of dark energy with gamma rays
NASA Astrophysics Data System (ADS)
Fairbairn, Malcolm
2013-08-01
Starlight in the Universe impedes the passage of high energy (e.g., TeV) gamma rays due to positron-electron pair production. The history of this stellar radiation field depends upon observations of the star formation rate which themselves can only be interpreted in the context of a particular cosmology. For different equations of state of dark energy, the star formation rate data suggest a different density of stellar photons at a particular redshift and a different probability of arrival of gamma rays from distant sources. In this work we aim to show that this effect can be used to constrain the equation of state of dark energy. The current work is a proof of concept and we outline the steps that would have to be taken to place the method in a rigorous statistical framework which could then be combined with other more mature methods such as fitting supernova luminosity distances.
Variable Equation of State for Generalized Dark Energy Model
Saibal Ray; Farook Rahaman; Utpal Mukhopadhyay; Ruby Sarkar
2010-03-30
We present a model for the present accelerating Universe and focus on the different important physical variables involved in the model under the phenomenological assumption $\\Lambda \\propto H^2$ with a prescription for equation of state parameter in the form $\\omega(t)=\\omega_0+\\frac{\\omega_1\\tau}{t^2}$, where $\\omega_0$ and $\\omega_1$ are two constants and $\\tau$ is a parameter having dimension of time $t^2$. General expressions for the density parameter $\\Omega$ and deceleration parameter $q$ are obtained which under specific bound reproduce some of the previous results. We explore physical features of these parameters which (i) provide the scenario of complete evolution of the cosmos with $\\omega(t)$ and (ii) agree mostly with the observational status of the present phase of the accelerating Universe.
Observational constraints on scalar field models of dark energy with barotropic equation of state
Sergijenko, Olga; Novosyadlyj, Bohdan [Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia str. 8, Lviv, 79005 (Ukraine); Durrer, Ruth, E-mail: olka@astro.franko.lviv.ua, E-mail: ruth.durrer@unige.ch, E-mail: novos@astro.franko.lviv.ua [Département de Physique Théorique and CAP, Université de Genève, 24 quai Ernest-Ansermet, CH-1211 Genève 4 (Switzerland)
2011-08-01
We constrain the parameters of dynamical dark energy in the form of a classical or tachyonic scalar field with barotropic equation of state jointly with other cosmological parameters using the following datasets: the CMB power spectra from WMAP7, the baryon acoustic oscillations in the space distribution of galaxies from SDSS DR7, the power spectrum of luminous red galaxies from SDSS DR7 and the light curves of SN Ia from 2 different compilations: Union2 (SALT2 light curve fitting) and SDSS (SALT2 and MLCS2k2 light curve fittings). It has been found that the initial value of dark energy equation of state parameter is constrained very weakly by most of the data while the other cosmological parameters are well constrained: their likelihoods and posteriors are similar, their forms are close to Gaussian (or half-Gaussian) and the confidence ranges are narrow. The most reliable determinations of the best-fit value and 1? confidence range for the initial value of the dark energy equation of state parameter are obtained from the combined datasets including SN Ia data from the full SDSS compilation with MLCS2k2 light curve fitting. In all such cases the best-fit value of this parameter is lower than the value of corresponding parameter for current epoch. Such dark energy loses its repulsive properties and in future the expansion of the Universe changes into contraction. We also perform a forecast for the Planck mock data and show that they narrow significantly the confidence ranges of cosmological parameters values, moreover, their combination with SN SDSS compilation with MLCS2k2 light curve fitting may exclude the fields with initial equation of state parameter > ?0.1 at 2? confidence level.
V. K. Oikonomou; N. Karagiannakis; Miok Park
2014-11-12
We study some aspects of cosmological evolution in a universe described by a viable curvature corrected exponential $F(R)$ gravity model, in the presence of matter fluids consisting of collisional matter and radiation. Particularly, we express the Friedmann-Robertson-Walker equations of motion in terms of parameters that are appropriate for describing the dark energy oscillations and compare the dark energy density and the dark energy equation of state parameter corresponding to collisional and non-collisional matter. In addition to these, and owing to the fact that the cosmological evolution of collisional and non-collisional matter universes, when quantified in terms of the Hubble parameter and the effective equation of states parameters, is very much alike, we further scrutinize the cosmological evolution study by extending the analysis to the study of matter perturbations in the matter domination era. We quantify this analysis in terms of the growth factor of matter perturbations, in which case the resulting picture of the cosmological evolution is clear, since collisional and non-collisional universes can be clearly distinguished. Interestingly enough, since it is known that the oscillations of the effective equation of state parameter around the phantom divide are undesirable and unwanted in $F(R)$ gravities, when these are considered for redshifts near the matter domination era and before, in the curvature corrected exponential model with collisional matter which we study here there exist oscillations that never cross the phantom divide. Therefore, this rather unwanted feature of the effective equation of state parameter is also absent in the collisional matter filled universe.
High-redshift investigation on the dark energy equation of state
NASA Astrophysics Data System (ADS)
Piedipalumbo, E.; Della Moglie, E.; De Laurentis, M.; Scudellaro, P.
2014-07-01
The understanding of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. Whether or not the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, many models have been proposed to solve this problem, both in the context of General Relativity and alternative theories of gravity. Actually, a further possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state (EOS), w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here, we investigate different models of dark energy, described through several parametrizations of the EOS. Our high-redshift analysis is based on the Union2 Type Ia supernovae data set (Suzuki et al.), the Hubble diagram constructed from some gamma-ray bursts luminosity-distance indicators, and Gaussian priors on the distance from the baryon acoustic oscillations, and the Hubble constant h (these priors have been included in order to help to break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the EOS parameters, we use the Markov Chain Monte Carlo Method. It turns out that, if exact flatness is assumed, the dark energy EOS is evolving for all the parametrizations that we considered. We finally compare our results with the ones obtained by previous cosmographic analyses performed on the same astronomical data sets, showing that the latter ones are sufficient to test and compare the new parametrizations.
Constraints on deflation from the equation of state of dark energy
Baum, Lauris; Frampton, Paul H; Matsuzaki, Shinya, E-mail: baum@physics.unc.edu, E-mail: frampton@physics.unc.edu, E-mail: synya@physics.unc.edu [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599 (United States)
2008-04-15
In cyclic cosmology based on phantom dark energy the requirement that our universe satisfy a CBE condition (comes back empty) imposes a lower bound on the number N{sub cp} of causal patches which separate just prior to turnaround. This bound depends on the dark energy equation of state w = p/{rho} = -1-{phi} with {phi}>0. More accurate measurement of {phi} will constrain N{sub cp}. The critical density {rho}{sub c} in the model has a lower bound {rho}{sub c}{>=}(10{sup 9} GeV){sup 4} or {rho}{sub c}{>=}(10{sup 18} GeV){sup 4} when the smallest bound state has size 10{sup -15} m, or 10{sup -35} m, respectively.
Dark energy as a fixed point of the Einstein Yang-Mills Higgs Equations
Rinaldi, Massimiliano
2015-01-01
We study the Einstein Yang-Mills Higgs equations in the $SO(3)$ representation on a isotropic and homogeneous flat Universe, in the presence of radiation and matter fluids. We map the equations of motion into a closed dynamical system of first-order differential equations and we find the equilibrium points. We show that there is only one stable fixed point that corresponds to an accelerated expanding Universe in the future. In the past, instead, there is an unstable fixed point that implies a stiff-matter domination. In between, we find three other unstable fixed points, corresponding, in chronological order, to radiation domination, to matter domination, and, finally, to a transition from decelerated expansion to accelerated expansion. We solve the system numerically and we confirm that there are smooth trajectories that correctly describe the evolution of the Universe, from a remote past dominated by radiation to a remote future dominated by dark energy, passing through a matter-dominated phase.
The Dark Energy Equation of State using Alternative High-z Cosmic Tracers
M. Plionis; R. Terlevich; S. Basilakos; F. Bresolin; E. Terlevich; J. Melnick; R. Chavez
2010-01-18
We propose to use alternative cosmic tracers to measure the dark energy equation of state and the matter content of the Universe [w(z) & Omega_m]. Our proposed method consists of two components: (a) tracing the Hubble relation using HII galaxies which can be detected up to very large redshifts, z~4, as an alternative to supernovae type Ia, and (b) measuring the clustering pattern of X-ray selected AGN at a median redshift of z~1. Each component of the method can in itself provide interesting constraints on the cosmological parameters, especially under our anticipation that we will reduce the corresponding random and systematic errors significantly. However, by joining their likelihood functions we will be able to put stringent cosmological constraints and break the known degeneracies between the dark energy equation of state (whether it is constant or variable) and the matter content of the universe and provide a powerful and alternative route to measure the contribution to the global dynamics and the equation of state of dark energy. A preliminary joint analysis of X-ray selected AGN (based on the largest to-date XMM survey; the 2XMM) and the currently largest SNIa sample (Hicken et al.), using as priors a flat universe and the WMAP5 normalization of the power-spectrum, provides: Omega_m=0.27+-0.02 and w=-0.96+-0.07. Equivalent and consistent results are provided by the joint analysis of X-ray selected AGN clustering and the latest Baryonic Acoustic Oscillation measures, providing: Omega_m=0.27+-0.02 and w=-0.97+-0.04.
Norbert Straumann
2003-11-26
After some remarks about the history and the mystery of the vacuum energy I shall review the current evidence for a cosmologically significant nearly homogeneous exotic energy density with negative pressure (`Dark Energy'). Special emphasis will be put on the recent polarization measurements by WMAP and their implications. I shall conclude by addressing the question: Do the current observations really imply the existence of a dominant dark energy component?
The State Equation of the Yang-Mills field Dark Energy Models
Wen Zhao; Yang Zhang
2008-11-28
In this paper, we study the possibility of building Yang-Mills(YM) field dark energy models with equation of state (EoS) crossing -1, and find that it can not be realized by the single YM field models, no matter what kind of lagrangian or initial condition. But the states of $-1-1$ to $<-1$, and it will go to the critical state of $\\omega=-1$ with the expansion of the Universe, which character is same with the single YM field models, and the Big Rip is naturally avoided.
Reconstruction of the deceleration parameter and the equation of state of dark energy
Gong Yungui; Wang Anzhong [College of Electronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China) and CASPER, Physics Department, Baylor University, Waco, Texas 76798 (United States); CASPER, Physics Department, Baylor University, Waco, Texas 76798 (United States)
2007-02-15
The new 182 gold supernova Ia data, the baryon acoustic oscillation measurement and the shift parameter determined from the Sloan Digital Sky Survey, and the three-year Wilkinson Microwave Anisotropy Probe data are combined to reconstruct the dark energy equation of state parameter w(z) and the deceleration parameter q(z). We find that the strongest evidence of acceleration happens around the redshift z{approx}0.2 and the stringent constraints on w(z) lie in the redshift range z{approx}0.2-0.5. At the sweet spot, -1.2
Sapone, Domenico; Kunz, Martin
2009-10-15
Dark energy perturbations are normally either neglected or else included in a purely numerical way, obscuring their dependence on underlying parameters like the equation of state or the sound speed. However, while many different explanations for the dark energy can have the same equation of state, they usually differ in their perturbations so that these provide a fingerprint for distinguishing between different models with the same equation of state. In this paper we derive simple yet accurate approximations that are able to characterize a specific class of models (encompassing most scalar-field models) which is often generically called 'dark energy'. We then use the approximate solutions to look at the impact of the dark energy perturbations on the dark matter power spectrum and on the integrated Sachs-Wolfe effect in the cosmic microwave background radiation.
Inference for the dark energy equation of state using Type IA supernova data
Christopher R. Genovese; Peter Freeman; Larry Wasserman; Robert C. Nichol; Christopher Miller
2009-01-01
The surprising discovery of an accelerating universe led cosmologists to posit the existence of “dark energy”—a mysterious energy field that permeates the universe. Understanding dark energy has become the central problem of modern cosmology. After describing the scientific background in depth, we formulate the task as a nonlinear inverse problem that expresses the comoving distance function in terms of the
dark matter dark energy inflation
Hu, Wayne
theory dark matter dark energy inflation The National Science Foundation The Kavli Foundation NSF Site Review November 28-29, 2005 #12;dark matter dark energy inflation NSF Site Visit November 28 - 29, 2005The National Science Foundation The Kavli Foundation The Theoretical Web UHE cosmic rays B
Detecting features in the dark energy equation of state: a wavelet approach
Hojjati, Alireza; Pogosian, Levon; Zhao, Gong-Bo E-mail: levon@sfu.ca
2010-04-01
We study the utility of wavelets for detecting the redshift evolution of the dark energy equation of state w(z) from the combination of supernovae (SNe), CMB and BAO data. We show that local features in w, such as bumps, can be detected efficiently using wavelets. To demonstrate, we first generate a mock supernovae data sample for a SNAP-like survey with a bump feature in w(z) hidden in, then successfully discover it by performing a blind wavelet analysis. We also apply our method to analyze the recently released ''Constitution'' SNe data, combined with WMAP and BAO from SDSS, and find weak hints of dark energy dynamics. Namely, we find that models with w(z) < ?1 for 0.2 < z < 0.5, and w(z) > ?1 for 0.5 < z < 1, are mildly favored at 95% confidence level. This is in good agreement with several recent studies using other methods, such as redshift binning with principal component analysis (PCA) (e.g. Zhao and Zhang, arXiv: 0908.1568)
Improved cosmological constraints on the curvature and equation of state of dark energy
NASA Astrophysics Data System (ADS)
Pan, Nana; Gong, Yungui; Chen, Yun; Zhu, Zong-Hong
2010-08-01
We apply the Constitution compilation of 397 supernova Ia, the baryon acoustic oscillation measurements including the A parameter, the distance ratio and the radial data, the five-year Wilkinson microwave anisotropy probe and the Hubble parameter data to study the geometry of the Universe and the property of dark energy by using the popular Chevallier-Polarski-Linder and Jassal-Bagla-Padmanabhan parameterizations. We compare the simple ?2 method of joined contour estimation and the Monte Carlo Markov chain method, and find that it is necessary to make the marginalized analysis on the error estimation. The probabilities of ?k and wa in the Chevallier-Polarski-Linder model are skew distributions, and the marginalized 1? errors are ?m = 0.279+0.015- 0.008, ?k = 0.005+0.006- 0.011, w0 = -1.05+0.23- 0.06 and wa = 0.5+0.3- 1.5. For the Jassal-Bagla-Padmanabhan model, the marginalized 1? errors are ?m = 0.281+0.015- 0.01, ?k = 0.000+0.007- 0.006, w0 = -0.96+0.25- 0.18 and wa = -0.6+1.9- 1.6. The equation of state parameter w(z) of dark energy is negative in the redshift range 0 <= z <= 2 at more than 3? level. The flat ?CDM model is consistent with the current observational data at the 1? level.
Bayesian model selection without evidences: application to the dark energy equation-of-state
Hee, Sonke; Hobson, Mike P; Lasenby, Anthony N
2015-01-01
A method is presented for Bayesian model selection without explicitly computing evidences, by using a combined likelihood and introducing an integer model selection parameter $n$ so that Bayes factors, or more generally posterior odds ratios, may be read off directly from the posterior of $n$. If the total number of models under consideration is specified a priori, the full joint parameter space $(\\theta, n)$ of the models is of fixed dimensionality and can be explored using standard MCMC or nested sampling methods, without the need for reversible jump MCMC techniques. The posterior on $n$ is then obtained by straightforward marginalisation. We demonstrate the efficacy of our approach by application to several toy models. We then apply it to constraining the dark energy equation-of-state using a free-form reconstruction technique. We show that $\\Lambda$CDM is significantly favoured over all extensions, including the simple $w(z){=}{\\rm constant}$ model.
NASA Astrophysics Data System (ADS)
Linden, Sebastian; Virey, Jean-Marc
2008-07-01
We test the robustness and flexibility of the Chevallier-Polarski-Linder (CPL) parametrization of the dark energy equation of state w(z)=w0+wa(z)/(1+z) in recovering a four-parameter steplike fiducial model. We constrain the parameter space region of the underlying fiducial model where the CPL parametrization offers a reliable reconstruction. It turns out that non-negligible biases leak into the results for recent (z<2.5) rapid transitions, but that CPL yields a good reconstruction in all other cases. The presented analysis is performed with supernova Ia data as forecasted for a space mission like SNAP/JDEM, combined with future expectations for the cosmic microwave background shift parameter R and the baryonic acoustic oscillation parameter A.
Thermodynamical properties of dark energy with the equation of state ?=?0+?1z
NASA Astrophysics Data System (ADS)
Zhang, Yongping; Yi, Zelong; Zhang, Tong-Jie; Liu, Wenbiao
2008-01-01
The thermodynamical properties of dark energy are usually investigated with the equation of state ?=?0+?1z. Recent observations show that our Universe is accelerating, and the apparent horizon and the event horizon vary with redshift z. Because definitions of the temperature and entropy of a black hole are used to describe the two horizons of the Universe, we examine the thermodynamical properties of the Universe, which is enveloped by the apparent horizon and the event horizon, respectively. We show that the first and the second laws of thermodynamics inside the apparent horizon in any redshift are satisfied, while they are broken down inside the event horizon in some redshifts. Therefore, the apparent horizon for the Universe may be the boundary of thermodynamical equilibrium for the Universe like the event horizon for a black hole.
Thermodynamical properties of dark energy with the equation of state $% ?=?_{0}+?_{1}z$
Yongping Zhang; Ze-Long Yi; Tong-Jie Zhang; Wenbiao Liu
2007-10-30
The thermodynamical properties of dark energy are usually investigated with the equation of state $\\omega =\\omega_{0}+\\omega_{1}z$. Recent observations show that our universe is accelerating, and the apparent horizon and the event horizon vary with redshift $z$. When definitions of the temperature and entropy of a black hole are used to the two horizons of the universe, we examine the thermodynamical properties of the universe which is enveloped by the apparent horizon and the event horizon respectively. We show that the first and the second laws of thermodynamics inside the apparent horizon in any redshift are satisfied, while they are broken down inside the event horizon in some redshift. Therefore, the apparent horizon for the universe may be the boundary of thermodynamical equilibrium for the universe like the event horizon for a black hole.
Zhang, Yongping; Zhang, Tong-Jie; Liu, Wenbiao
2007-01-01
The thermodynamical properties of dark energy are usually investigated with the equation of state $\\omega =\\omega_{0}+\\omega_{1}z$. Recent observations show that our universe is accelerating, and the apparent horizon and the event horizon vary with redshift $z$. When definitions of the temperature and entropy of a black hole are used to the two horizons of the universe, we examine the thermodynamical properties of the universe which is enveloped by the apparent horizon and the event horizon respectively. We show that the first and the second laws of thermodynamics inside the apparent horizon in any redshift are satisfied, while they are broken down inside the event horizon in some redshift. Therefore, the apparent horizon for the universe may be the boundary of thermodynamical equilibrium for the universe like the event horizon for a black hole.
Speliotopoulos, Achilles D
2010-01-01
Recently, an extension of the geodesic equations of motion using the Dark Energy length scale was proposed. Here, we apply this extension to the analyzing the motion of test particles at the galactic scale and longer. A cosmological check of the extension is made using the observed rotational velocity curves and core sizes of 1393 spiral galaxies. We derive the density profile of a model galaxy using this extension, and with it, we calculate $\\sigma_8$ to be $0.73_{\\pm 0.12}$; this is within experimental error of the WMAP value of $0.761_{-0.048}^{+0.049}$. We then calculate $R_{200}$ to be $206_{\\pm 53}$ kpc, which is in reasonable agreement with observations.
Chasing the phantom: A closer look at type Ia supernovae and the dark energy equation of state
NASA Astrophysics Data System (ADS)
Shafer, Daniel L.; Huterer, Dragan
2014-03-01
Some recent observations provide >2? evidence for phantom dark energy—a value of the dark energy equation of state less than the cosmological-constant value of -1. We focus on constraining the equation of state by combining current data from the most mature geometrical probes of dark energy: type Ia supernovae (SNe Ia) from the Supernova Legacy Survey (SNLS3), the Supernova Cosmology Project (Union2.1), and the Pan-STARRS1 survey (PS1); cosmic microwave background measurements from Planck and WMAP9; and a combination of measurements of baryon acoustic oscillations. The combined data are consistent with w=-1 for the Union2.1 sample, though they present moderate (˜1.9?) evidence for a phantom value when either the SNLS3 or PS1 sample is used instead. We study the dependence of the constraints on the redshift, stretch, color, and host galaxy stellar mass of SNe, but we find no unusual trends. In contrast, the constraints strongly depend on any external H0 prior: a higher adopted value for the direct measurement of the Hubble constant (H0?71 km/s/Mpc) leads to ?2? evidence for phantom dark energy. Given Planck data, we can therefore make the following statement at 2? confidence: either the SNLS3 and PS1 data have systematics that remain unaccounted for or the Hubble constant is below 71 km/s/Mpc; else the dark energy equation of state is indeed phantom.
Dark-energy thermodynamic models
Besprosvany, Jaime [Instituto de Fisica, Universidad Nacional Autonoma de Mexico, Apartado Postal 20-364, Mexico 01000, Distrito Federal (Mexico); Izquierdo, German [Institut de Mathematiques de Bourgogne, Universite de Bourgogne, 9 Av. Alain Savary, 21078 Dijon Cedex (France)
2010-12-07
We study cosmological consequences of dark-energy thermodynamic models. The assumption that dark energy is conformed of quanta, and an extensivity argument generalize its equation of state. This implies that dark energy and another key component exchange energy. The energy densities of dark energy and the other component then tend asymptotically to a constant, thus explaining the coincidence of dark matter and dark energy today. On the other hand, a model of non-relativistic particles in a Bose-Einstein condensate, with a short-range attractive interaction, produces acceleration. It is shown that the phantom-acceleration regime, at the beginning of the universe, solves the horizon problem.
Petiteau, Antoine; Babak, Stanislav; Sesana, Alberto [Max-Planck-Institut fuer Gravitationsphysik, Albert-Einstein-Institut, Am Muhlenberg 1, D-14476 Golm (Germany)
2011-05-10
Gravitational wave (GW) signals from coalescing massive black hole (MBH) binaries could be used as standard sirens to measure cosmological parameters. The future space-based GW observatory Laser Interferometer Space Antenna (LISA) will detect up to a hundred of those events, providing very accurate measurements of their luminosity distances. To constrain the cosmological parameters, we also need to measure the redshift of the galaxy (or cluster of galaxies) hosting the merger. This requires the identification of a distinctive electromagnetic event associated with the binary coalescence. However, putative electromagnetic signatures may be too weak to be observed. Instead, we study here the possibility of constraining the cosmological parameters by enforcing statistical consistency between all the possible hosts detected within the measurement error box of a few dozen of low-redshift (z < 3) events. We construct MBH populations using merger tree realizations of the dark matter hierarchy in a {Lambda}CDM universe, and we use data from the Millennium simulation to model the galaxy distribution in the LISA error box. We show that, assuming that all the other cosmological parameters are known, the parameter w describing the dark energy equation of state can be constrained to a 4%-8% level (2{sigma} error), competitive with current uncertainties obtained by type Ia supernovae measurements, providing an independent test of our cosmological model.
Cosmo-dynamics and dark energy with non-linear equation of state: a quadratic model
Ananda, K N; Ananda, Kishore N.; Bruni, Marco
2005-01-01
We investigate the general relativistic dynamics of Robertson-Walker models with a non-linear equation of state (EoS), focusing on the quadratic case P = P_0 + \\alpha \\rho + \\beta \\rho^2. This may be taken to represent the Taylor expansion of any arbitrary barotropic EoS, P(\\rho). With the right combination of P_0, \\alpha and \\beta, it serves as a simple phenomenological model for dark energy, or even unified dark matter. Indeed we show that this simple model for the EoS can produce a large variety of qualitatively different dynamical behaviors that we classify using dynamical systems theory. An almost universal feature is that accelerated expansion phases are mostly natural for these non-linear EoS's. These are often asymptotically de Sitter thanks to the appearance of an effective cosmological constant. Other interesting possibilities that arise from the quadratic EoS are closed models that can oscillate with no singularity, models that bounce between infinite contraction/expansion and models which evolve f...
Cosmo-dynamics and dark energy with non-linear equation of state: a quadratic model
Kishore N. Ananda; Marco Bruni
2005-12-08
We investigate the general relativistic dynamics of Robertson-Walker models with a non-linear equation of state (EoS), focusing on the quadratic case P = P_0 + \\alpha \\rho + \\beta \\rho^2. This may be taken to represent the Taylor expansion of any arbitrary barotropic EoS, P(\\rho). With the right combination of P_0, \\alpha and \\beta, it serves as a simple phenomenological model for dark energy, or even unified dark matter. Indeed we show that this simple model for the EoS can produce a large variety of qualitatively different dynamical behaviors that we classify using dynamical systems theory. An almost universal feature is that accelerated expansion phases are mostly natural for these non-linear EoS's. These are often asymptotically de Sitter thanks to the appearance of an effective cosmological constant. Other interesting possibilities that arise from the quadratic EoS are closed models that can oscillate with no singularity, models that bounce between infinite contraction/expansion and models which evolve from a phantom phase, asymptotically approaching a de Sitter phase instead of evolving to a "Big Rip". In a second paper we investigate the effects of the quadratic EoS in inhomogeneous and anisotropic models, focusing in particular on singularities.
Keith A. Olive
2010-01-27
A brief overview of our current understanding of abundance and properties of dark energy and dark matter is presented. A more focused discussion of supersymmetric dark matter follows. Included is a frequentist approach to the supersymmetric parameter space and consequences for the direct detection of dark matter.
Dark Group: Dark Energy and Dark Matter
Axel de la Macorra
2004-02-03
We study the possibility that a dark group, a gauge group with particles interacting with the standard model particles only via gravity, is responsible for containing the dark energy and dark matter required by present day observations. We show that it is indeed possible and we determine the constrains for the dark group. The non-perturbative effects generated by a strong gauge coupling constant can de determined and a inverse power law scalar potential IPL for the dark meson fields is generated parameterizing the dark energy. On the other hand it is the massive particles, e.g. dark baryons, of the dark gauge group that give the corresponding dark matter. The mass of the dark particles is of the order of the condensation scale $\\Lambda_c$ and the temperature is smaller then the photon's temperature. The dark matter is of the warm matter type. The only parameters of the model are the number of particles of the dark group. The allowed values of the different parameters are severely restricted. The dark group energy density at $\\Lambda_c$ must be $ \\Omdgc \\leq 0.17$ and the evolution and acceptable values of dark matter and dark energy leads to a constrain of $\\Lmc$ and the IPL parameter $n$ giving $\\Lambda_c=O(1-10^3) eV$ and $0.28 \\leq n \\leq 1.04$.
NASA Astrophysics Data System (ADS)
Liang, Shi-Dong; Harko, Tiberiu
2015-04-01
Based on the analogy with superconductor physics we consider a scalar-vector-tensor gravitational model, in which the dark energy action is described by a gauge invariant electromagnetic type functional. By assuming that the ground state of the dark energy is in a form of a condensate with the U(1) symmetry spontaneously broken, the gauge invariant electromagnetic dark energy can be described in terms of the combination of a vector and of a scalar field (corresponding to the Goldstone boson), respectively. The gravitational field equations are obtained by also assuming the possibility of a nonminimal coupling between the cosmological mass current and the superconducting dark energy. The cosmological implications of the dark energy model are investigated for a Friedmann-Robertson-Walker homogeneous and isotropic geometry for two particular choices of the electromagnetic type potential, corresponding to a pure electric type field, and to a pure magnetic field, respectively. The time evolutions of the scale factor, matter energy density and deceleration parameter are obtained for both cases, and it is shown that in the presence of the superconducting dark energy the Universe ends its evolution in an exponentially accelerating vacuum de Sitter state. By using the formalism of the irreversible thermodynamic processes for open systems we interpret the generalized conservation equations in the superconducting dark energy model as describing matter creation. The particle production rates, the creation pressure and the entropy evolution are explicitly obtained.
The Dark Energy Survey Collaboration
2005-10-12
We describe the Dark Energy Survey (DES), a proposed optical-near infrared survey of 5000 sq. deg of the South Galactic Cap to ~24th magnitude in SDSS griz, that would use a new 3 sq. deg CCD camera to be mounted on the Blanco 4-m telescope at Cerro Telolo Inter-American Observatory (CTIO). The survey data will allow us to measure the dark energy and dark matter densities and the dark energy equation of state through four independent methods: galaxy clusters, weak gravitational lensing tomography, galaxy angular clustering, and supernova distances. These methods are doubly complementary: they constrain different combinations of cosmological model parameters and are subject to different systematic errors. By deriving the four sets of measurements from the same data set with a common analysis framework, we will obtain important cross checks of the systematic errors and thereby make a substantial and robust advance in the precision of dark energy measurements.
Roland de Putter; Eric V. Linder
2008-08-01
Exploring the diversity of dark energy dynamics, we discover a calibration relation, a uniform stretching of the amplitude of the equation of state time variation with scale factor. This defines homogeneous families of dark energy physics. The calibration factor has a close relation to the standard time variation parameter w_a, and we show that the new, calibrated w_a describes observables, i.e. distance and Hubble parameter as a function of redshift, typically to an accuracy level of 10^{-3}. We discuss implications for figures of merit for dark energy science programs.
Yong-Seon Song; Lloyd Knox
2003-12-08
We study how parameter error forecasts for tomographic cosmic shear observations are affected by sky coverage, density of source galaxies, inclusion of CMB experiments, simultaneou fitting of non--dark energy parameters, and the parametrization of the history of the dark energy equation-of-state parameter w(z). We find tomographic shear-shear power spectra on large angular scales (l0.04.
Cosmic chronometers: constraining the equation of state of dark energy. I: H(z) measurements
Stern, Daniel [Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 169-506, Pasadena CA-91109 (United States); Jimenez, Raul; Verde, Licia [ICREA and Institute of Sciences of the Cosmos (ICC), University of Barcelona, Barcelona 08028 (Spain); Kamionkowski, Marc [California Institute of Technology, Mail Code 350-17, Pasadena, California 91125 (United States); Stanford, S. Adam, E-mail: stern@thisvi.jpl.nasa.gov, E-mail: raul@icc.ub.edu, E-mail: licia@icc.ub.edu, E-mail: kamion@tapir.caltech.edu, E-mail: stanford@physics.ucdavis.edu [University of California, Davis, CA 95616 (United States)
2010-02-01
We present new determinations of the cosmic expansion history from red-envelope galaxies. We have obtained for this purpose high-quality spectra with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with high-quality, publicly available archival spectra from the SPICES and VVDS surveys. We improve over our previous expansion history measurements in Simon et al. (2005) by providing two new determinations of the expansion history: H(z) = 97±62 km sec{sup ?1} Mpc{sup ?1} at z ? 0.5 and H(z) = 90±40 km sec{sup ?1} Mpc{sup ?1} at z ? 0.9. We discuss the uncertainty in the expansion history determination that arises from uncertainties in the synthetic stellar-population models. We then use these new measurements in concert with cosmic-microwave-background (CMB) measurements to constrain cosmological parameters, with a special emphasis on dark-energy parameters and constraints to the curvature. In particular, we demonstrate the usefulness of direct H(z) measurements by constraining the dark-energy equation of state parameterized by w{sub 0} and w{sub a} and allowing for arbitrary curvature. Further, we also constrain, using only CMB and H(z) data, the number of relativistic degrees of freedom to be 4±0.5 and their total mass to be < 0.2 eV, both at 1?.
Nonparametric reconstruction of the dark energy equation of state from diverse data sets
NASA Astrophysics Data System (ADS)
Holsclaw, Tracy; Alam, Ujjaini; Sansó, Bruno; Lee, Herbie; Heitmann, Katrin; Habib, Salman; Higdon, David
2011-10-01
The cause of the accelerated expansion of the Universe poses one of the most fundamental questions in physics today. In the absence of a compelling theory to explain the observations, a first task is to develop a robust phenomenological approach: If the acceleration is driven by some form of dark energy, then the phenomenology is determined by the form of the dark energy equation of state w(z) as a function of redshift. A major aim of ongoing and upcoming cosmological surveys is to measure w and its evolution at high accuracy. Since w(z) is not directly accessible to measurement, powerful reconstruction methods are needed to extract it reliably from observations. We have recently introduced a new reconstruction method for w(z) based on Gaussian process modeling. This method can capture nontrivial w(z) dependences and, most importantly, it yields controlled and unbiased error estimates. In this paper we extend the method to include a diverse set of measurements: baryon acoustic oscillations, cosmic microwave background measurements, and supernova data. We analyze currently available data sets and present the resulting constraints on w(z), finding that current observations are in very good agreement with a cosmological constant. In addition, we explore how well our method captures nontrivial behavior of w(z) by analyzing simulated data assuming high-quality observations from future surveys. We find that the baryon acoustic oscillation measurements by themselves already lead to remarkably good reconstruction results and that the combination of different high-quality probes allows us to reconstruct w(z) very reliably with small error bounds.
Effective equation of state for running vacuum: `mirage' quintessence and phantom dark energy
NASA Astrophysics Data System (ADS)
Basilakos, Spyros; Solà, Joan
2014-02-01
Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not incompatible with a phantom phase near our time. This has been the case in the years of Wilkinson Microwave Anisotropy Probe observations, in combination with the remaining cosmological observables. Such situations did not completely disappear from the data collected from the Planck satellite mission. In it the EoS analysis may still be interpreted as suggesting ?D ? -1, and so a mildly evolving DE cannot be discarded. In our opinion, the usual ansatzs made on the structure of the EoS for dynamical DE models (e.g. quintessence and the like) is too simplified. In this work, we examine in detail some of these issues and suggest that a general class of models with a dynamical vacuum energy density could explain the persistent phantom anomaly, despite this there is no trace of real phantom behaviour in them. The spurious or `mirage' effect is caused by an attempt to describe them as if the DE would be caused by fundamental phantom scalar fields. Remarkably, the effective DE behaviour can also appear as quintessence in transit to phantom, or vice versa.
Unified Description of Dark Energy and Dark Matter
Walter Petry
2008-11-09
Dark energy in the universe is assumed to be vacuum energy. The energy-momentum of vacuum is described by a scale-dependent cosmological constant. The equations of motion imply for the density of matter (dust) the sum of the usual matter density (luminous matter) and an additional matter density (dark matter) similar to the dark energy. The scale-dependent cosmological constant is given up to an exponent which is approximated by the experimentally decided density parameters of dark matter and dark energy. This yields that dark matter is one third of dark energy for all times implying an explanation of the coincidence of dark matter and dark energy. In the final state, the universe becomes dark, consisting of dark matter and dark energy.
Interacting Agegraphic Dark Energy
Hao Wei; Rong-Gen Cai
2009-01-10
A new dark energy model, named "agegraphic dark energy", has been proposed recently, based on the so-called K\\'{a}rolyh\\'{a}zy uncertainty relation, which arises from quantum mechanics together with general relativity. In this note, we extend the original agegraphic dark energy model by including the interaction between agegraphic dark energy and pressureless (dark) matter. In the interacting agegraphic dark energy model, there are many interesting features different from the original agegraphic dark energy model and holographic dark energy model. The similarity and difference between agegraphic dark energy and holographic dark energy are also discussed.
Dark energy without dark energy
Pedro F. Gonzalez-Diaz
2006-08-29
It is proposed that the current acceleration of the universe is not originated by the existence of a mysterious dark energy fluid nor by the action of extra terms in the gravity Lagrangian, but just from the sub-quantum potential associated with the CMB particles. The resulting cosmic scenario corresponds to a benigner phantom model which is free from the main problems of the current phantom approaches.
Michael Doran; Georg Robbers
2006-06-30
We propose a novel parameterization of the dark energy density. It is particularly well suited to describe a non-negligible contribution of dark energy at early times and contains only three parameters, which are all physically meaningful: the fractional dark energy density today, the equation of state today and the fractional dark energy density at early times. As we parameterize Omega_d(a) directly instead of the equation of state, we can give analytic expressions for the Hubble parameter, the conformal horizon today and at last scattering, the sound horizon at last scattering, the acoustic scale as well as the luminosity distance. For an equation of state today w_0 < -1, our model crosses the cosmological constant boundary. We perform numerical studies to constrain the parameters of our model by using Cosmic Microwave Background, Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that the fractional dark energy density at early times Omega_early < 0.06. This bound tightens considerably to Omega_early < 0.04 when the latest Boomerang data is included. We find that both the gold sample of Riess et. al. and the SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer w_0 < -1, but are well compatible with a cosmological constant.
Spherical collapse with dark energy
Irit Maor
2006-02-20
I discuss the work of Maor and Lahav [1], in which the inclusion of dark energy into the spherical collapse formalism is reviewed. Adopting a phenomenological approach, I consider the consequences of - a) allowing the dark energy to cluster, and, b) including the dark energy in the virialization process. Both of these issues affect the final state of the system in a fundamental way. The results suggest a potentially differentiating signature between a true cosmological constant and a dynamic form of dark energy. This signature is unique in the sense that it does not depend on a measurement of the value of the equation of state of dark energy.
Vinod B. Johri; P. K. Rath
2007-02-24
The dynamical age of the universe depends upon the rate of the expansion of the universe, which explicitly involves the dark energy equation of state parameter $w(z)$. Consequently, the evolution of $w(z)$ has a direct imprint on the age of the universe. We have shown that the dynamical age of the universe as derived from CMB data can be used as an authentic criterion, being independent of the priors like the present value of the Hubble constant $H_{0}$ and the cosmological density parameter $\\Omega_{M}^{0}$, to constrain the range of admissible values of $w$ for quiessence models and to test the physically viable parametrizations of the equation of state $w(z) $ in kinessence models. An upper bound on variation of dark energy density is derived and a relation between cosmological density parameters and the transition redshift is established.
Dark Matter, Dark Energy and the Chaplygin Gas
Neven Bilic; Gary B. Tupper; Raoul D. Viollier
2002-07-19
We formulate a Zel'dovich-like approximation for the Chaplygin gas equation of state P = -A/rho, and sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M-theory.
Modeling Dark Matter and Dark Energy
NASA Astrophysics Data System (ADS)
Ludwick, Kevin J.
We study various models of dark matter and dark energy. We first examine the implications of the assumption that black holes act as dark matter. Assuming dark matter in galactic halos is composed solely of black holes, and using observational constraints, we calculate the number of halo black holes and the total entropy due to them. We then study the prospect of dark energy with a non-constant density. We analyze several parameterizations of dark energy density from the literature and one of our own, in particular focusing on the value of redshift at which cosmic acceleration due to dark energy begins. In considering the properties of dark energy densities that monotonically increase over time, we present two new categorizations of dark energy models that we dub "little rip" and "pseudo-rip" models, and both avoid future singularities in the cosmic scale factor. The dark energy density of a little rip model continually increases for all future time, and a pseudo-rip model's dark energy density asymptotically approaches a maximum value. These two types of models, big rip models, and models that have constant dark energy densities comprise all categories of dark energy density with monotonic growth in the future. A little rip leads to the dissociation of all bound structures in the universe, and a pseudo-rip occurs when all bound structures at or below a certain threshold dissociate. We present explicit parameterizations of the little rip and pseudo-rip models that fit supernova data well, and we calculate the times at which particular bound structures rip apart. In looking at different applications of these models, we show that coupling between dark matter and dark energy with an equation of state for a little rip can change the usual evolution of a little rip model into an asymptotic de Sitter expansion. We also give conditions on minimally coupled phantom scalar field models and scalar-tensor models that indicate whether or not they lead to a little rip or a pseudo-rip.
On dark energy isocurvature perturbation
Liu, Jie; Zhang, Xinmin [Institute of High Energy Physics, Chinese Academy of Science, P.O. Box 918-4, Beijing 100049 (China); Li, Mingzhe, E-mail: liujie@ihep.ac.cn, E-mail: limz@nju.edu.cn, E-mail: xmzhang@ihep.ac.cn [Department of Physics, Nanjing University, Nanjing 210093 (China)
2011-06-01
Determining the equation of state of dark energy with astronomical observations is crucially important to understand the nature of dark energy. In performing a likelihood analysis of the data, especially of the cosmic microwave background and large scale structure data the dark energy perturbations have to be taken into account both for theoretical consistency and for numerical accuracy. Usually, one assumes in the global fitting analysis that the dark energy perturbations are adiabatic. In this paper, we study the dark energy isocurvature perturbation analytically and discuss its implications for the cosmic microwave background radiation and large scale structure. Furthermore, with the current astronomical observational data and by employing Markov Chain Monte Carlo method, we perform a global analysis of cosmological parameters assuming general initial conditions for the dark energy perturbations. The results show that the dark energy isocurvature perturbations are very weakly constrained and that purely adiabatic initial conditions are consistent with the data.
Francisco S. N. Lobo
2006-01-17
The gravastar picture is an alternative model to the concept of a black hole, where there is an effective phase transition at or near where the event horizon is expected to form, and the interior is replaced by a de Sitter condensate. In this work, a generalization of the gravastar picture is explored, by considering a matching of an interior solution governed by the dark energy equation of state, $\\omega\\equiv p/ \\rhoSchwarzschild vacuum solution at a junction interface. The motivation for implementing this generalization arises from the fact that recent observations have confirmed an accelerated cosmic expansion, for which dark energy is a possible candidate. Several relativistic dark energy stellar configurations are analyzed by imposing specific choices for the mass function. The first case considered is that of a constant energy density, and the second choice, that of a monotonic decreasing energy density in the star's interior. The dynamical stability of the transition layer of these dark energy stars to linearized spherically symmetric radial perturbations about static equilibrium solutions is also explored. It is found that large stability regions exist that are sufficiently close to where the event horizon is expected to form, so that it would be difficult to distinguish the exterior geometry of the dark energy stars, analyzed in this work, from an astrophysical black hole.
Tachyonic Teleparallel Dark Energy
A. Banijamali; B. Fazlpour
2012-06-13
Teleparallel gravity is an equivalent formulation of general relativity in which instead of the Ricci scalar $R$, one uses the torsion scalar $T$ for the Lagrangian density. Recently teleparallel dark energy has been proposed by Geng et al. in (Geng et al., 2011). They have added quintessence scalar field, allowing also a non-minimal coupling with gravity in the Lagrangian of teleparallel gravity and found that such a non-minimally coupled quintessence theory has a richer structure than the same one in the frame work of general relativity. In the present work we are interested in tachyonic teleparallel dark energy in which scalar field is responsible for dark energy in the frame work of torsion gravity. We find that such a non-minimally coupled tachyon gravity can realize the crossing of the phantom divide line for the effective equation of state. Using the numerical calculations we display such a behavior of the model explicitly.
NASA Astrophysics Data System (ADS)
Sánchez, E.
2006-11-01
A new proposed optical-near infrared survey of 5000 square degrees of the South Galactic Cap is presented. To perform it, a new CCD camera of 3 deg2 field of view will be mounted on the Blanco 4m telescope at Cerro Tololo (Chile). The goal of the survey is the measurement of the dark matter and dark energy densities and the determination of the dark energy equation of state. Four independent methods will be used: galaxy clusters counts and distributions, weak gravitational lensing tomography, angular distribution of galaxies and supernovae Ia distances. Obtaining the four measurements from the same data set will allow important cross-checks of the systematic uncertainties to obtain a robust and precise determination of the parameters.
THEORY OF DARK ENERGY AND DARK MATTER MARCO HERNANDEZ, TIAN MA, AND SHOUHONG WANG
THEORY OF DARK ENERGY AND DARK MATTER MARCO HERNANDEZ, TIAN MA, AND SHOUHONG WANG Dedicated to the the presence of dark energy and dark matter. With the field equations, we show that gravity can display both attractive and repulsive behavior, and the dark matter and dark energy are just a property of gravity caused
Reconstructing and deconstructing dark energy
Linder, Eric V.
2004-06-07
The acceleration of the expansion of the universe, ascribed to a dark energy, is one of the most intriguing discoveries in science. In addition to precise, systematics controlled data, clear, robust interpretation of the observations is required to reveal the nature of dark energy. Even for the simplest question: is the data consistent with the cosmological constant? there are important subtleties in the reconstruction of the dark energy properties. We discuss the roles of analysis both in terms of the Hubble expansion rate or dark energy density {rho}DE(z) and in terms of the dark energy equation of state w(z), arguing that each has its carefully defined place. Fitting the density is best for learning about the density, but using it to probe the equation of state can lead to instability and bias.
Thermodynamical properties of dark energy
Gong Yungui; Wang Bin; Wang Anzhong [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China) and CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States); Department of Physics, Fudan University, Shanghai 200433 (China); CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States)
2007-06-15
We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature T{approx}a{sup -n} and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state w>-1 and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation, and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.
R. Chan; M. F. A. da Silva; J. F. Villas da Rocha
2008-09-12
Since the discovery of the accelerated expansion of the universe, it was necessary to introduce a new component of matter distribution called dark energy. The standard cosmological model considers isotropy of the pressure and assumes an equation of state $p=\\omega \\rho$, relating the pressure $p$ and the energy density $\\rho$. The interval of the parameter $\\omega$ defines the kind of matter of the universe, related to the fulfillment, or not, of the energy conditions of the fluid. The recent interest in this kind of fluid with anisotropic pressure, in the scenario of the gravitational collapse and star formation, imposes a carefull analysis of the energy conditions and the role of the components of the pressure. Here, in this work, we show an example where the classification of dark energy for isotropic pressure fluids is used incorrectly for anisotropic fluids. The correct classification and its consequences are presented.
Beyond two dark energy parameters.
Sarkar, Devdeep; Sullivan, Scott; Joudaki, Shahab; Amblard, Alexandre; Holz, Daniel E; Cooray, Asantha
2008-06-20
Our ignorance of dark energy is generally described by a two-parameter equation of state. In these approaches, a particular ad hoc functional form is assumed, and only two independent parameters are incorporated. We propose a model-independent, multiparameter approach to fitting dark energy and show that next-generation surveys will constrain the equation of state in three or more independent redshift bins to better than 10%. Future knowledge of dark energy will surpass two numbers (e.g., [w{0},w{1}] or [w{0},w{a}]), and we propose a more flexible approach to the analysis of present and future data. PMID:18643568
Cosmological Consequences of a Dark Energy Component with Super-Negative Equation of State
R. R. Caldwell
It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state
Phenomenological models of dark energy interacting with dark matter
NASA Astrophysics Data System (ADS)
Tamanini, Nicola
2015-08-01
An interaction between dark matter and dark energy is usually introduced by a phenomenological modification of the matter conservation equations, while the Einstein equations are left unchanged. Starting from some general and fundamental considerations, in this work it is shown that a coupling in the dark sector is likely to introduce new terms also in the gravitational dynamics. Specifically in the cosmological background equations a bulk dissipative pressure, characterizing viscous effects and able to suppress structure formation at small scales, should appear from the dark coupling. At the level of the perturbations the analysis presented in this work reveals instead the difficulties in properly defining the dark sector interaction from a phenomenological perspective.
On phenomenological models of dark energy interacting with dark matter
Tamanini, Nicola
2015-01-01
An interaction between dark matter and dark energy is usually introduced by a phenomenological modification of the matter conservation equations, while the Einstein equations are left unchanged. Starting from some general and fundamental considerations, in this work it is shown that a coupling in the dark sector is likely to introduce new terms also in the gravitational dynamics. Specifically in the cosmological background equations a bulk dissipative pressure, characterizing viscous effects and able to suppress structure formation at small scales, should appear from the dark coupling. At the level of the perturbations the analysis presented in this work reveals instead the difficulties in properly defining the dark sector interaction from a phenomenological perspective.
Ruth A. Daly; S. G. Djorgovski
2004-05-04
One of the goals of current cosmological studies is the determination of the expansion and acceleration rates of the universe as functions of redshift, and the determination of the properties of the dark energy that can explain these observations. Here the expansion and acceleration rates are determined directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. We use the latest set of distances to SN standard candles from Riess et al. (2004), supplemented by data on radio galaxy standard ruler sizes, as described by Daly and Djorgovski (2003, 2004). We find that the universe transitions from acceleration to deceleration at a redshift of about 0.4. The standard "concordance model" provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. The expansion and acceleration rates are then combined with a theory of gravity to determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. Adopting General Relativity as the correct theory of gravity, the redshift trends for the pressure, energy density, and equation of state of the dark energy out to redshifts of about one are determined, and are found to be generally consistent with the concordance model.
Linder, Eric V.
2004-04-01
The physical process leading to the acceleration of the expansion of the universe is unknown. It may involve new high energy physics or extensions to gravitation. Calling this generically dark energy, we examine the consistencies and relations between these two approaches, showing that an effective equation of state function w(z) is broadly useful in describing the properties of the dark energy. A variety of cosmological observations can provide important information on the dynamics of dark energy and the future looks bright for constraining dark energy, though both the measurements and the interpretation will be challenging. We also discuss a more direct relation between the spacetime geometry and acceleration, via ''geometric dark energy'' from the Ricci scalar, and superacceleration or phantom energy where the fate of the universe may be more gentle than the Big Rip.
Raul Jimenez; Licia Verde; Tommaso Treu; Daniel Stern
2003-02-27
We place tight constraints on the redshift-averaged, effective value of the equation of state of dark energy, w, using only the absolute ages of Galactic stars and the observed position of the first peak in the angular power spectrum of the CMB. We find w -1, this finding suggests that within our uncertainties, dark energy is indistinguishable from a classical vacuum energy term. We detect a correlation between the ages of the oldest galaxies and their redshift. This opens up the possibility of measuring w(z) by computing the relative ages of the oldest galaxies in the universe as a function of redshift, dz/dt. We show that this is a realistic possibility by computing dz/dt at z~0 from SDSS galaxies and obtain an independent estimate for the Hubble constant, H_0 = 69 \\pm 12 km s-1 Mpc-1. The small number of galaxies considered at z>0.2 does not yield, currently, a precise determination of w(z), but shows that the age--redshift relation is consistent with a Standard LCDM universe with $w=-1$.
Burra G. Sidharth
2014-12-30
Though the concept of a dark energy driven accelerating universe was introduced by the author in 1997, to date dark energy itself, as described below has remained a paradigm. A model for the cosmological constant is suggested.
Measuring the speed of dark: Detecting dark energy perturbations
Putter, Roland de [Berkeley Lab and University of California, Berkeley, California 94720 (United States); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan, 48109 (United States); Linder, Eric V. [Berkeley Lab and University of California, Berkeley, California 94720 (United States); Institute for the Early Universe, Ewha Womans University, Seoul (Korea, Republic of)
2010-05-15
The nature of dark energy can be probed not only through its equation of state but also through its microphysics, characterized by the sound speed of perturbations to the dark energy density and pressure. As the sound speed drops below the speed of light, dark energy inhomogeneities increase, affecting both cosmic microwave background and matter power spectra. We show that current data can put no significant constraints on the value of the sound speed when dark energy is purely a recent phenomenon, but can begin to show more interesting results for early dark energy models. For example, the best fit model for current data has a slight preference for dynamics [w(a){ne}-1], degrees of freedom distinct from quintessence (c{sub s{ne}}1), and early presence of dark energy [{Omega}{sub de}(a<<1){ne}0]. Future data may open a new window on dark energy by measuring its spatial as well as time variation.
Holographic Dark Energy Density
NASA Astrophysics Data System (ADS)
Saadat, Hassan
2011-06-01
In this article we consider the cosmological model based on the holographic dark energy. We study dark energy density in Universe with arbitrary spatially curvature described by the Friedmann-Robertson-Walker metric. We use Chevallier-Polarski-Linder parametrization to specify dark energy density.
Weak lensing and dark energy: The impact of dark energy on nonlinear dark matter clustering
Joudaki, Shahab; Cooray, Asantha; Holz, Daniel E. [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
2009-07-15
We examine the influence of percent-level dark energy corrections to the nonlinear matter power spectrum on constraints of the dark energy equation of state from future weak lensing probes. We explicitly show that a poor approximation (off by > or approx.10%) to the nonlinear corrections causes a > or approx. 1{sigma} bias on the determination of the dark energy equation of state. Future weak lensing surveys must therefore incorporate dark energy modifications to the nonlinear matter power spectrum accurate to the percent-level, to avoid introducing significant bias in their measurements. For the WMAP5 cosmology, the more accurate power spectrum is more sensitive to dark energy properties, resulting in a factor of 2 improvement in dark energy equation of state constraints. We explore the complementary constraints on dark energy from future weak lensing and supernova surveys. A space-based, Joint Dark Energy Mission-like survey measures the equation of state in five independent redshift bins to {approx}10%, while this improves to {approx}5% for a wide-field ground-based survey like the Large Synoptic Survey Telescope. These constraints are contingent upon our ability to control weak lensing systematic uncertainties to the sub-percent level.
A Field Theory Model for Dark Matter and Dark Energy in Interaction
Sandro Micheletti; Elcio Abdalla; Bin Wang
2009-05-18
We propose a field theory model for dark energy and dark matter in interaction. Comparing the classical solutions of the field equations with the observations of the CMB shift parameter, BAO, lookback time and Gold supernovae sample, we observe a possible interaction between dark sectors with energy decay from dark energy into dark matter. The observed interaction provides an alleviation to the coincidence problem.
Dark Energy From Fifth Dimension
H. Alavirad; N. Riazi
2008-01-21
Observational evidence for the existence of dark energy is strong. Here we suggest a model which is based on a modified gravitational theory in 5D and interpret the 5th dimension as a manifestation of dark energy in the 4D observable universe. We also obtain an equation of state parameter which varies with time. Finally, we match our model with observations by choosing the free parameters of the model.
Entropy bounds and dark energy
Stephen D. H. Hsu
2004-01-01
Entropy bounds render quantum corrections to the cosmological constant ? finite. Under certain assumptions, the natural value of ? is of order the observed dark energy density ?10?10 eV4, thereby resolving the cosmological constant problem. We note that the dark energy equation of state in these scenarios is w?p\\/?=0 over cosmological distances, and is strongly disfavored by observational data. Alternatively, ?
Seokcheon Lee
2005-04-28
We introduce a simple parametrization of the dark energy equation of state, \\omega, which is motivated from theory and recent experimental data. The theory is related to the tracker solution to alleviate the fine tuning problem. Recent experimental data indicates that the present value of \\omega is close to -1. We analyze the evolution of \\omega from the separation of CMB peaks and the time variation of the fine structure constant, \\alpha. We find that -1.00 \\leq \\omega^{(0)} \\leq -0.971_{-0.027}^{+0.017} and 1.76_{-0.42}^{+0.29} \\times 10^{-4} \\leq (d \\omega / dz)_{z=0} \\leq 0.041_{-0.037}^{+0.016} at 95% confidence level (CL).
Ruth Durrer; Roy Maartens
2007-12-05
Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy. Speculative ideas from string theory may hold some promise, but it is fair to say that no convincing model has yet been proposed. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity (or dark gravity) provides a new angle on the problem, but also faces serious difficulties, including in all known cases severe fine-tuning and the problem of explaining why the vacuum energy does not gravitate. The lack of an adequate theoretical framework for the late-time acceleration of the universe represents a deep crisis for theory -- but also an exciting challenge for theorists. It seems likely that an entirely new paradigm is required to resolve this crisis.
On virialization with dark energy
NASA Astrophysics Data System (ADS)
Maor, Irit; Lahav, Ofer
2005-07-01
We review the inclusion of dark energy in the formalism of spherical collapse, and the virialization of a two-component system, made of matter and dark energy. We compare two approaches in previous studies. The first assumes that only the matter component virializes, e.g. as in the case of a classic cosmological constant. The second approach allows the full system to virialize as a whole. We show that the two approaches give fundamentally different results for the final state of the system. This might be a signature discriminating between the classic cosmological constant which cannot virialize and a dynamical dark energy mimicking a cosmological constant. This signature is independent of the measured value of the equation of state. An additional issue which we address is energy non-conservation of the system, which originates from the homogeneity assumption for the dark energy. We propose a way to take this energy loss into account.
Simon DeDeo
2006-02-23
We explore the physics of a gas of particles interacting with a condensate that spontaneously breaks Lorentz invariance. The equation of state of this gas varies from 1/3 to less than -1 and can lead to the observed cosmic acceleration. The particles are always stable. In our particular class of models these particles are fermions with a chiral coupling to the condensate. They may behave as relativistic matter at early times, produce a brief period where they dominate the expansion with wsmall parameters in our models, which generically lead to dark energy clustering and, depending on the choice of parameters, smoothing of small scale power.
B. H. J. McKellar; T. Goldman; G. J. Stephenson, Jr.; P. M. Alsing
2009-08-06
We present a model in which the equation of state parameter w approaches -1 near a particular value of z, and has significant negative values in a restricted range of z. For example, one can have w ~ -1 near z = 1, and w > -0.2 from z = 0 to z = 0.3, and for z > 9. The ingredients of the model are neutral fermions (which may be neutrinos, neutralinos, etc) which are very weakly coupled to a light scalar field. This model emphasises the importance of the proposed studies of the properties of dark energy into the region z > 1.
How many dark energy parameters?
Linder, Eric V.; Huterer, Dragan
2005-05-16
For exploring the physics behind the accelerating universe a crucial question is how much we can learn about the dynamics through next generation cosmological experiments. For example, in defining the dark energy behavior through an effective equation of state, how many parameters can we realistically expect to tightly constrain? Through both general and specific examples (including new parametrizations and principal component analysis) we argue that the answer is 42 - no, wait, two. Cosmological parameter analyses involving a measure of the equation of state value at some epoch (e.g., w_0) and a measure of the change in equation of state (e.g., w') are therefore realistic in projecting dark energy parameter constraints. More elaborate parametrizations could have some uses (e.g., testing for bias or comparison with model features), but do not lead to accurately measured dark energy parameters.
Dark Energy in Global Brane Universe
Yongli Ping; Lixin Xu; Chengwu Zhang; Hongya Liu
2007-12-20
We discuss the exact solutions of brane universes and the results indicate the Friedmann equations on the branes are modified with a new density term. Then, we assume the new term as the density of dark energy. Using Wetterich's parametrization equation of state (EOS) of dark energy, we obtain the new term varies with the red-shift z. Finally, the evolutions of the mass density parameter $\\Omega_2$, dark energy density parameter $\\Omega_x$ and deceleration parameter q_2 are studied.
Dark Energy from Transplanckian Physics
NASA Astrophysics Data System (ADS)
Bastero-Gil, Mar; Mersini, Laura
2003-03-01
Observational evidence suggests that our universe is presently dominated by a dark energy component and undergoing accelerated expansion. The idea of the transplanckian dark energy (TDE) is based on the freeze-out mechanism of the ultralow frequency modes, ?(k) of very short distances, by the expansion of the background universe, ?(k) ? H. We then derive the equation of state for the TDE model, which has the distinctive feature of being continually time-dependent. The explanation of the coincidence puzzle relies entirely on the intrinsic time-evolution of the TDE equation of state.
Instability of agegraphic dark energy models
Kyoung Yee Kim; Hyung Won Lee; Yun Soo Myung
2007-09-18
We investigate the agegraphic dark energy models which were recently proposed to explain the dark energy-dominated universe. For this purpose, we calculate their equation of states and squared speeds of sound. We find that the squared speed for agegraphic dark energy is always negative. This means that the perfect fluid for agegraphic dark energy is classically unstable. Furthermore, it is shown that the new agegraphic dark energy model could describe the matter (radiation)-dominated universe in the far past only when the parameter $n$ is chosen to be $n>n_c$, where the critical values are determined to be $n_c=2.6878(2.5137752)$ numerically. It seems that the new agegraphic dark energy model is no better than the holographic dark energy model for the description of the dark energy-dominated universe, even though it resolves the causality problem.
Matter Field, Dark Matter and Dark Energy
Masayasu Tsuge
2009-03-24
A model concerning particle theory and cosmology is proposed. Matter field, dark matter and dark energy are created by an energy flow from space to primordial matter fields at the phase transition in the early universe.
Cold Dark Matter from Dark Energy
Aharon Davidson; David Karasik; Yoav Lederer
2001-01-01
Dark energy\\/matter unification is first demonstrated within the framework of a simplified model. Geodetic evolution of a cosmological constant dominated bubble Universe, free of genuine matter, is translated into a specific FRW cosmology whose effectively induced dark component highly resembles the cold dark matter ansatz. The realistic extension constitutes a dark soliton which bridges past (radiation and\\/or matter dominated) and
Direct reconstruction of dark energy.
Clarkson, Chris; Zunckel, Caroline
2010-05-28
An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With so few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space. We present a new nonparametric method which can accurately reconstruct a wide variety of dark energy behavior with no prior assumptions about it. It is simple, quick and relatively accurate, and involves no expensive explorations of parameter space. The technique uses principal component analysis and a combination of information criteria to identify real features in the data, and tailors the fitting functions to pick up trends and smooth over noise. We find that we can constrain a large variety of w(z) models to within 10%-20% at redshifts z?1 using just SNAP-quality data. PMID:20867085
Neven Bilic
2010-09-27
Thermodynamic properties of dark energy are discussed assuming that dark energy is described in terms of a selfinteracting complex scalar. We first show that, under certain assumptions, selfinteracting complex scalar field theories are equivalent to purely kinetic k-essence models. Then we analyze the themal properties of k-essence and in particular we show that dark-energy in the phantom regime does not necessarily yield negative entropy.
The vacuum's dark particles behave like dark matter and dark energy
NASA Astrophysics Data System (ADS)
Haller, John
2015-04-01
Building on the governing hypothesis that self-information is equal to action, I solve for the time step of the vacuum. The resulting equations (both quantum diffusion and Friedmann's equations) argue that a dark particle, or special black hole, exists at hbar or twice the reduced Planck mass where the Hawking temperature breaks down. It is hypothesized that if neutral hydrogen is nearby the dark particles are able to couple with the background field and thus have a density that looks like dark matter. If hydrogen is not around, the dark particles become frozen leading to a constant density of black body radiation similar to dark energy. If the Universe's dark particles (away from neutral hydrogen) became frozen during the re-ionization of the Universe's history, its BBR density is well within confidence ranges for the cosmological constant. This hypothesis can also explain the recent observations that dark matter decays into dark energy.
Introduction to Dark Energy and Dark Matter
Paul H. Frampton
2005-06-28
In an introductory manner, the nature of dark energy is addressed, how it is observed and what further tests are needed to reconstruct its properties. Several theoretical approaches to dark energy will be discussed. Finally, the dark matter, especially WIMPs, is introduced.
Dynamics of dark energy in collapsing halo of dark matter
Tsizh, M
2015-01-01
We investigate the non-linear evolution of spherical density and velocity perturbations of dark matter and dark energy in the expanding Universe. For that we have used the conservation and Einstein equations to describe the evolution of gravitationally coupled inhomogeneities of dark matter, dark energy and radiation from linear stage in the early Universe to the non-linear one at the current epoch. The simple method of the numerical integration of the system of non-linear differential equations for evolution of the central part of halo is proposed. The results are presented for halo of cluster ($k=2$ Mpc$^{-1}$) and supercluster scales ($k=0.2$ Mpc$^{-1}$) and show that quintessential scalar field dark energy with small value of effective speed of sound $c_s<0.1$ can give noticeable impact on the formation of large scale structures in the expanding Universe.
Dynamics of dark energy in collapsing halo of dark matter
NASA Astrophysics Data System (ADS)
Tsizh, M.; Novosyadlyj, B.
2015-09-01
We investigate the non-linear evolution of spherical density and velocity perturbations of dark matter and dark energy in the expanding Universe. For this we have used the conservation and Einstein equations to describe the evolution of gravitationally coupled inhomogeneities of dark matter, dark energy and radiation from the linear stage in the early Universe to the non-linear stage at the current epoch. A simple method of numerical integration of the system of non-linear differential equations for evolution of the central part of halo is proposed. The results are presented for the halo of cluster (k=2 Mpc^{-1}) and supercluster scales (k=0.2 Mpc^{-1}) and show that a quintessential scalar field dark energy with a low value of effective speed of sound c_s<0.1 can have a notable impact on the formation of large-scale structures in the expanding Universe.
Varun Sahni
2005-01-01
I briefly review our current understanding of dark matter and dark energy. The first part of this review focusses on issues pertaining to dark matter including observational evidence for its existence, current constraints and the a...\\/a H3 = 1\\\\stackrel{...}{a}\\/a H^3 = 1 for LCDM, which is helpful for differentiating between LCDM and rival dark energy models. The review ends with
The Fully Quantized Axion and Dark Energy
Dylan Tanner
2012-12-17
This letter reviews the exact evolution equation for the axion effective potential with the axion scale factor f and phenomenological consequences of the flat effective potential solution are discussed. It is shown that the corresponding vacuum energy can be consistent with Dark Energy, and we compare this result to other studies relating the axion and Dark Energy.
Neutrino models of dark energy
NASA Astrophysics Data System (ADS)
Peccei, R. D.
2005-01-01
I consider a scenario proposed by Fardon, Nelson, and Weiner where dark energy and neutrinos are connected. As a result, neutrino masses are not constant but depend on the neutrino number density. By examining the full equation of state for the dark sector, I show that in this scenario the dark energy is equivalent to having a cosmological constant, but one that “runs” as the neutrino mass changes with temperature. Two examples are examined that illustrate the principal features of the dark sector of this scenario. In particular, the cosmological constant is seen to be negligible for most of the evolution of the Universe, becoming important only when neutrinos become nonrelativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented.
Neutrino models of dark energy
Peccei, R.D. [Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California, 90095 (United States)
2005-01-15
I consider a scenario proposed by Fardon, Nelson, and Weiner where dark energy and neutrinos are connected. As a result, neutrino masses are not constant but depend on the neutrino number density. By examining the full equation of state for the dark sector, I show that in this scenario the dark energy is equivalent to having a cosmological constant, but one that 'runs' as the neutrino mass changes with temperature. Two examples are examined that illustrate the principal features of the dark sector of this scenario. In particular, the cosmological constant is seen to be negligible for most of the evolution of the Universe, becoming important only when neutrinos become nonrelativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented.
The Phase Transition of Dark Energy
Wei Wang; Yuanxing Gui; Ying Shao
2006-12-05
Considering that the universe is filled with the nonrelativistic matter and dark energy and each component is respectively satisfied with its conservation condition in the absence of their interaction, we give the change rate of the fractional density and the density of dark energy from the conservation condition. It is clear that the fractional density of dark energy will monotonously increase and gradually become the dominating contribution to the universe as the redshift becomes low. Combining the evolutional trend of the state equation of dark energy and the change rate of the density of dark energy we find that the density of dark energy will decrease up to a minimum and whereafter it will increase again as the redshift becomes low. This can be regarded as the phase transition of dark energy from the quintessence phase to the phantom phase.
Scattering of dark matter and dark energy
Simpson, Fergus [SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)
2010-10-15
We demonstrate how the two dominant constituents of the Universe, dark energy and dark matter, could possess a large scattering cross section without considerably impacting observations. Unlike interacting models which invoke energy exchange between the two fluids, the background cosmology remains unaltered, leaving fewer observational signatures. Following a brief review of the scattering cross sections between cosmologically significant particles, we explore the implications of an elastic interaction between dark matter and dark energy. The growth of large scale structure is suppressed, yet this effect is found to be weak due to the persistently low dark energy density. Thus we conclude that the dark matter-dark energy cross section may exceed the Thomson cross section by several orders of magnitude.
Generalizing the cosmic energy equation
Shtanov, Yuri; Sahni, Varun
2010-11-15
We generalize the cosmic energy equation to the case when massive particles interact via a modified gravitational potential of the form {phi}(a,r), which is allowed to explicitly depend upon the cosmological time through the expansion factor a(t). Using the nonrelativistic approximation for particle dynamics, we derive the equation for the cosmological expansion which has the form of the Friedmann equation with a renormalized gravitational constant. The generalized Layzer-Irvine cosmic energy equation and the associated cosmic virial theorem are applied to some recently proposed modifications of the Newtonian gravitational interaction between dark-matter particles. We also draw attention to the possibility that the cosmic energy equation may be used to probe the expansion history of the universe thereby throwing light on the nature of dark matter and dark energy.
A model of holographic dark energy
Miao Li
2004-01-01
A model for holographic dark energy is proposed, following the idea that the short distance cut-off is related to the infrared cut-off. We assume that the infrared cut-off relevant to the dark energy is the size of the event horizon. With the input ??=0.73, we predict the equation of state of the dark energy at the present time be characterized
Dark Energy Coupled with Dark Matter in Viscous Fluid Cosmology
I. Brevik; V. V. Obukhov; A. V. Timoshkin
2014-10-10
We investigate cosmological models with two interacting fluids: dark energy and dark matter in flat Friedmann-Robertson-Walker universe. The interaction between dark energy and dark matter is described in terms of the parameters present in the inhomogeneous equation of state when allowance is made for bulk viscosity, for the Little Rip, the Pseudo Rip, and the bounce universes. We obtain analytic representation for characteristic properties in these cosmological models, in particular the bulk viscosity $\\zeta=\\zeta(H,t)$ as function of Hubble parameter and time. We discuss the corrections of thermodynamical parameters in the equations of state due coupling between the viscous fluid and dark matter. Some common properties of these corrections are elucidated.
Brane-Bulk energy exchange and agegraphic dark energy
Ahmad Sheykhi
2010-02-06
We consider the agegraphic models of dark energy in a braneworld scenario with brane-bulk energy exchange. We assume that the adiabatic equation for the dark matter is satisfied while it is violated for the agegraphic dark energy due to the energy exchange between the brane and the bulk. Our study shows that with the brane-bulk interaction, the equation of state parameter of agegraphic dark energy on the brane, $w_D$, can have a transition from normal state where $w_D >-1 $ to the phantom regime where $w_D energy always satisfies $w^{\\mathrm{eff}}_D\\geq-1$.
Edmund J. Copeland; M. Sami; Shinji Tsujikawa
2006-01-01
We review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark
Professor Sean Carroll
2006-11-13
General relativity is inconsistent with cosmological observations unless we invoke components of dark matter and dark energy that dominate the universe. While it seems likely that these exotic substances really do exist, the alternative is worth considering: that Einstein's general relativity breaks down on cosmological scales. I will discuss models of modified gravity, tests in the solar system and elsewhere, and consequences for cosmology.
Supernova Constraints on Braneworld Dark Energy
Ujjaini Alam; Varun Sahni
2002-01-01
Braneworld models of dark energy are examined in the light of observations of high redshift type Ia supernovae. Braneworld models admit several novel and even exotic possibilities which include: (i) The effective equation of state of dark energy can be more negative than in LCDM: $w \\\\leq -1$; (ii) A class of braneworld models can encounter a `quiescent' future singularity
The Quintom Model of Dark Energy
Bo Feng
2006-02-07
In this paper I give a brief review on the recently proposed new scenario of dark energy model dubbed $Quintom$. Quintom describes the dynamical dark energy models where the equation of state getting across the cosmological constant boundary during evolutions. I discuss some aspects on the quintom model buildings and the observational consequences.
Post-Planck dark energy constraints
NASA Astrophysics Data System (ADS)
Hazra, Dhiraj Kumar; Majumdar, Subhabrata; Pal, Supratik; Panda, Sudhakar; Sen, Anjan A.
2015-04-01
We constrain plausible dark energy models using the recently published cosmic microwave background (CMB) temperature anisotropy data from Planck together with WMAP9 low-? polarization data and the data from low redshift surveys. To circumvent the limitations of any particular equation of state toward describing all existing dark energy models, we work with three different equations of state covering a wider class of dark energy models and hence provide more robust and generic constraints on the dark energy behavior. We show that a possible tension exists between constraints from CMB and non-CMB observations when one allows for both phantom and nonphantom behavior for the dark energy. Further, we reconstruct the equation of state of dark energy as a function of redshift using the combined CMB and non-CMB data and show that cosmological constant behavior is disallowed at the 68.3% confidence level. A fully nonphantom history is also disallowed at the 68.3% confidence level, and a considerable fine-tuning is also needed to keep it inside the 95.5% confidence limit. This result might motivate one to construct phantom models for dark energy, which may be achievable in the presence of higher derivative operators as in string theory. However, for a theoretical model that allows only nonphantom behavior, both CMB and non-CMB data sets agree on the dark energy constraint with the mean equation of state being very close to the cosmological constant.
NASA Astrophysics Data System (ADS)
Debnath, Ujjal
2014-12-01
Here we briefly discuss the Galileon gravity theory and modified Friedmann equations. By considering new holographic dark energy (NHDE) in the framework of Galileon gravity, we found the energy density, pressure, equation of state and the deceleration parameter in terms of the scale factor. Subsequently, we study the correspondence between the NHDE in the framework of Galileon gravity with other dark energies like quintessence, k-essence, tachyon, dilaton, hessence and DBI-essence dark energies and construct the scalar field and corresponding scalar potentials which describe the dynamics of the scalar fields graphically. All the dark energy models, the scalar field and the potential decrease due to the evolution of the universe.
Burra G. Sidharth
2015-01-12
Some seventy five years ago, the concept of dark matter was introduced by Zwicky to explain the anomaly of galactic rotation curves, though there is no clue to its identity or existence to date. In 1997, the author had introduced a model of the universe which went diametrically opposite to the existing paradigm which was a dark matter assisted decelarating universe. The new model introduces a dark energy driven accelarating universe though with a small cosmological constant. The very next year this new picture was confirmed by the Supernova observations of Perlmutter, Riess and Schmidt. These astronomers got the 2011 Nobel Prize for this dramatic observation. All this is discussed briefly, including the fact that dark energy may obviate the need for dark matter.
Coupled tachyonic dark energy: a dynamical analysis
Landim, Ricardo C G
2015-01-01
In this paper we present a dynamical analysis for a coupled tachyonic dark energy with dark matter. The tachyonic field $\\phi$ is considered in the presence of barothropic fluids (matter and radiation) and the autonomous system due to the evolution equations is studied. The three cosmological eras (radiation, matter and dark energy) are described through the critical points, for a generic potential $V(\\phi)$.
Thermodynamical description of the ghost dark energy model
M. Honarvaryan; A. Sheykhi; H. Moradpour
2015-05-02
In this paper, we point out thermodynamical description of ghost dark energy and its generalization to the early universe. Thereinafter, we find expressions for the entropy changes of these dark energy candidates. In addition, considering thermal fluctuations, thermodynamics of the dark energy component interacting with a dark matter sector is addressed. {We will also find the effects of considering the coincidence problem on the mutual interaction between the dark sectors, and thus the equation of state parameter of dark energy.} Finally, we derive a relation between the mutual interaction of the dark components of the universe, accelerated with the either ghost dark energy or its generalization, and the thermodynamic fluctuations.
Thermodynamical description of the ghost dark energy model
Honarvaryan, M; Moradpour, H
2015-01-01
In this paper, we point out thermodynamical description of ghost dark energy and its generalization to the early universe. Thereinafter, we find expressions for the entropy changes of these dark energy candidates. In addition, considering thermal fluctuations, thermodynamics of the dark energy component interacting with a dark matter sector is addressed. {We will also find the effects of considering the coincidence problem on the mutual interaction between the dark sectors, and thus the equation of state parameter of dark energy.} Finally, we derive a relation between the mutual interaction of the dark components of the universe, accelerated with the either ghost dark energy or its generalization, and the thermodynamic fluctuations.
Constraining dark energy fluctuations with supernova correlations
Blomqvist, Michael [The Oskar Klein Centre for Cosmoparticle Physics, Department of Astronomy, Stockholm University, AlbaNova University Center, SE–106 91 Stockholm (Sweden); Enander, Jonas; Mörtsell, Edvard, E-mail: michaelb@astro.su.se, E-mail: enander@fysik.su.se, E-mail: edvard@fysik.su.se [The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova University Center, SE–106 91 Stockholm (Sweden)
2010-10-01
We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state w??1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime.
Interacting holographic dark energy with logarithmic correction
Jamil, Mubasher; Farooq, M. Umar, E-mail: mjamil@camp.nust.edu.pk, E-mail: mufarooq@yahoo.com [Center for Advanced Mathematics and Physics, National University of Sciences and Technology, Rawalpindi, 46000 (Pakistan)
2010-03-01
The holographic dark energy (HDE) is considered to be the most promising candidate of dark energy. Its definition is motivated from the entropy-area relation which depends on the theory of gravity under consideration. Recently a new definition of HDE is proposed with the help of quantum corrections to the entropy-area relation in the setup of loop quantum cosmology. Employing this new definition, we investigate the model of interacting dark energy and derive its effective equation of state. Finally we establish a correspondence between generalized Chaplygin gas and entropy-corrected holographic dark energy.
Statefinder Diagnostic for Dilaton Dark Energy
Z. G. Huang; X. M. Song; H. Q. Lu; W. Fang
2008-05-07
Statefinder diagnostic is a useful method which can differ one dark energy model from the others. The Statefinder pair $\\{r, s\\}$ is algebraically related to the equation of state of dark energy and its first time derivative. We apply in this paper this method to the dilaton dark energy model based on Weyl-Scaled induced gravitational theory. We investigate the effect of the coupling between matter and dilaton when the potential of dilaton field is taken as the Mexican hat form. We find that the evolving trajectory of our model in the $r-s$ diagram is quite different from those of other dark energy models.
Holographic dark energy interacting with dark matter in a Closed Universe
Norman Cruz; Samuel Lepe; Francisco Pena; Joel Saavedra
2008-07-24
A cosmological model of an holographic dark energy interacting with dark matter throughout a decaying term of the form $Q=3(\\lambda_1\\rho_{DE} + \\lambda_2\\rho_m) H$ is investigated. General constraint on the parameters of the model are found when accelerated expansion is imposed and we found a phantom scenarios, without any reference to a specific equation of state for the dark energy. The behavior of equation of stated for dark energy is also discussed.
Dark Energy and Dark Matter in Stars Physic
Plamen Fiziev
2014-11-02
We present the basic equations and relations for the relativistic static spherically symmetric stars (SSSS) in the model of minimal dilatonic gravity (MDG) which is {\\em locally} equivalent to the f(R) theories of gravity and gives an alternative description of the effects of dark matter and dark energy. The results for the simplest form of the relativistic equation of state (EOS) of neutron matter are represented. Our approach overcomes the well-known difficulties of the physics of SSSS in the f(R) theories of gravity introducing two novel EOS for cosmological energy-pressure densities and dilaton energy-pressure densities, as well as proper boundary conditions.
Nair, Remya; Jhingan, Sanjay, E-mail: remya_phy@yahoo.com, E-mail: sanjay.jhingan@gmail.com [Centre for Theoretical Physics, Jamia Millia Islamia, New Delhi 110025 (India)
2013-02-01
We look for evidence for the evolution in dark energy density by employing Principal Component Analysis (PCA). Distance redshift data from supernovae and baryon acoustic oscillations (BAO) along with WMAP7 distance priors are used to put constraints on curvature parameter ?{sub k} and dark energy parameters. The data sets are consistent with a flat Universe. The constraints on the dark energy evolution parameters obtained from supernovae (including CMB distance priors) are consistent with a flat ?CDM Universe. On the other hand, in the parameter estimates obtained from the addition of BAO data the second principal component, which characterize a non-constant contribution from dark energy, is non-zero at 1?. This could be a systematic effect and future BAO data holds key to making more robust claims.
T. Padmanabhan
2008-01-01
I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding\\u000a the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence\\u000a for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a\\u000a candidate and emphasizes
Eric V. Linder
2006-04-11
Non-negligible dark energy density at high redshifts would indicate dark energy physics distinct from a cosmological constant or ``reasonable'' canonical scalar fields. Such dark energy can be constrained tightly through investigation of the growth of structure, with limits of \\la2% of total energy density at z\\gg1 for many models. Intermediate dark energy can have effects distinct from its energy density; the dark ages acceleration can be constrained to last less than 5% of a Hubble e-fold time, exacerbating the coincidence problem. Both the total linear growth, or equivalently \\sigma_8, and the shape and evolution of the nonlinear mass power spectrum for zenergy behavior over the entire range z=0-1100.
Interacting agegraphic dark energy models in non-flat universe
Ahmad Sheykhi
2009-09-12
A so-called "agegraphic dark energy" was recently proposed to explain the dark energy-dominated universe. In this Letter, we generalize the agegraphic dark energy models to the universe with spatial curvature in the presence of interaction between dark matter and dark energy. We show that these models can accommodate $w_D = -1 $ crossing for the equation of state of dark energy. In the limiting case of a flat universe, i.e. $k = 0$, all previous results of agegraphic dark energy in flat universe are restored.
Thermodynamics of dark energy interacting with dark matter and radiation
Jamil, Mubasher [Center for Advanced Mathematics and Physics, National University of Sciences and Technology, Rawalpindi, 46000 (Pakistan); Saridakis, Emmanuel N. [Department of Physics, University of Athens, GR-15771 Athens (Greece); Setare, M. R. [Department of Science, Payame Noor University, Bijar (Iran, Islamic Republic of)
2010-01-15
We investigate the validity of the generalized second law of thermodynamics, in the cosmological scenario where dark energy interacts with both dark matter and radiation. Calculating separately the entropy variation for each fluid component and for the apparent horizon itself, we show that the generalized second law is always and generally valid, independently of the specific interaction form, of the fluids equation-of-state parameters and of the background geometry.
How clustering dark energy affects matter perturbations
NASA Astrophysics Data System (ADS)
Mehrabi, A.; Basilakos, S.; Pace, F.
2015-09-01
The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed c2_eff and for c2_eff=0 dark energy clusters in a similar fashion to dark matter while for c2_eff=1 it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, wd = const and w_d=w_0+w_1(z/1+z) with c2_eff as a free parameter and we try to constrain the dark energy effective sound speed using current available data including Type Ia supernovae, baryon acoustic oscillation, cosmic microwave background shift parameter (Planck and WMAP), Hubble parameter, big bang nucleosynthesis and the growth rate of structures f?8(z). At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that c2_eff=const. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at c2_eff=0; however, the dark energy sound speed is degenerate with respect to the cosmological parameters, namely ?m and wd.
Dipolar dark matter and dark energy
Luc Blanchet; Alexandre Le Tiec
2009-01-01
In previous work [L. Blanchet and A. Le Tiec, Phys. Rev. DPRVDAQ1550-7998 78, 024031 (2008)10.1103\\/PhysRevD.78.024031], a model of dark matter and dark energy based on the concept of gravitational polarization was investigated. This model was shown to recover the concordance cosmological scenario (Lambda-cold dark matter) at cosmological scales, and the phenomenology of the modified Newtonian dynamics at galactic scales. In
Interacting Dark Matter and Dark Energy
Glennys R. Farrar; P. J. E. Peebles
2003-01-01
We discuss models for the cosmological dark sector in which the energy\\u000adensity of a scalar field approximates Einstein's cosmological constant and the\\u000ascalar field value determines the dark matter particle mass by a Yukawa\\u000acoupling. A model with one dark matter family can be adjusted so the\\u000aobservational constraints on the cosmological parameters are close to but\\u000adifferent from
Marc Kamionkowski
2007-01-01
This is a short review, aimed at a general audience, of several current subjects of research in cosmology. The topics discussed include the cosmic microwave background (CMB), with particular emphasis on its relevance for testing inflation; dark matter, with a brief review of astrophysical evidence and more emphasis on particle candidates; and cosmic acceleration and some of the ideas that
Massimo Pietroni
2005-09-07
The two most popular candidates for dark energy, i.e. a cosmological constant and quintessence, are very difficult to distinguish observationally, mostly because the quintessence field does not have sizable fluctuations. We study a scalar field model for dark energy in which the scalar field is invariant under reflection symmetry, phi -> -phi. Under general assumptions, there is a phase transition at late times (z < 0.5). Before the phase transition, the field behaves as a cosmological constant. After the phase transition, a time-dependent scalar condensate forms, the field couples with dark matter and develops sizable perturbations tracking those of dark matter. The background cosmological evolution is in agreement with existing observations, but might be distinguished from that of a cosmological constant by future Supernovae surveys. The growth of cosmological perturbations carries the imprint of the phase transition, however a non-linear approach has to be developed in order to study it quantitatively.
Interacting dark energy: the role of microscopic feedback in the dark sector
Avelino, P P
2015-01-01
We investigate the impact on the classical dynamics of dark matter particles and dark energy of a non-minimal coupling in the dark sector, assuming that the mass of the dark matter particles is coupled to a dark energy scalar field. We show that standard results can only be recovered if the space-time variation of the dark energy scalar field is sufficiently smooth on the characteristic length scale of the dark matter particles, and we determine the associated constraint dependent on both the mass and radius of the dark matter particles and the coupling to the dark energy scalar field. We further show, using field theory numerical simulations, that a violation of such constraint results in a microscopic feedback effect strongly affecting the dynamics of dark matter particles, with a potential impact on structure formation and on the space-time evolution of the dark energy equation of state.
Interacting dark energy: the role of microscopic feedback in the dark sector
P. P. Avelino
2015-03-10
We investigate the impact on the classical dynamics of dark matter particles and dark energy of a non-minimal coupling in the dark sector, assuming that the mass of the dark matter particles is coupled to a dark energy scalar field. We show that standard results can only be recovered if the space-time variation of the dark energy scalar field is sufficiently smooth on the characteristic length scale of the dark matter particles, and we determine the associated constraint dependent on both the mass and radius of the dark matter particles and the coupling to the dark energy scalar field. We further show, using field theory numerical simulations, that a violation of such constraint results in a microscopic feedback effect strongly affecting the dynamics of dark matter particles, with a potential impact on structure formation and on the space-time evolution of the dark energy equation of state.
Unravelling the Dark Matter - Dark Energy Paradigm
Reginald T Cahill
2009-01-26
The standard LambdaCDM model of cosmology is usually understood to arise from demanding that the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric satisfy the General Relativity dynamics for spacetime metrics. The FLRW data-based dominant parameter values, Omega_Lambda=0.73 and Omega_m=0.27 for the dark energy and dark matter+matter, respectively, are then determined by fitting the supernova red-shift data. However in the pressure-less flat-space case the LambdaCDM model is most easily derived from Newtonian gravity, and which was based on the special case of planetary motion in the solar system. Not surprisingly when extended to galactic rotations and cosmology Newtonian dynamics is found to be wanting, and the fix-up involves introducing dark matter and dark energy, as shown herein. However a different theory of gravity leads to a different account of galactic rotations and cosmology, and does not require dark matter nor dark energy to fit the supernova data. It is shown that fitting the LambdaCDM model to this new model, and so independently of the actual supernova data, requires the LambdaCDM model parameters to be those given above. Hence we conclude that dark energy and dark matter are no more than mathematical artifacts to fix-up limitations of Newtonian gravity. Various other data are also briefly reviewed to illustrate other successful tests of this new theory of gravity.
Generalized Model for $?$-Dark Energy
Utpal Mukhopadhyay; P. C. Ray; Saibal Ray; S. B. Datta Choudhury
2008-02-07
Einstein field equations under spherically symmetric space-times are considered here in connection to dark energy investigation. A set of solutions are obtained for a kinematical $\\Lambda$ model, viz., $\\Lambda \\sim (\\dot a/a)^2$ without assuming any {\\it a priori} value for the curvature constant and the equation of state parameter $\\omega$. Some interesting results, such as the nature of cosmic density $\\Omega$ and deceleration parameter $q$, have been obtained with the consideration of two-fluid structure instead of usual uni-fluid cosmological model.
Measuring the dark matter equation of state
NASA Astrophysics Data System (ADS)
Serra, Ana Laura; Domínguez Romero, Mariano Javier L.
2011-07-01
The nature of the dominant component of galaxies and clusters remains unknown. While the astrophysics community supports the cold dark matter (CDM) paradigm as a clue factor in the current cosmological model, no direct CDM detections have been performed. Faber & Visser have suggested a simple method for measuring the dark matter equation of state that combines kinematic and gravitational lensing data to test the widely adopted assumption of pressureless dark matter. Following this formalism, we have measured the dark matter equation of state for the first time using improved techniques. We have found that the value of the equation-of-state parameter is consistent with pressureless dark matter within the errors. Nevertheless, the measured value is lower than expected because, typically, the masses determined with lensing are larger than those obtained through kinematic methods. We have tested our techniques using simulations and we have also analysed possible sources of error that could invalidate or mimic our results. In light of this result, we can now suggest that understanding the nature of requires a complete general relativistic analysis.
Dark energy from quantum wave function collapse of dark matter
A. S. Majumdar; D. Home; S. Sinha
2009-09-03
Dynamical wave function collapse models entail the continuous liberation of a specified rate of energy arising from the interaction of a fluctuating scalar field with the matter wave function. We consider the wave function collapse process for the constituents of dark matter in our universe. Beginning from a particular early era of the universe chosen from physical considerations, the rate of the associated energy liberation is integrated to yield the requisite magnitude of dark energy around the era of galaxy formation. Further, the equation of state for the liberated energy approaches $w \\to -1$ asymptotically, providing a mechanism to generate the present acceleration of the universe.
Inferring "Dark Matter" and "Dark Energy" from Videos , Sinisa Todorovic
Todorovic, Sinisa
Inferring "Dark Matter" and "Dark Energy" from Videos Dan Xie , Sinisa Todorovic , and Song.g., grass lawns). Therefore, functional objects can be viewed as "dark matter", emanating "dark energy" that affects people's trajectories in the video. To detect "dark mat- ter" and infer their "dark energy" field
Schwarzschild black hole in dark energy background
Ngangbam Ishwarchandra; Ng. Ibohal; K. Yugindro Singh
2014-09-27
In this paper we present an exact solution of Einstein's field equations describing the Schwarzschild black hole in dark energy background. It is also regarded as an embedded solution that the Schwarzschild black hole is embedded into the dark energy space producing Schwarzschild-dark energy black hole. It is found that the space-time geometry of Schwarzschild-dark energy solution is non-vacuum Petrov type $D$ in the classification of space-times. We study the energy conditions (like weak, strong and dominant conditions) for the energy-momentum tensor of the Schwarzschild-dark energy solution. We also find that the energy-momentum tensor of the Schwarzschild-dark energy solution violates the strong energy condition due to the negative pressure leading to a repulsive gravitational force of the matter field in the space-time. It is shown that the time-like vector field for an observer in the Schwarzschild-dark energy space is expanding, accelerating, shearing and non-rotating. We investigate the surface gravity and the area of the horizons for the Schwarzschild-dark energy black hole.
Lincoln, Don
2014-04-15
Scientists were shocked in 1998 when the expansion of the universe wasn't slowing down as expected by our best understanding of gravity at the time; the expansion was speeding up! That observation is just mind blowing, and yet it is true. In order to explain the data, physicists had to resurrect an abandoned idea of Einstein's now called dark energy. In this video, Fermilab's Dr. Don Lincoln tells us a little about the observations that led to the hypothesis of dark energy and what is the status of current research on the subject.
Lincoln, Don
2014-08-07
Scientists were shocked in 1998 when the expansion of the universe wasn't slowing down as expected by our best understanding of gravity at the time; the expansion was speeding up! That observation is just mind blowing, and yet it is true. In order to explain the data, physicists had to resurrect an abandoned idea of Einstein's now called dark energy. In this video, Fermilab's Dr. Don Lincoln tells us a little about the observations that led to the hypothesis of dark energy and what is the status of current research on the subject.
Kazuharu Bamba; Antonio Lopez-Revelles; R. Myrzakulov; S. D. Odintsov; L. Sebastiani
2012-11-13
A generic feature of viable $F(R)$ gravity is investigated: It is demonstrated that during the matter dominated era the large frequency oscillations of the effective dark energy may influence the behavior of higher derivatives of the Hubble parameter with the risk to produce some singular unphysical solutions at high redshift. This behavior is explicitly analyzed for realistic $F(R)$ models, in particular, exponential gravity and a power form model. To stabilize such oscillations, we consider the additional modification of the models via a correction term which does not destroy the viability properties. A detailed analysis on the future evolution of the universe and the evolution history of the growth index of the matter density perturbations are performed. Furthermore, we explore two applications of exponential gravity to the inflationary scenario. We show how it is possible to obtain different numbers of $e$-folds during the early-time acceleration by making different choices of the model parameters in the presence of ultrarelativistic matter, which destabilizes inflation and eventually leads to the exit from the inflationary stage. We execute the numerical analysis of inflation in two viable exponential gravity models. It is proved that at the end of the inflation, the effective energy density and curvature of the universe decrease and thus a unified description between inflation and the $\\Lambda$CDM-like dark energy dominated era can be realized.
NASA Astrophysics Data System (ADS)
Xing, Lili; Chen, Jianbin; Gui, Yuanxing; Schlegel, Eric M.; Lu, Jianbo
We investigate the validity of the thermodynamical properties of the universe in a new parametric model of dark energy with the equation of state w = w0 + w1 · z(1 + z)/(1 + z2). In the spatially homogeneous and isotropic universe, assuming that the temperature and entropy in cosmology is as in a black hole, we examine the thermodynamical properties of the universe bounded by the apparent horizon and the event horizon respectively. By analysis, we find that the first and the second laws of thermodynamics are valid inside the apparent horizon, while they break down inside the event horizon.
Accelerating Universe with spacetime torsion but without dark matter and dark energy
A. V. Minkevich
2009-01-01
It is shown that cosmological equations for homogeneous isotropic models deduced in the framework of the Poincaré gauge theory of gravity by certain restrictions on indefinite parameters of gravitational Lagrangian take at asymptotics the same form as cosmological equations of general relativity theory for ?CDM-model. Terms related to dark matter and dark energy in cosmological equations of standard theory for
Cosmic steps in modeling dark energy
Wang Tower [Center for High-Energy Physics, Peking University, Beijing 100871 (China)
2009-11-15
Past and recent data analyses gave some hints of steps in dark energy. Considering dark energy as a dynamical scalar field, we investigate several models with various steps: a step in the scalar potential, a step in the kinetic term, a step in the energy density, and a step in the equation-of-state parameter w. These toy models provide a workable mechanism to generate steps and features of dark energy. Remarkably, a single real scalar can cross w=-1 dynamically with a step in the kinetic term.
Urban, Federico R.; Zhitnitsky, Ariel R.
2010-08-30
We review two mechanisms rooted in the infrared sector of QCD which, by exploiting the properties of the QCD ghost, as introduced by Veneziano, provide new insight on the cosmological dark energy problem, first, in the form of a Casimir-like energy from quantising QCD in a box, and second, in the form of additional, time-dependent, vacuum energy density in an expanding universe. Based on [1, 2].
R. R. Caldwell
2002-01-01
It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state
CAPUT DARK ENERGY TOPICS, 2013 1. The Cosmological Constant
Weijgaert, Rien van de
CAPUT DARK ENERGY TOPICS, 2013 1 #12;1. The Cosmological Constant - The acceleration as curvature term in the Einstein field equation and not a form of dark energy. Provide a critical discussion., Rovelli C., 2010 Is dark energy really a mystery ? Nature, 466, 321 (July 2010) - Padmanabhan T., 2003
A new type of dark energy model
Zhang, Yi [Department of Astronomy, Beijing Normal University, Beijing 100875 (China); Li, Hui, E-mail: zhangyia@cqupt.edu.cn, E-mail: lihui@ytu.edu.cn [Department of Physics, Yantai University, Yantai 264005 (China)
2010-06-01
In this paper, we propose a general form of the equation of state (EoS) which is the function of the fractional dark energy density ?{sub d}. At least, five related models, the cosmological constant model, the holographic dark energy model, the agegraphic dark energy model, the modified holographic dark energy model and the Ricci scalar holographic dark energy model are included in this form. Furthermore, if we consider proper interactions, the interactive variants of those models can be included as well. The phase-space analysis shows that the scaling solutions may exist both in the non-interacting and interacting cases. And the stability analysis of the system could give out the attractor solution which could alleviate the coincidence problem.
Dark Energy from Entanglement Entropy
NASA Astrophysics Data System (ADS)
Capozziello, Salvatore; Luongo, Orlando
2013-08-01
We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy. The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras. As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe. The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing a priori any cosmological constant. In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, V? a 3 and V? H -3, and find two models which fairly well approximate the current cosmic speed up. The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the quantum decoherence.
Dark energy from entanglement entropy
Capozziello, Salvatore
2013-01-01
We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy. The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras. As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe. The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing {\\it a priori} any cosmological constant. In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, $V\\propto a^3$ and $V\\propto H^{-3}$, and find two models which fairly well approximate the current cosmic speed up. The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the qua...
Dark energy from entanglement entropy
Salvatore Capozziello; Orlando Luongo
2013-03-06
We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy. The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras. As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe. The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing {\\it a priori} any cosmological constant. In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, $V\\propto a^3$ and $V\\propto H^{-3}$, and find two models which fairly well approximate the current cosmic speed up. The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the quantum decoherence.
Dark Energy: Recent Developments
Norbert Straumann
2006-04-26
A six parameter cosmological model, involving a vacuum energy density that is extremely tiny compared to fundamental particle physics scales, describes a large body of increasingly accurate astronomical data. In a first part of this brief review we summarize the current situation, emphasizing recent progress. An almost infinitesimal vacuum energy is only the simplest candidate for a cosmologically significant nearly homogeneous exotic energy density with negative pressure, generically called Dark Energy. If general relativity is assumed to be also valid on cosmological scales, the existence of such a dark energy component that dominates the recent universe is now almost inevitable. We shall discuss in a second part the alternative possibility that general relativity has to be modified on distances comparable to the Hubble scale. It will turn out that observational data are restricting theoretical speculations more and more. Moreover, some of the recent proposals have serious defects on a fundamental level (ghosts, acausalities, superluminal fluctuations).
Burra G. Sidharth
2008-08-05
In the light of recent developments in Dark Energy, we consider the electron in a such a background field and show that at the Compton wavelength the electron is stable, in that the Cassini inward pressure exactly counterbalances the outward Coulomb repulsive pressure thus answering a problem of the earlier electron theory.
Varun Sahni; Alexei Starobinsky
2006-01-01
This review summarizes recent attempts to reconstruct the expansion history of the universe and to probe the nature of dark energy. Reconstruction methods can be broadly classified into parametric and non-parametric approaches. It is encouraging that, even with the limited observational data currently available, different approaches give consistent results for the reconstruction of the Hubble parameter H(z) and the effective
Roles of dark energy perturbations in dynamical dark energy models: can we ignore them?
Park, Chan-Gyung; Hwang, Jai-chan; Lee, Jae-heon; Noh, Hyerim
2009-10-01
We show the importance of properly including the perturbations of the dark energy component in the dynamical dark energy models based on a scalar field and modified gravity theories in order to meet with present and future observational precisions. Based on a simple scaling scalar field dark energy model, we show that observationally distinguishable substantial differences appear by ignoring the dark energy perturbation. By ignoring it the perturbed system of equations becomes inconsistent and deviations in (gauge-invariant) power spectra depend on the gauge choice. PMID:19905618
Investigating Dark Energy with Black Hole Binaries
Laura Mersini-Houghton; Adam Kelleher
2009-06-08
The accelerated expansion of the universe is ascribed to the existence of dark energy. Black holes accretion of dark energy induces a mass change proportional to the energy density and pressure of the background dark energy fluid. The time scale during which the mass of black holes changes considerably is too long relative to the age of the universe, thus beyond detection possibilities. We propose to take advantage of the modified black hole masses for exploring the equation of state $w[z]$ of dark energy, by investigating the evolution of supermassive black hole binaries on a dark energy background. Deriving the signatures of dark energy accretion on the evolution of binaries, we find that dark energy imprints on the emitted gravitational radiation and on the changes in the orbital radius of the binary can be within detection limits for certain supermassive black hole binaries. In this talk I describe how binaries can provide a useful tool in obtaining complementary information on the nature of dark energy, based on the work done with A.Kelleher.
Rui Neves; Cenalo Vaz
2006-05-18
In the Randall-Sundrum scenario, we analyse the dynamics of an AdS5 braneworld when conformal matter fields propagate in five dimensions. We show that conformal fields of weight -4 are associated with stable geometries which describe the dynamics of inhomogeneous dust, generalized dark radiation and homogeneous polytropic dark energy on a spherically symmetric 3-brane embedded in the compact AdS5 orbifold. We discuss aspects of the radion stability conditions and of the localization of gravity in the vicinity of the brane.
Wechsler, Risa
2007-10-30
What is the Universe made of? This question has been asked as long as humans have been questioning, and astronomers and physicists are finally converging on an answer. The picture which has emerged from numerous complementary observations over the past decade is a surprising one: most of the matter in the Universe isn't visible, and most of the Universe isn't even made of matter. In this talk, I will explain what the rest of this stuff, known as 'Dark Energy' is, how it is related to the so-called 'Dark Matter', how it impacts the evolution of the Universe, and how we can study the dark universe using observations of light from current and future telescopes.
Dragan Huterer
2002-01-01
We study the power of upcoming weak lensing surveys to probe dark energy. Dark energy modifies the distance-redshift relation as well as the matter power spectrum, both of which affect the weak lensing convergence power spectrum. Some dark-energy models predict additional clustering on very large scales, but this probably cannot be detected by weak lensing alone due to cosmic variance.
Dark energy: myths and reality
V. N. Lukash; V. A. Rubakov
2008-07-10
We discuss the questions related to dark energy in the Universe. We note that in spite of the effect of dark energy, large-scale structure is still being generated in the Universe and this will continue for about ten billion years. We also comment on some statements in the paper ``Dark energy and universal antigravitation'' by A.D. Chernin [4].
Salvador Robles-Perez; Prado Martin-Moruno; Alberto Rozas-Fernandez; Pedro F. Gonzalez-Diaz
2007-01-23
We present cosmic solutions corresponding to universes filled with dark and phantom energy, all having a negative cosmological constant. All such solutions contain infinite singularities, successively and equally distributed along time, which can be either big bang/crunchs or big rips singularities. Classicaly these solutions can be regarded as associated with multiverse scenarios, being those corresponding to phantom energy that may describe the current accelerating universe.
Probing the time dependence of dark energy
Barboza Edésio Jr, M. [Departamento de Física, Universidade do Estado do Rio Grande do Norte, Rua Professor Antônio Campos s/n, Mossoró (Brazil); Alcaniz, J.S., E-mail: edesiobarboza@uern.br, E-mail: alcaniz@on.br [Departamento de Astronomia, Observatório Nacional, Rua General José Cristino 77, Rio de Janeiro (Brazil)
2012-02-01
A new method to investigate a possible time-dependence of the dark energy equation of state w is proposed. We apply this methodology to a combination of data involving one of the most recent type Ia supernova sample (SNLS3) along with the current baryon acoustic oscillation and H(z) measurements. We show that current observations cannot rule out a non-evolving dark energy component (dw/dz = 0). The approach developed here reduces considerably the so-called smearing effect on w determinations and may be useful to probe a possible evolving dark energy component when applied to upcoming observational data.
Dark energy models through nonextensive Tsallis' statistics
NASA Astrophysics Data System (ADS)
Barboza, Edésio M.; Nunes, Rafael da C.; Abreu, Everton M. C.; Ananias Neto, Jorge
2015-10-01
The accelerated expansion of the Universe is one of the greatest challenges of modern physics. One candidate to explain this phenomenon is a new field called dark energy. In this work we have used the Tsallis nonextensive statistical formulation of the Friedmann equation to explore the Barboza-Alcaniz and Chevalier-Polarski-Linder parametric dark energy models and the Wang-Meng and Dalal vacuum decay models. After that, we have discussed the observational tests and the constraints concerning the Tsallis nonextensive parameter. Finally, we have described the dark energy physics through the role of the q-parameter.
Dark energy FRW cosmology - dynamical system reconstruction
Marek Szydlowski; Aleksandra Kurek
2007-11-02
We develop a simple method of dark energy reconstruction using a geometrical form of the luminosity-distance relation. In this method the FRW dynamical system with dark energy is reconstructed instead of the equation of state parameter. We give several examples which illustrate the usefulness of our method in fitting the redshift transition from the decelerating to accelerating phase as the value of the Hubble function at the transition.
Can dark energy evolve to the phantom?
Alexander Vikman
2005-01-01
Dark energy with the equation of state w(z) rapidly evolving from the dustlike (w~=0 at z˜1) to the phantomlike (-1.2≲w≲-1 at z~=0) has been recently proposed as the best fit for the supernovae Ia data. Assuming that a dark energy component with an arbitrary scalar-field Lagrangian p(phi,?muphi) dominates in the flat Friedmann universe, we analyze the possibility of a dynamical
Universal Forces and the Dark Energy Problem
Afsar Abbas
2007-04-01
The Dark Energy problem is forcing us to re-examine our models and our understanding of relativity and space-time. Here a novel idea of Fundamental Forces is introduced. This allows us to perceive the General Theory of Relativity and Einstein's Equation from a new pesrpective. In addition to providing us with an improved understanding of space and time, it will be shown how it leads to a resolution of the Dark Energy problem.
Black hole and holographic dark energy
Yun Soo Myung
2007-04-11
We discuss the connection between black hole and holographic dark energy. We examine the issue of the equation of state (EOS) for holographic energy density as a candidate for the dark energy carefully. This is closely related to the EOS for black hole, because the holographic dark energy comes from the black hole energy density. In order to derive the EOS of a black hole, we may use its dual (quantum) systems. Finally, a regular black hole without the singularity is introduced to describe an accelerating universe inside the cosmological horizon. Inspired by this, we show that the holographic energy density with the cosmological horizon as the IR cutoff leads to the dark energy-dominated universe with $\\omega_{\\rm \\Lambda}=-1$.
Braneworld models of dark energy
NASA Astrophysics Data System (ADS)
Sahni, Varun; Shtanov, Yuri
2003-11-01
We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop effects induced by matter fields residing on the brane. Spatially flat braneworld models can enter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the luminosity distance in LCDM. Whereas models with dL leq dL(LCDM) imply w = p/rho geq -1 and have frequently been discussed in the literature, models with dL > dL(LCDM) have traditionally been ignored, perhaps because, within the general-relativistic framework, the luminosity distance has this property only if the equation of state of matter is strongly negative (w < -1). Within the conventional framework, `phantom energy' with w < -1 is beset with a host of undesirable properties, which makes this model of dark energy unattractive. Braneworld models, on the other hand, have the capacity to endow dark energy with exciting new possibilities (including w < -1) without suffering from the problems faced by phantom energy. For a subclass of parameter values, braneworld dark energy and the acceleration of the universe are transient phenomena. In these models, the universe, after the current period of acceleration, re-enters the matter-dominated regime so that the deceleration parameter q(t) rightarrow 0.5 when t >> t0, where t0 is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.
Dai, D.-C.; Stojkovic, Dejan; Dutta, Sourish
2009-09-15
We examine a dark energy model where a scalar unparticle degree of freedom plays the role of quintessence. In particular, we study a model where the unparticle degree of freedom has a standard kinetic term and a simple mass potential, the evolution is slowly rolling and the field value is of the order of the unparticle energy scale ({lambda}{sub u}). We study how the evolution of w depends on the parameters B (a function of unparticle scaling dimension d{sub u}), the initial value of the field {phi}{sub i} (or equivalently, {lambda}{sub u}) and the present matter density {omega}{sub m0}. We use observational data from type Ia supernovae, baryon acoustic oscillations and the cosmic microwave background to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, unparticle dark energy model is very attractive, since unparticles (being bound states of fundamental fermions) are protected from radiative corrections. Further, coupling of unparticles to the standard model fields can be arbitrarily suppressed by raising the fundamental energy scale M{sub F}, making the unparticle dark energy model free of most of the problems that plague conventional scalar field quintessence models.
Cosmological Evolution of Pilgrim Dark Energy
Sharif, M
2015-01-01
We study pilgrim dark energy model by taking IR cut-offs as particle and event horizons as well as conformal age of the universe. We derive evolution equations for fractional energy density and equation of state parameters for pilgrim dark energy. The phantom cosmic evolution is established in these scenarios which is well supported by the cosmological parameters such as deceleration parameter, statefinder parameters and phase space of $\\omega_\\vartheta$ and $\\omega'_\\vartheta$. We conclude that the consistent value of parameter $\\mu$ is $\\mu<0$ in accordance with the current Planck and WMAP$9$ results.
Is this the end of dark energy?
Barboza, Edésio M; Abreu, Éverton M C; Neto, Jorge Ananias
2015-01-01
In this paper we investigate the limits imposed by thermodynamics to a dark energy fluid. We obtain the heat capacities and the compressibilities for a dark energy fluid. These thermodynamical variables are easily accessible experimentally for any terrestrial fluid. The thermal and mechanical stabilities require these quantities to be positive. We show that such requirements forbid the existence of a cosmic fluid with negative constant EoS parameter which excludes vacuum energy as a candidate to explain the cosmic acceleration. We also show that the current observational data from SN Ia, BAO and $H(z)$ are in conflict with the physical constraints that a general dark energy fluid with a time-dependent EoS parameter must obey which can be interpreted as an evidence against the dark energy hypothesis. Although our result excludes the vacuum energy, a geometrical cosmological term as originally introduced by Einstein in the field equations remains untouched.
Dark Energy, Inflation and Extra Dimensions
Paul J. Steinhardt; Daniel Wesley
2008-12-07
We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation-of-state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in thecompact dimensions and vary with time in precise synchrony with the matter and energy density in the non-compact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications.
Dark Energy: fiction or reality?
NASA Astrophysics Data System (ADS)
Triay, Roland
2010-06-01
Is Dark Energy justified as an alternative to the cosmological constant ? in order to explain the acceleration of the cosmic expansion? It turns out that a straightforward dimensional analysis of Einstein equation provides us with clear evidences that the geometrical nature of ? is the only viable source to this phenomenon, in addition of the application of Ockham's razor principle. This contribution is primarily a review of the main stream in the interpretation of ? because it is at the origin of such a research program.
Dark Energy: fiction or reality?
Triay, Roland [Centre de Physique Theorique, CNRS Luminy Case 907, 13288 Marseille Cedex 9 (France)
2010-06-15
Is Dark Energy justified as an alternative to the cosmological constant LAMBDA in order to explain the acceleration of the cosmic expansion? It turns out that a straightforward dimensional analysis of Einstein equation provides us with clear evidences that the geometrical nature of LAMBDA is the only viable source to this phenomenon, in addition of the application of Ockham's razor principle. This contribution is primarily a review of the main stream in the interpretation of LAMBDA because it is at the origin of such a research program.
Dark energy from ? -attractors
NASA Astrophysics Data System (ADS)
Linder, Eric V.
2015-06-01
A class of inflation theories called ? -attractors has been investigated recently with interesting properties interpolating between quadratic potentials, the Starobinsky model, and an attractor limit. Here we examine their use for late-time cosmic acceleration. We generalize the class and demonstrate how it can interpolate between thawing and freezing dark energy, and reduce the fine-tuning of initial conditions, allowing w ?-1 for a prolonged period or as a de Sitter attractor.
Effective theory of interacting dark energy
NASA Astrophysics Data System (ADS)
Gleyzes, Jérôme; Langlois, David; Mancarella, Michele; Vernizzi, Filippo
2015-08-01
We present a unifying treatment of dark energy and modified gravity that allows distinct conformal-disformal couplings of matter species to the gravitational sector. In this very general approach, we derive the conditions to avoid ghost and gradient instabilities. We compute the equations of motion for background quantities and linear perturbations. We illustrate our formalism with two simple scenarios, where either cold dark matter or a relativistic fluid is nonminimally coupled. This extends previous studies of coupled dark energy to a much broader spectrum of gravitational theories.
The Dark Side: from Dark Energy & Dark Matter to Washington and Science Policy
Collar, Juan I.
The Dark Side: from Dark Energy & Dark Matter to Washington and Science Policy Presenter: Michael: The Map Room (www.maproom.com )1949 N. Hoyne #12;The Dark Side: from Dark Energy and Dark Matter questions --- what is the dark matter that holds our galaxy and every structure in the Universe together
Probing Dark Energy with Black Hole Binaries
Laura Mersini-Houghton; Adam Kelleher
2008-08-25
The equation of state (EoS) of dark energy $w$ remains elusive despite enormous experimental efforts to pin down its value and its time variation. Yet it is the single most important handle we have in our understanding of one of the most mysterious puzzle in nature, dark energy. This letter proposes a new method for measuring the EoS of dark energy by using the gravitational waves (GW) of black hole binaries. The method described here offers an alternative to the standard way of large scale surveys. It is well known that the mass of a black hole changes due to the accretion of dark energy but at an extremely slow rate. However, a binary of supermassive black holes (SBH) radiates gravitational waves with a power proportional to the masses of these accreting stars and thereby carries information on dark energy. These waves can propagate through the vastness of structure in the universe unimpeded. The orbital changes of the binary, induced by the energy loss from gravitational radiation, receive a large contribution from dark energy accretion. This contribution is directly proportional to $(1+w)$ and is dominant for SBH binaries with separation $R \\ge 1000$ parsec, thereby accelerating the merging process for $w > -1$ or ripping the stars apart for phantom dark energy with $w < -1$. Such orbital changes, therefore $w$, can be detected with LIGO and LISA near merging time, or with X-ray and radio measurements of Chandra and VLBA experiments.
From Asymptotic Safety to Dark Energy
Changrim Ahn; Chanju Kim; Eric V. Linder
2011-06-07
We consider renormalization group flow applied to the cosmological dynamical equations. A consistency condition arising from energy-momentum conservation links the flow parameters to the cosmological evolution, restricting possible behaviors. Three classes of cosmological fixed points for dark energy plus a barotropic fluid are found: a dark energy dominated universe, which can be either accelerating or decelerating depending on the RG flow parameters, a barotropic dominated universe where dark energy fades away, and solutions where the gravitational and potential couplings cease to flow. If the IR fixed point coincides with the asymptotically safe UV fixed point then the dark energy pressure vanishes in the first class, while (only) in the de Sitter limit of the third class the RG cutoff scale becomes the Hubble scale.
Conformal Higgs model: predicted dark energy density
R. K. Nesbet
2014-11-03
Postulated universal Weyl conformal scaling symmetry provides an alternative to the $\\Lambda$CDM paradigm for cosmology. Recent applications to galactic rotation velocities, Hubble expansion, and a model of dark galactic halos explain qualitative phenomena and fit observed data without invoking dark matter. Significant revision of theory relevant to galactic collisions and clusters is implied, but not yet tested. Dark energy is found to be a consequence of conformal symmetry for the Higgs scalar field of electroweak physics. The present paper tests this implication. The conformal Higgs model acquires a gravitational effect described by a modified Friedmann cosmic evolution equation, shown to fit cosmological data going back to the cosmic microwave background epoch. The tachyonic mass parameter of the Higgs model becomes dark energy in the Friedmann equation. A dynamical model of this parameter, analogous to the Higgs mechanism for gauge boson mass, is derived and tested here. An approximate calculation yields a result consistent with the empirical magnitude inferred from Hubble expansion.
Probing gravitation, dark energy, and acceleration
Linder, Eric V.
2004-02-20
The acceleration of the expansion of the universe arises from unknown physical processes involving either new fields in high energy physics or modifications of gravitation theory. It is crucial for our understanding to characterize the properties of the dark energy or gravity through cosmological observations and compare and distinguish between them. In fact, close consistencies exist between a dark energy equation of state function w(z) and changes to the framework of the Friedmann cosmological equations as well as direct spacetime geometry quantities involving the acceleration, such as ''geometric dark energy'' from the Ricci scalar. We investigate these interrelationships, including for the case of super acceleration or phantom energy where the fate of the universe may be more gentle than the Big Rip.
Piattella, O.F.; Martins, D.L.A.; Casarini, L. E-mail: denilsonluizm@gmail.com
2014-10-01
We consider a cosmological model of the late universe constituted by standard cold dark matter plus a dark energy component with constant equation of state w and constant effective speed of sound. By neglecting fluctuations in the dark energy component, we obtain an equation describing the evolution of sub-horizon cold dark matter perturbations through the epoch of dark matter-dark energy equality. We explore its analytic solutions and calculate an exact w-dependent correction for the dark matter growth function, logarithmic growth function and growth index parameter through the epoch considered. We test our analytic approximation with the numerical solution and find that the discrepancy is less than 1% for 0k = during the cosmic evolution up to a = 100.
Zero Cosmological Constant and Nonzero Dark Energy from Holographic Principle
Jae-Weon Lee
2013-04-13
It is shown that the first law of thermodynamics and the holographic principle applied to an arbitrary large cosmic causal horizon naturally demand the zero cosmological constant and non-zero dynamical dark energy in the form of the holographic dark energy. Semiclassical analysis shows that the holographic dark energy has a parameter $d=1$ and an equation of state comparable to current observational data, if the entropy of the horizon saturates the Bekenstein-Hawking bound. This result indicates that quantum field theory should be modified at large scale to explain dark energy. The relations among dark energy, quantum vacuum energy and entropic gravity are also discussed.
Dark Energy and Neutrino Model in SUSY
Ryo Takahashi; Morimitsu Tanimoto
2006-10-30
We discuss the effect of the supersymmetry breaking on the Mass Varying Neutrinos(MaVaNs) scenario. Especially, the effect mediated by the gravitational interaction between the hidden sector and the dark energy sector is studied. A model including a chiral superfield in the dark sector and the right-handed neutrino superfield is proposed. Evolutions of the neutrino mass and the equation of state parameter are presented in the model.
Interacting Dark Energy: Decay into Fermions
A. de la Macorra
2007-02-08
A dark energy component is responsible for the present stage of acceleration of our universe. If no fine tuning is assumed on the dark energy potential then it will end up dominating the universe at late times and the universe will not stop this stage of acceleration. On the other hand, the equation of state of dark energy seems to be smaller than -1 as suggested by the cosmological data. We take this as an indication that dark energy does indeed interact with another fluid (we consider fermion fields) and we determine the interaction through the cosmological data and extrapolate it into the future. We study the conditions under which a dark energy can dilute faster or decay into the fermion fields. We show that it is possible to live now in an accelerating epoch dominated by the dark energy and without introducing any fine tuning parameters the dark energy can either dilute faster or decaying into fermions in the future. The acceleration of the universe will then cease.
Can Holographic dark energy increase the mass of the wormhole?
Surajit Chattopadhyay; Davood Momeni; Aziza Altaibayeva; Ratbay Myrzakulov
2014-11-26
In this work, we have studied accretion of dark energy (DE) onto Morris- Thorne wormhole with three different forms, namely, holographic dark energy, holographic Ricci dark energy and modified holographic Ricci dark energy . Considering the scale factor in power-law form we have observed that as the holographic dark energy accretes onto wormhole, the mass of the wormhole is decreasing. In the next phase we considered three parameterization schemes that are able to get hold of quintessence as well as phantom phases. Without any choice of scale factor we reconstructed Hubble parameter from conservation equation and dark energy densities and subsequently got the mass of the wormhole separately for accretion of the three dark energy candidates. It was observed that if these dark energies accrete onto the wormhole, then for quintessence stage, wormhole mass decreases up to a certain finite value and then again increases to aggressively during phantom phase of the universe.
Dark energy and extended dark matter halos
NASA Astrophysics Data System (ADS)
Chernin, A. D.; Teerikorpi, P.; Valtonen, M. J.; Dolgachev, V. P.; Domozhilova, L. M.; Byrd, G. G.
2012-03-01
The cosmological mean matter (dark and baryonic) density measured in the units of the critical density is ?m = 0.27. Independently, the local mean density is estimated to be ?loc = 0.08-0.23 from recent data on galaxy groups at redshifts up to z = 0.01-0.03 (as published by Crook et al. 2007, ApJ, 655, 790 and Makarov & Karachentsev 2011, MNRAS, 412, 2498). If the lower values of ?loc are reliable, as Makarov & Karachentsev and some other observers prefer, does this mean that the Local Universe of 100-300 Mpc across is an underdensity in the cosmic matter distribution? Or could it nevertheless be representative of the mean cosmic density or even be an overdensity due to the Local Supercluster therein. We focus on dark matter halos of groups of galaxies and check how much dark mass the invisible outer layers of the halos are able to host. The outer layers are usually devoid of bright galaxies and cannot be seen at large distances. The key factor which bounds the size of an isolated halo is the local antigravity produced by the omnipresent background of dark energy. A gravitationally bound halo does not extend beyond the zero-gravity surface where the gravity of matter and the antigravity of dark energy balance, thus defining a natural upper size of a system. We use our theory of local dynamical effects of dark energy to estimate the maximal sizes and masses of the extended dark halos. Using data from three recent catalogs of galaxy groups, we show that the calculated mass bounds conform with the assumption that a significant amount of dark matter is located in the invisible outer parts of the extended halos, sufficient to fill the gap between the observed and expected local matter density. Nearby groups of galaxies and the Virgo cluster have dark halos which seem to extend up to their zero-gravity surfaces. If the extended halo is a common feature of gravitationally bound systems on scales of galaxy groups and clusters, the Local Universe could be typical or even an overdense region, with a low density contrast ~1.
Dark matter, dark energy and gravity
NASA Astrophysics Data System (ADS)
Robson, B. A.
2015-02-01
Within the framework of the Generation Model (GM) of particle physics, gravity is identified with the very weak, universal and attractive residual color interactions acting between the colorless particles of ordinary matter (electrons, neutrons and protons), which are composite structures. This gravitational interaction is mediated by massless vector bosons (hypergluons), which self-interact so that the interaction has two additional features not present in Newtonian gravitation: (i) asymptotic freedom and (ii) color confinement. These two additional properties of the gravitational interaction negate the need for the notions of both dark matter and dark energy.
Dark matter and dark energy: The critical questions
Michael S. Turner
2002-01-01
Stars account for only about 0.5% of the content of the Universe; the bulk of the Universe is optically dark. The dark side of the Universe is comprised of: at least 0.1% light neutrinos; 3.5% {+-} 1% baryons; 29% {+-} 4% cold dark matter; and 66% {+-} 6% dark energy. Now that we have characterized the dark side of the
Falsifying Field-based Dark Energy Models
Genly Leon; Yoelsy Leyva; Emmanuel N. Saridakis; Osmel Martin; Rolando Cardenas
2009-12-02
We survey the application of specific tools to distinguish amongst the wide variety of dark energy models that are nowadays under investigation. The first class of tools is more mathematical in character: the application of the theory of dynamical systems to select the better behaved models, with appropriate attractors in the past and future. The second class of tools is rather physical: the use of astrophysical observations to crack the degeneracy of classes of dark energy models. In this last case the observations related with structure formation are emphasized both in the linear and non-linear regimes. We exemplify several studies based on our research, such as quintom and quinstant dark energy ones. Quintom dark energy paradigm is a hybrid construction of quintessence and phantom fields, which does not suffer from fine-tuning problems associated to phantom field and additionally it preserves the scaling behavior of quintessence. Quintom dark energy is motivated on theoretical grounds as an explanation for the crossing of the phantom divide, i.e. the smooth crossing of the dark energy state equation parameter below the value -1. On the other hand, quinstant dark energy is considered to be formed by quintessence and a negative cosmological constant, the inclusion of this later component allows for a viable mechanism to halt acceleration. We comment that the quinstant dark energy scenario gives good predictions for structure formation in the linear regime, but fails to do that in the non-linear one, for redshifts larger than one. We comment that there might still be some degree of arbitrariness in the selection of the best dark energy models.
Goldman, Terry J [Los Alamos National Laboratory; Mckellar, B H J [UNIV 0F MELBOURNE; Stephenson, G J [UNM; Alsing, P M [UNM
2009-01-01
We present a model in which the parameter w approaches -1 near a particular value of z, and has significant negative values in a restricted range of z. For example, one can have w {approx} -1 near z = 1, and w > -0.2 from z = 0 to z = 0.3, and for z > 9. The ingredients of the model are neutral fermions (which may be neutrinos, neutralinos, etc) which are very weakly coupled to a light scalar field. This model emphasizes the importance of the proposed studies of the properties of dark energy into the region z > 1.
NASA Astrophysics Data System (ADS)
Frampton, Paul H.
2002-07-01
A long-standing problem of theoretical physics is the exceptionally small value of the cosmological constant Lambda approx 10-120 measured in natural Planckian units. Here we derive this tiny number from a toroidal string cosmology based on closed strings. In this picture the dark energy arises from the correlation between momentum and winding modes that for short distances has an exponential fall-off with increasing values of the momenta. The freeze-out by the expansion of the background universe for these transplanckian modes may be interpreted as a frozen condensate of the closed-string modes in the three non-compactified spatial dimensions.
NSDL National Science Digital Library
In this lesson, students simulate an experiment in which the discovery of dark energy can be made by plotting modern supernova distances on a Hubble Diagram. Data is provided in an Excel spreadsheet (see related resources). In order to complete this activity, students should be familiar with Hubble's Law and the concepts of absolute luminosity, apparent luminosity, and Doppler shift (particularly redshift). This activity can be done using either a computer graphing program or manually with graph paper. This lesson is part of the "Cosmic Times" teacher's guide and is intended to be used in conjunction with the 2006 Cosmic Times Poster.
Dark energy and particle mixing
A. Capolupo; S. Capozziello; G. Vitiello
2008-08-30
We show that the vacuum condensate due to particle mixing is responsible of a dynamically evolving dark energy. In particular, we show that values of the adiabatic index close to -1 for vacuum condensates of neutrinos and quarks imply, at the present epoch, contributions to the vacuum energy compatible with the estimated upper bound on the dark energy.
The Logotropic Dark Fluid as a unification of dark matter and dark energy
Pierre-Henri Chavanis
2015-04-29
We propose a heuristic unification of dark matter and dark energy in terms of a single dark fluid with a logotropic equation of state $P=A\\ln(\\rho/\\rho_P)$, where $\\rho$ is the rest-mass density, $\\rho_P$ is the Planck density, and $A$ is the logotropic temperature. The energy density $\\epsilon$ is the sum of a rest-mass energy term $\\rho c^2$ mimicking dark matter and an internal energy term $u(\\rho)=-P(\\rho)-A$ mimicking dark energy. The logotropic temperature is approximately given by $A \\simeq \\rho_{\\Lambda}c^2/\\ln(\\rho_P/\\rho_{\\Lambda})\\simeq\\rho_{\\Lambda}c^2/[123 \\ln(10)]$, where $\\rho_{\\Lambda}$ is the cosmological density. More precisely, we obtain $A=2.13\\times 10^{-9} \\, {\\rm g}\\, {\\rm m}^{-1}\\, {\\rm s}^{-2}$ that we interpret as a fundamental constant. At the cosmological scale, this model fullfills the same observational constraints as the $\\Lambda$CDM model. However, it has a nonzero velocity of sound and a nonzero Jeans length which, at the beginning of the matter era, is about $\\lambda_J=40.4\\, {\\rm pc}$, in agreement with the minimum size of the dark matter halos observed in the universe. At the galactic scale, the logotropic pressure balances gravitational attraction and solves the cusp problem and the missing satellite problem. The logotropic equation of state generates a universal rotation curve that agrees with the empirical Burkert profile of dark matter halos up to the halo radius. In addition, it implies that all the dark matter halos have the same surface density $\\Sigma_0=\\rho_0 r_h=141\\, M_{\\odot}/{\\rm pc}^2$ and that the mass of dwarf galaxies enclosed within a sphere of fixed radius $r_{u}=300\\, {\\rm pc}$ has the same value $M_{300}=1.93\\times 10^{7}\\, M_{\\odot}$, in remarkable agreement with the observations.
On the similarity of Information Energy to Dark Energy
M. P. Gough; T. D. Carozzi; A. M. Buckley
2006-06-19
Information energy is shown here to have properties similar to those of dark energy. The energy associated with each information bit of the universe is found to be defined identically to the characteristic energy of a cosmological constant. Two independent methods are used to estimate the universe information content of ~10^91 bits, a value that provides an information energy total comparable to that of the dark energy. Information energy is also found to have a significantly negative equation of state parameter, w energy.
Dark Matter, Dark Energy and General Relativity
NASA Astrophysics Data System (ADS)
Leibovitz, Jacques
2013-04-01
From an earlier model of DM, we derived: An equation of state for DM. An equation governing the coupled distributions of baryonic and dark matters; galactic flat rotation curves; the Tully-Fisher relation; Milgrom's MOND relation; and a mechanism for the accelerated expansion of the Universe (J of Modern Physics, Dec. 2011). In the present paper, we add the derivation of: a mechanism for the formation of a super massive black hole at the center of every large galaxy; a rational for the finding (by Oort) that the Sun mass on the Solar Neighborhood scale is twice that found on the Solar System scale. Results are discussed; we conclude that general relativity may neither be applied to DM nor to the description of the Universe and its evolution. The correct description must be based not on General Relativity but on Newton's Gravitation, on the proposed model of DM, and on Einstein's relativity of 1905.
On cosmic acceleration without dark energy
Edward W. Kolb; Sabino Matarrese; Antonio Riotto
2006-01-01
We elaborate on the proposal that the observed acceleration of the Universe is the result of the backreaction of cosmological perturbations, rather than the effect of a negative-pressure dark-energy fluid or a modification of general relativity. Through the effective Friedmann equations describing an inhomogeneous Universe after smoothing, we demonstrate that acceleration in our local Hubble patch is possible even if
National Radio Astronomy Observatory Dark Energy
Groppi, Christopher
National Radio Astronomy Observatory Dark Energy: Constraints from Astronomy, Answers from Physics? Jim Condon #12;National Radio Astronomy Observatory UVa/NRAO DE Lunch Talk 2005 Nov. 30 Constraining Astronomy Observatory UVa/NRAO DE Lunch Talk 2005 Nov. 30 Friedmann Equations for Expansion a = distance
From Dark Energy and Dark Matter to Dark Metric
S. Capozziello; M. De Laurentis; M. Francaviglia; S. Mercadante
2008-08-04
It is nowadays clear that General Relativity cannot be the definitive theory of Gravitation due to several shortcomings that come out both from theoretical and experimental viewpoints. At large scales (astrophysical and cosmological) the attempts to match it with the latest observational data lead to invoke Dark Energy and Dark Matter as the bulk components of the cosmic fluid. Since no final evidence, at fundamental level, exists for such ingredients, it is clear that General Relativity presents shortcomings at infrared scales. On the other hand, the attempts to formulate more general theories than Einstein's one give rise to mathematical difficulties that need workarounds that, in turn, generate problems from the interpretative viewpoint. We present here a completely new approach to the mathematical objects in terms of which a theory of Gravitation may be written in a first-order (a' la Palatini) formalism, and introduce the concept of Dark Metric which could completely bypass the introduction of disturbing concepts as Dark Energy and Dark Matter.
Generalized ghost dark energy in Brans-Dicke theory
A. Sheykhi; E. Ebrahimi; Y. Yosefi
2013-07-23
It was argued that the vacuum energy of the Veneziano ghost field of QCD, in a time-dependent background, can be written in the general form, $H + O(H^2)$, where $H$ is the Hubble parameter. Based on this, a phenomenological dark energy model whose energy density is of the form $\\rho=\\alpha H+\\beta H^{2}$ was recently proposed to explain the dark energy dominated universe. In this paper, we investigate this generalized ghost dark energy model in the setup of Brans-Dicke cosmology. We study the cosmological implications of this model. In particular, we obtain the equation of state and the deceleration parameters and a differential equation governing the evolution of this dark energy model. It is shown that the equation of state parameter of the generalized ghost dark energy can cross the phantom line ($w_D=-1$) in some range of the parameters spaces.
Quintessence Ghost Dark Energy Model
Ahamd Sheykhi; Ali Bagheri
2011-06-17
A so called "ghost dark energy" was recently proposed to explain the present acceleration of the universe expansion. The energy density of ghost dark energy, which originates from Veneziano ghost of QCD, is proportional to the Hubble parameter, $\\rho_D=\\alpha H$, where $\\alpha$ is a constant which is related to the QCD mass scale. In this paper, we establish the correspondence between ghost dark energy and quintessence scalar field energy density. This connection allows us to reconstruct the potential and the dynamics of the quintessence scalar field according to the evolution of ghost energy density.
Cosmology with dark energy decaying through its chemical-potential contribution
J. Besprosvany
2007-12-29
The consideration of dark energy's quanta, required also by thermodynamics, introduces its chemical potential into the cosmological equations. Isolating its main contribution, we obtain solutions with dark energy decaying to matter or radiation. When dominant, their energy densities tend asymptotically to a constant ratio, explaining today's dark energy-dark matter coincidence, and in agreement with supernova redshift data.
Dynamical system analysis for DBI dark energy interacting with dark matter
Nilanjana Mahata; Subenoy Chakraborty
2015-01-19
A dynamical system analysis related to Dirac Born Infeld (DBI) cosmological model has been investigated in this present work. For spatially flat FRW space time, the Einstein field equation for DBI scenario has been used to study the dynamics of DBI dark energy interacting with dark matter. The DBI dark energy model is considered as a scalar field with a nonstandard kinetic energy term. An interaction between the DBI dark energy and dark matter is considered through a phenomenological interaction between DBI scalar field and the dark matter fluid. The field equations are reduced to an autonomous dynamical system by a suitable redefinition of the basic variables. The potential of the DBI scalar field is assumed to be exponential. Finally, critical points are determined, their nature have been analyzed and corresponding cosmological scenario has been discussed.
Interacting dark matter and holographic dark energy in an anisotropic universe
NASA Astrophysics Data System (ADS)
Adhav, K. S.; Tayade, G. B.; Bansod, A. S.
2014-09-01
In this paper we have studied the anisotropic and homogeneous Bianchi type-I universe filled with interacting Dark matter and Holographic dark energy. Here we discussed two models, in first model the solutions of the field equations are obtained for constant value of deceleration parameter where as in the second model the solutions of the field equations are obtained for special form of deceleration parameter. It is shown that for suitable choice of interaction between dark matter and holographic dark energy there is no coincidence problem (unlike ?CDM). Also, in all the resulting models the anisotropy of expansion dies out very quickly and attains isotropy after some finite time. The Statefinder diagnostic is applied to both the models in order to distinguish between our dark energy models with other existing dark energy models. The physical and geometrical aspects of the models are also discussed.
Amplitude of dark energy perturbations
Gordon, Christopher; Wands, David [Kavli Institute for Cosmological Physics, Enrico Fermi Institute and Department of Astronomy and Astrophysics, University of Chicago, Chicago Illinois 60637 (United States); Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 2EG (United Kingdom)
2005-06-15
We propose a model which produces dark energy perturbations large enough to explain the lack of power seen at the quadrupole scale in the cosmic microwave background. If the dark energy is frozen from horizon exit during inflation until dark energy domination, then it is not possible to have perturbations in the dark energy which are large enough. We propose using a tachyonic amplification mechanism to overcome this. The dark energy is taken to be a complex scalar field, where the radial field has a Mexican hat potential. During inflation, the radial component is trapped near the maximum of its potential. At the end of inflation, it rolls down to the minimum. The dark energy today is taken to be a pseudo-Nambu-Goldstone boson. The perturbations generated during inflation are amplified by the rolling of the radial field. We also examine the use of the variable decay mechanism in order to generate an anticorrelation between the dark energy perturbations and the curvature perturbation. We show that using this mechanism then constrains the properties of the dark energy and its evolution from redshift one until today.
"Dark energy" as conformal dynamics of space
D. Burlankov
2006-10-23
The exact solution for dynamic of conform-flat space homogeneous since dynamic equation is given. Conform mode of space metric changing in Global time theory has negative energy density. Swap of energy to this mode from another ones lead to increasing of Universe homogeneity although probability of this swap from local objects is negligibly small. Conform mode is corresponding to "dark energy" in observation astronomy.
Fake evolution of dark energy from observation data
Qi, Shi
2015-01-01
The equation of state (EOS) of the dark energy is the key parameter to study the nature of the dark energy from the observation. Though the dark energy is found to be well consistent with the cosmological constant with a constant EOS of $-1$, weak evidences from different observation data and analyses show that dark energy models with an evolving EOS slightly less than $-1$ at some medium redshifts and greater than $-1$ at high redshifts are more favored. In this paper, It is shown that how such a pattern of an evolving dark energy EOS can be just biases arising from the statistical method widely adopted in data analyses together with the dependence of the cosmic expansion on the dark energy EOS. The issue is actually not limited to dark energy or cosmology. It represents a class of mathematical problems of Bayesian analysis. It should be paid attention to in similar data analyses to avoid biases in drawing conclusions.
Dark Matter and Dark Energy: Summary and Future Directions
John Ellis
2003-04-10
This paper reviews the progress reported at this Royal Society Discussion Meeting and advertizes some possible future directions in our drive to understand dark matter and dark energy. Additionally, a first attempt is made to place in context the exciting new results from the WMAP satellite, which were published shortly after this Meeting. In the first part of this review, pieces of observational evidence shown here that bear on the amounts of dark matter and dark energy are reviewed. Subsequently, particle candidates for dark matter are mentioned, and detection strategies are discussed. Finally, ideas are presented for calculating the amounts of dark matter and dark energy, and possibly relating them to laboratory data.
Holographic Dark Energy Like in $f(R)$ Gravity
Kh. Saaidi; A. Aghamohammadi
2010-10-12
We investigate the corresponding relation between $f(R)$ gravity and holographic dark energy. We introduce a kind of energy density from $f(R)$ which has role of the same as holographic dark energy. We obtain the differential equation that specify the evolution of the introduced energy density parameter based on varying gravitational constant. We find out a relation for the equation of state parameter to low redshifts which containing varying $G$ correction.
Gravitational collapse due to dark matter and dark energy in the brane world scenario
Soma Nath; Subenoy Chakraborty; Ujjal Debnath
2005-12-21
Gravitational collapse of FRW brane world embedded in a conformaly flat bulk is considered for matter cloud consists of dark matter and dark energy with equation of state $p=\\epsilon \\rho$ $(\\epsilonmatter and dark energy is being considered first separately and then a combination of them both with and without interaction. In some cases the collapse leads to black hole in some other cases naked singularity appears.
Dark Energy - Dark Matter Unification: Generalized Chaplygin Gas Model
Orfeu Bertolami
2005-04-14
We review the main features of the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and discuss how it admits an unique decomposition into dark energy and dark matter components once phantom-like dark energy is excluded. In the context of this approach we consider structure formation and show that unphysical oscillations or blow-up in the matter power spectrum are not present. Moreover, we demonstrate that the dominance of dark energy occurs about the time when energy density fluctuations start evolving away from the linear regime.
Non-linear dark energy clustering
Anselmi, Stefano; Ballesteros, Guillermo; Pietroni, Massimo E-mail: ballesteros@pd.infn.it
2011-11-01
We consider a dark energy fluid with arbitrary sound speed and equation of state and discuss the effect of its clustering on the cold dark matter distribution at the non-linear level. We write the continuity, Euler and Poisson equations for the system in the Newtonian approximation. Then, using the time renormalization group method to resum perturbative corrections at all orders, we compute the total clustering power spectrum and matter power spectrum. At the linear level, a sound speed of dark energy different from that of light modifies the power spectrum on observationally interesting scales, such as those relevant for baryonic acoustic oscillations. We show that the effect of varying the sound speed of dark energy on the non-linear corrections to the matter power spectrum is below the per cent level, and therefore these corrections can be well modelled by their counterpart in cosmological scenarios with smooth dark energy. We also show that the non-linear effects on the matter growth index can be as large as 10–15 per cent for small scales.
Fingerprinting dark energy. II. Weak lensing and galaxy clustering tests
Sapone, Domenico [Departamento de Fisica Teorica and Instituto de Fisica Teorica, Universidad Autonoma de Madrid IFT-UAM/CSIC, Cantoblanco, Madrid (Spain); Kunz, Martin [Departement de Physique Theorique, Universite de Geneve, 24 quai Ernest Ansermet, CH-1211 Geneve 4 (Switzerland); Institut d'Astrophysique Spatiale, Universite Paris-Sud XI, Orsay 91405 (France); Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom); Amendola, Luca [University of Heidelberg, Philosophenweg 16, 69120 Heidelberg (Germany); INAF/Rome (Italy)
2010-11-15
The characterization of dark energy is a central task of cosmology. To go beyond a cosmological constant, we need to introduce at least an equation of state and a sound speed and consider observational tests that involve perturbations. If dark energy is not completely homogeneous on observable scales, then the Poisson equation is modified and dark matter clustering is directly affected. One can then search for observational effects of dark energy clustering using dark matter as a probe. In this paper we exploit an analytical approximate solution of the perturbation equations in a general dark energy cosmology to analyze the performance of next-decade large-scale surveys in constraining equation of state and sound speed. We find that tomographic weak lensing and galaxy redshift surveys can constrain the sound speed of the dark energy only if the latter is small, of the order of c{sub s} < or approx. 0.01 (in units of c). For larger sound speeds the error grows to 100% and more. We conclude that large-scale structure observations contain very little information about the perturbations in canonical scalar field models with a sound speed of unity. Nevertheless, they are able to detect the presence of cold dark energy, i.e. a dark energy with nonrelativistic speed of sound.
Ricci Dark Energy in Brans-Dicke theory
Chao-Jun Feng
2008-06-04
A holographic dark energy from Ricci scalar curvature called Ricci dark energy was proposed recently. In this model the future event horizon area is replaced by the inverse of the Ricci scalar curvature. We study the evolution of equation of state of the Ricci dark energy and the transition from decelerated to accelerated expansion of the universe in the Brans-Dicke theory, which is a natural extension of general relativity. We find that the current acceleration of our universe is well explained.
Optimizing New Dark Energy Experiments
Tyson, J. Anthony
2013-08-26
Next generation “Stage IV” dark energy experiments under design during this grant, and now under construction, will enable the determination of the properties of dark energy and dark matter to unprecedented precision using multiple complementary probes. The most pressing challenge in these experiments is the characterization and understanding of the systematic errors present within any given experimental configuration and the resulting impact on the accuracy of our constraints on dark energy physics. The DETF and the P5 panel in their reports recommended “Expanded support for ancillary measurements required for the long-term program and for projects that will improve our understanding and reduction of the dominant systematic measurement errors.” Looking forward to the next generation Stage IV experiments we have developed a program to address the most important potential systematic errors within these experiments. Using data from current facilities it has been feasible and timely to undertake a detailed investigation of the systematic errors. In this DOE grant we studied of the source and impact of the dominant systematic effects in dark energy measurements, and developed new analysis tools and techniques to minimize their impact. Progress under this grant is briefly reviewed in this technical report. This work was a necessary precursor to the coming generations of wide-deep probes of the nature of dark energy and dark matter. The research has already had an impact on improving the efficiencies of all Stage III and IV dark energy experiments.
Examining the viability of phantom dark energy
NASA Astrophysics Data System (ADS)
Ludwick, Kevin J.
2015-09-01
In the standard cosmological framework of the 0th-order Friedmann-Lemaître-Robertson-Walker (FLRW) metric and the use of perfect fluids in the stress-energy tensor, dark energy with an equation-of-state parameter w <-1 (known as phantom dark energy) implies negative kinetic energy and vacuum instability when modeled as a scalar field. However, the accepted values for present-day w from Planck and WMAP9 include a significant range of values less than -1 . We find that it is not as obvious as one might think that phantom dark energy has negative kinetic energy categorically. Analogously, we find that field models of quintessence dark energy (w?>-1 ) do not necessarily have positive kinetic energy categorically. Staying within the confines of observational constraints and general relativity, for which there is good experimental validation, we consider a few reasonable departures from the standard 0th-order framework in an attempt to see if negative kinetic energy can be avoided in these settings despite an apparent w <-1 . We consider a more accurate description of the universe through the perturbing of the isotropic and homogeneous FLRW metric and the components of the stress-energy tensor, and we consider dynamic w and primordial isocurvature and adiabatic perturbations. We find that phantom dark energy does not necessarily have negative kinetic energy for all relevant length scales at all times, and we also find that, by the same token, quintessence dark energy does not necessarily have positive kinetic energy for all relevant length scales at all times.
The Localized Energy Distribution of Dark Energy Star Solutions
Paul Halpern; Michael Pecorino
2013-03-05
We examine the question of energy localization for an exact solution of Einstein's equations with a scalar field corresponding to the phantom energy interpretation of dark energy. We apply three different energy-momentum complexes, the Einstein, Papapetrou and M{\\o}ller prescriptions, to the exterior metric and determine the energy distribution for each. Comparing the results, we find that the three prescriptions yield identical energy distributions.
Existence of Negative Gravity Material Identification of Dark Energy
Gilson, J G
2006-01-01
A solution to Einstein's field equations via the Friedman equations is shown to produce a cosmological model that is in exact agreement with the measurements made by the dark energy astronomers. All the essential physical parameters are obtained as epoch dependent functions all in closed form. The equations of state are obtained for total density, non-dark energy density and dark energy density. An interpretation of the structure involving a dark energy mass distribution that is twice the usual value is shown to clarify greatly the physical significance of the mathematics. It is asserted that the astronomer's measurements together with the mathematical model proves that the universe is permeated uniformly with a positive mass density that caries a negative gravitational constant, -G, characteristic. This mass component is identified with the dark energy content of the universe that has been postulated to explain the observed acceleration. Another result implied by the model is that there is twice the amount o...
Dark Matter and Dark Energy huncheng@math.mit.edu
Cheng, Hung
Dark Matter and Dark Energy Hung Cheng huncheng@math.mit.edu January 17, 2008 Abstract We suggest that a candidate for dark matter is a meson with spin one the existence of which is dictated by local scale invariance pro- posed by Herman Weyl.. Dark Matter and Scale Invariance Hung Cheng Department of Mathematics
Dynamics of teleparallel dark energy
NASA Astrophysics Data System (ADS)
Wei, Hao
2012-06-01
Recently, Geng et al. proposed to allow a non-minimal coupling between quintessence and gravity in the framework of teleparallel gravity, motivated by the similar one in the framework of General Relativity (GR). They found that this non-minimally coupled quintessence in the framework of teleparallel gravity has a richer structure, and named it "teleparallel dark energy". In the present work, we note that there might be a deep and unknown connection between teleparallel dark energy and Elko spinor dark energy. Motivated by this observation and the previous results of Elko spinor dark energy, we try to study the dynamics of teleparallel dark energy. We find that there exist only some dark-energy-dominated de Sitter attractors. Unfortunately, no scaling attractor has been found, even when we allow the possible interaction between teleparallel dark energy and matter. However, we note that w at the critical points is in agreement with observations (in particular, the fact that w=-1 independently of ? is a great advantage).
Holographic Dark Energy with Time Varying n^2 Parameter in Non-Flat Universe
Bushra Majeed; Mubasher Jamil; Azad A. Siddiqui
2014-11-01
We consider a holographic dark energy model, with a varying parameter, n, which evolves slowly with time. We obtain the differential equation describing evolution of the dark energy density parameter, $\\Omega_d$, for the flat and non-flat FRW universes. The equation of state parameter in this generalized version of holographic dark energy depends on n.
Fate of the phantom dark energy universe in semiclassical gravity
Jaume Haro; Jaume Amoros; Emilio Elizalde
2011-01-01
The fate of the phantom dark energy universe in semiclassical gravity is investigated. Quantum corrections coming from massless fields conformally coupled with gravity are considered, to see if they can lead to avoidance of the big rip singularity, which shows up in a flat Friedmann-Robertson-Walker universe, filled with phantom dark energy and modeled by an equation of state of the
Properties of singularities in the (phantom) dark energy universe
Shinichi Nojiri; Sergei D. Odintsov; Shinji Tsujikawa
2005-01-01
The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an
Faculty Research Seminar of the Dark Energy
Hu, Wayne
Smith · Takemi Okamoto #12;Dark Energy Observables #12;Making Light of the Dark Side · Line element: ds2;CMB Background #12;Dark Energy Tests · Standard Candles · Standard Ruler Sound horizon (2%); matter
Optimized supernova constraints on dark energy evolution
Christian Stephan-Otto
2006-07-20
A model-independent method to study the possible evolution of dark energy is presented. Optimal estimates of the dark energy equation of state w are obtained from current supernovae data from Riess et al. (2004) following a principal components approach. We assess the impact of varying the number of piecewise constant estimates of w using a model selection method, the Bayesian information criterion, and compare the most favored models with some parametrizations commonly used in the literature. Although data seem to prefer a cosmological constant, some models are only moderately disfavored by our selection criterion: a constant w, w linear in the scale factor, w linear in redshift and the two-parameter models introduced here. Among these, the models we find by optimization are slightly preferred. However, current data do not allow us to draw a conclusion on the possible evolution of dark energy. Interestingly, the best fits for all varying-w models exhibit a w<-1 at low redshifts.
Quantum Informational Dark Energy: Dark energy from forgetting
Jae-Weon Lee; Jungjai Lee; Hyeong-Chan Kim
2008-04-06
We suggest that dark energy has a quantum informational origin. Landauer's principle associated with the erasure of quantum information at a cosmic horizon implies the non-zero vacuum energy having effective negative pressure. Assuming the holographic principle, the minimum free energy condition, and the Gibbons-Hawking temperature for the cosmic event horizon we obtain the holographic dark energy with the parameter $d\\simeq 1$, which is consistent with the current observational data. It is also shown that both the entanglement energy and the horizon energy can be related to Landauer's principle.
Wormhole solutions supported by interacting dark matter and dark energy
Vladimir Folomeev; Vladimir Dzhunushaliev
2014-03-10
We show that the presence of a nonminimal interaction between dark matter and dark energy may lead to a violation of the null energy condition and to the formation of a configuration with nontrivial topology (a wormhole). In this it is assumed that both dark matter and dark energy satisfy the null energy condition, a violation of which takes place only in the inner high-density regions of the configuration. This is achieved by assuming that, in a high-density environment, a nonminimal coupling function changes its sign in comparison with the case where dark matter and dark energy have relatively low densities which are typical for a cosmological background. For this case, we find regular static, spherically symmetric solutions describing wormholes supported by dark matter nonminimally coupled to dark energy in the form of a quintessence scalar field.
Alternatives to dark matter and dark energy
Philip D. Mannheim
2006-01-01
We review the underpinnings of the standard Newton–Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy–momentum tensor which serves as the source of gravity. We identify the
Numerical study of halo concentrations in dark-energy cosmologies
Klaus Dolag; Matthias Bartelmann; Francesca Perrotta; Carlo Baccigalupi; Lauro Moscardini; Massimo Meneghetti; Giuseppe Tormen
2004-01-01
We study the concentration parameters, their mass dependence and redshift evolution, of dark-matter halos in different dark-energy cosmologies with constant and time-variable equation of state, and compare them with ``standard'' LambdaCDM and OCDM models. We find that previously proposed algorithms for predicting halo concentrations can be well adapted to dark-energy models. When centred on the analytically expected values, halo concentrations
The CMB as a dark energy probe
Carlo Baccigalupi; Viviana Acquaviva
2006-06-03
We give a brief review of the known effects of a dynamical vacuum cosmological component, the dark energy, on the anisotropies of the cosmic microwave background (CMB). We distinguish between a "classic" class of observables, used so far to constrain the average of the dark energy abundance in the redshift interval in which it is relevant for acceleration, and a "modern" class, aiming at the measurement of its differential redshift behavior. We show that the gravitationally lensed CMB belongs to the second class, as it can give a measure of the dark energy abundance at the time of equality with matter, occurring at about redshift 0.5. Indeed, the dark energy abundance at that epoch influences directly the lensing strength, which is injected at about the same time, if the source is the CMB. We illustrate this effect focusing on the curl (BB) component of CMB polarization, which is dominated by lensing on arcminute angular scales. An increasing dark energy abundance at the time of equality with matter, parameterized by a rising first order redshift derivative of its equation of state today, makes the BB power dropping with respect to a pure LambdaCDM cosmology, keeping the other cosmological parameters and primordial amplitude fixed. We briefly comment on the forthcoming probes which might measure the lensing power on CMB.
Aaron Trout
2012-08-15
In this paper, we give a conceptual explanation of dark energy as a small negative residual scalar curvature present even in empty spacetime. This curvature ultimately results from postulating a discrete spacetime geometry, very closely related to that used in the dynamical triangulations approach to quantum gravity. In this model, there are no states which have total scalar curvature exactly zero. Moreover, numerical evidence in dimension three suggests that, at a fixed volume, the number of discrete-spacetime microstates strongly increases with decreasing curvature. Because of the resulting entropic force, any dynamics which push empty spacetime strongly toward zero scalar curvature would instead produce typically observed states with a small negative curvature. This provides a natural explanation for the empirically observed small positive value for the cosmological constant (Lambda is about 10^(-121) in Planck units.) In fact, we derive the very rough estimate Lambda=10^(-187) from a simple model containing only the two (highly-degenerate) quantum states with total scalar-curvature closest to zero.
NASA Astrophysics Data System (ADS)
Nugent, Peter
2006-07-01
Astronomers have begun to measure the fundamental parameters of cosmology through the observation of very distant Type Ia supernovae. Over the past decade more than 300 spectroscopically confirmed high-redshift supernovae have been discovered. These supernovae are used as standardized candles to measure the history of the expansion of the universe. Under the current standard model for cosmology these measurements indicate the presence of a heretofore unknown dark energy causing a recent acceleration in the expansion of the universe. At this time supernova measurements of the cosmological parameters are no longer limited by statistical uncertainties, rather systematic uncertainties are the dominant source of error. These include the effects of evolution (further back in time do the supernovae behave the same way?), the effect of intergalactic dust on the brightness of the supernovae and the relationship between supernovae and their environments. Here I present exciting new developments in the field of cosmology using Type II-P supernovae as standardized candles and the prospect of using them to independently measure the cosmological parameters.
Dynamical behavior of interacting dark energy in loop quantum cosmology
Kui Xiao; Jian-Yang Zhu
2010-06-28
The dynamical behaviors of interacting dark energy in loop quantum cosmology are discussed in this paper. Based on defining three dimensionless variables, we simplify the equations of the fixed points. The fixed points for interacting dark energy can be determined by the Friedmann equation coupled with the dynamical equations {in Einstein cosmology}. But in loop quantum cosmology, besides the Friedmann equation, the conversation equation also give a constrain on the fixed points. The difference of stability properties for the fixed points in loop quantum cosmology and the ones in Einstein cosmology also have been discussed.
Dark Energy Rules the Universe
Linder, Eric
2008-01-01
Berkeley Lab theoretical physicist Eric Linder previews his Nov. 24, 2008 talk on the mystery of dark energy. Catch his full lecture here: http://www.osti.gov/sciencecinema/servlets/purl/1007511?format=mp4
Extraterrestrial life contradicts dark energy
NASA Astrophysics Data System (ADS)
Gibson, Carl H.
2012-10-01
Extraterrestrial life contradicts the Cold Dark Matter (CDM) Hierarchical Clustering (HC) model for cosmology, as well as its dark energy extension (by the 2011 Nobel Prize in Physics) to include an accelerating expansion of the universe (?CDMHC). The expansion is driven by the antigravitational property of dark energy that justified Einstein's cosmological constant (?). CDM stars appear only after a dark-age period lasting 300 Myr, rendering cosmic scale extraterrestrial life problematic. Turbulence stresses of Hydro-Gravitational-Dynamics (HGD) cosmology during the big bang are powerful but temporary, so CDM and dark energy ??are unnecessary. Superclusters fragment at 0.03 Myr. Hydrogen planets in proto-globular-star-cluster (PGC) clumps fragment protogalaxies at the transition to gas (0.3 Myr). The density at 0.03 Myr is preserved by old globular clusters (OGC) as a fossil of first fragmentation. Infrared observations support the HGD prediction (Gibson 1996) and quasar microlensing observation (Schild 1996) that the dark matter of galaxies is Earth-mass gas planets in dense PGC clumps. Water oceans seeded by dust of the first exploding stars at 2 Myr hosted extraterrestrial life spread on cosmic scales. Life anywhere falsifies dark energy.
Reconstructing interaction between dark energy and dark matter using Gaussian Processes
Cai, Rong-Gen; Yang, Tao
2015-01-01
We present a non-parametric approach to reconstruct the interaction between dark energy and dark matter directly from SNIa Union 2.1 data using Gaussian Processes, which is a fully Bayesian approach for smoothing data. In this method, once the equation of state ($w$) of dark energy is specified, the interaction can be reconstructed with respect to redshift. For the decaying vacuum energy case with $w=-1$, the reconstructed interaction is consistent with the $\\Lambda$CDM model, namely, there is no evidence for the interaction. This also holds for the constant $w$ cases from $-0.9$ to $-1.1$ and for the CPL parameterization case. If the equation of state deviates obviously from $-1$, the reconstructed interaction exits at $95\\%$ confidence level. This shows the degeneracy between the interaction and the equation of state of dark energy when they get constraints from the observational data.
Reconstructing the interaction between dark energy and dark matter using Gaussian Processes
Tao Yang; Zong-Kuan Guo; Rong-Gen Cai
2015-06-25
We present a nonparametric approach to reconstruct the interaction between dark energy and dark matter directly from SNIa Union 2.1 data using Gaussian processes, which is a fully Bayesian approach for smoothing data. In this method, once the equation of state ($w$) of dark energy is specified, the interaction can be reconstructed as a function of redshift. For the decaying vacuum energy case with $w=-1$, the reconstructed interaction is consistent with the standard $\\Lambda$CDM model, namely, there is no evidence for the interaction. This also holds for the constant $w$ cases from $-0.9$ to $-1.1$ and for the Chevallier-Polarski-Linder (CPL) parametrization case. If the equation of state deviates obviously from $-1$, the reconstructed interaction exists at $95\\%$ confidence level. This shows the degeneracy between the interaction and the equation of state of dark energy when they get constraints from the observational data.
Dark Energy Constraints from the Cosmic Age and Supernova
Bo Feng; Xiulian Wang; Xinmin Zhang
2004-12-23
Using the low limit of cosmic ages from globular cluster and the white dwarfs: $t_0 > 12$Gyr, together with recent new high redshift supernova observations from the HST/GOODS program and previous supernova data, we give a considerable estimation of the equation of state for dark energy, with uniform priors as weak as $0.2age limit plays a significant role in lowering the upper bound on the variation amplitude of dark energy equation of state. We propose in this paper a new scenario of dark energy dubbed Quintom, which gives rise to the equation of state larger than -1 in the past and less than -1 today, satisfying current observations. In addition we've also considered the implications of recent X-ray gas mass fraction data on dark energy, which favors a negative running of the equation of state.
Stable dark energy stars: An alternative to black holes?
Francisco S. N. Lobo
2006-12-05
In this work, a generalization of the Mazur-Mottola gravastar model is explored, by considering a matching of an interior solution governed by the dark energy equation of state, $\\omega\\equiv p/ \\rhoenergy is a possible candidate.
Dark Energy and the Accelerating Universe
Weijgaert, Rien van de
Dark Energy and the Accelerating Universe Joshua A. Frieman,1,2 Michael S. Turner,2 and Dragan% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound acceleration could arise from the repulsive gravity of dark energy--for example, the quantum energy
Dark energy, dark matter and the Chaplygin gas
R. Colistete Jr.; J. C. Fabris; S. V. B. Goncalves; P. E. de Souza
2002-10-23
The possibility that the dark energy may be described by the Chaplygin gas is discussed. Some observational constraints are established. These observational constraints indicate that a unified model for dark energy and dark matter through the employement of the Chaplygin gas is favored.
Dark energy from gravitoelectromagnetic inflation?
Federico Agustin Membiela; Mauricio Bellini
2008-07-29
Gravitoectromagnetic Inflation (GI) was introduced to describe in an unified manner, electromagnetic, gravitatory and inflaton fields from a 5D vacuum state. On the other hand, the primordial origin and evolution of dark energy is today unknown. In this letter we show using GI that the zero modes of some redefined vector fields $B_i=A_i/a$ produced during inflation, could be the source of dark energy in the universe.
Anisotropic charged dark energy star
Das, Kanika
2014-01-01
As the stars carry electrical charges, we present in this paper a model for charged dark energy star which is singularity free. We take Krori-Barua space time. We assume that the radial pressure exerted on the system due to the presence of dark energy is proportional to the isotropic perfect fluid matter density and the difference between tangential and radial pressure is proportional to the square of the electric field intensity. The solution satisfies the physical conditions inside the star
Is there Supernova Evidence for Dark Energy Metamorphosis ?
Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky
2004-07-10
We reconstruct the equation of state $w(z)$ of dark energy (DE) using a recently released data set containing 172 type Ia supernovae without assuming the prior $w(z) \\geq -1$ (in contrast to previous studies). We find that dark energy evolves rapidly and metamorphoses from dust-like behaviour at high $z$ ($w \\simeq 0$ at $z \\sim 1$) to a strongly negative equation of state at present ($w \\lleq -1$ at $z \\simeq 0$). Dark energy metamorphosis appears to be a robust phenomenon which manifests for a large variety of SNe data samples provided one does not invoke the weak energy prior $\\rho + p \\geq 0$. Invoking this prior considerably weakens the rate of growth of $w(z)$. These results demonstrate that dark energy with an evolving equation of state provides a compelling alternative to a cosmological constant if data are analysed in a prior-free manner and the weak energy condition is not imposed by hand.
Is there Supernova Evidence for Dark Energy Metamorphosis ?
Alam, U; Saini, T D; Starobinsky, A A; Alam, Ujjaini; Sahni, Varun; Saini, Tarun Deep
2003-01-01
We reconstruct the equation of state w(z) of dark energy (DE) using a recently released data set containing 172 type Ia supernovae without imposing any priors on w(z) (in contrast to previous studies). We find that dark energy evolves rapidly and metamorphoses from dust-like behaviour at high z ($w \\simeq 0$ at $z \\sim 1$) to a strongly negative equation of state at present ($w \\lleq -1$ at $z \\simeq 0$). Dark energy metamorphosis appears to be a robust phenomenon which manifests for a large variety of Sn data samples provided one does not invoke the weak energy prior $\\rho + p \\geq 0$. Invoking this prior considerably weakens the rate of growth of w(z). These results demonstrate that dark energy with an evolving equation of state provides a compelling alternative to a cosmological constant if data are analysed in a prior-free manner and the weak energy condition is not imposed by hand.
Phase-space analysis of teleparallel dark energy
Xu, Chen [College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China); Saridakis, Emmanuel N. [Physics Division, National Technical University of Athens, 15780 Zografou Campus, Athens (Greece); Leon, Genly, E-mail: xuc1990@126.com, E-mail: Emmanuel_Saridakis@baylor.edu, E-mail: genly@uclv.edu.cu [Department of Mathematics, Universidad Central de Las Villas, Santa Clara, CP 54830 (Cuba)
2012-07-01
We perform a detailed dynamical analysis of the teleparallel dark energy scenario, which is based on the teleparallel equivalent of General Relativity, in which one adds a canonical scalar field, allowing also for a nonminimal coupling with gravity. We find that the universe can result in the quintessence-like, dark-energy-dominated solution, or to the stiff dark-energy late-time attractor, similarly to standard quintessence. However, teleparallel dark energy possesses an additional late-time solution, in which dark energy behaves like a cosmological constant, independently of the specific values of the model parameters. Finally, during the evolution the dark energy equation-of-state parameter can be either above or below -1, offering a good description for its observed dynamical behavior and its stabilization close to the cosmological-constant value.
Towards understanding the nature of dark energy
NASA Astrophysics Data System (ADS)
Fang, Wenjuan
2009-10-01
Dark energy turns out to be one of the most intriguing problems in modern cosmology. In this thesis, we study several projects related to unveiling the nature of dark energy. As a negative pressure component, the most important property of dark energy is its equation of state w. We study the prospect of constraining w through a future weak-lensing survey by combining two methods: the tomographic shear-shear correlations and the number counts of the shear-selected galaxy clusters. We calculate the covariance between the two observables and find it negligible. By forecasting the constraining power through the Fisher matrix formalism, we find that each method has serious parameter degeneracy, hut their combination results in appreciable complementarity. In order to use the abundance of galaxy clusters to constrain dark energy, the primary systematic effect is uncertainty in the mass-observable scaling relations. For the X-ray clusters, we calibrate the preheating model for the intracluster gas by comparing its predictions for the X-ray Luminosity-Temperature relations with those of two cluster samples observed at different redshifts. We find that the required entropy level increases with time, which indicates the time-dependence of feedback on the intracluster gas cannot be discarded. Instead of a missing unknown energy component, the dark energy problem may indicate that gravity deviates from GR on cosmological scales. We investigate the self-accelerating Dvali-Gabadadze-Porrati (DGP) model which explains the accelerated expansion of the universe from the perspective of modified gravity. By implementing the parameterized post-Friedmann (PPF) formalism into standard cosmological tools, we can calculate the growth of structure in DGP efficiently. This allows us to perform a thorough Markov Chain Monte Carlo analysis of the model, given the current observations of the anisotropies of the cosmic microwave background, magnitude of the supernovae and the Hubble constant. Our results show that this model cannot fit both the geometry and growth data simultaneously, with its best-fit nominally 5sigma poorer than that of the concordance ACDM model. If dark energy is not a cosmological constant, it clusters in space. The clustering properties are well described for the scalar field dark energy models. However, if w evolves across the phantom divide defined by w = -1, to avoid instabilities in the perturbations, the dark energy cannot be realized by a single scalar field, but a composite of multiple fields, the construction of which is cumbersome. We introduce a PPF description for these smooth dark energy models to avoid this difficulty. By comparing with true scalar field models, we find that the PPF description gives accurate results.
Anisotropic cosmological models with perfect fluid and dark energy
Bijan Saha
2004-12-16
We consider a self-consistent system of Bianchi type-I (BI) gravitational field and a binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be the one obeying the usual equation of state, i.e., $p = \\zeta \\ve$, with $\\zeta \\in [0, 1]$ whereas, the dark energy density is considered to be either the quintessence or the Chaplygin gas. Exact solutions to the corresponding Einstein equations are obtained.
Dark Energy and Dark Matter as Inertial Effects
Serkan Zorba
2012-10-20
A globally rotating model of the universe is postulated. It is shown that dark energy and dark matter are cosmic inertial effects resulting from such a cosmic rotation, corresponding to centrifugal and a combination of centrifugal and the Coriolis forces, respectively. The physics and the cosmological and galactic parameters obtained from the model closely match those attributed to dark energy and dark matter in the standard {\\Lambda}-CDM model.
Dark energy and dark matter from primordial QGP
NASA Astrophysics Data System (ADS)
Vaidya, Vaishali; Upadhyaya, G. K.
2015-07-01
Coloured relics servived after hadronization might have given birth to dark matter and dark energy. Theoretical ideas to solve mystery of cosmic acceleration, its origin and its status with reference to recent past are of much interest and are being proposed by many workers. In the present paper, we present a critical review of work done to understand the earliest appearance of dark matter and dark energy in the scenario of primordial quark gluon plasma (QGP) phase after Big Bang.
A Possible Origin of Dark Matter, Dark Energy, and Particle-Antiparticle Asymmetry
A. C. V. V. de Siqueira
2010-09-30
In this paper we present a possible origin of dark matter and dark energy from a solution of the Einstein's equation to a primordial universe, which was presented in a previous paper. We also analyze the Dirac's equation in this primordial universe and present the possible origin of the particle-antiparticle asymmetry. We also present ghost primordial particles as candidates to some quantum vacuum contituents.
Reissner-Nordstrom black hole in dark energy background
Ngangbam Ishwarchandra; Ng. Ibohal; K. Yugindro Singh
2014-11-29
In this paper we propose a stationary solution of Einstein's field equations describing Reissner-Nordstrom black hole in dark energy background. It is to be regarded as the Reissner-Nordstrom black hole is embedded into the dark energy solution producing Reissner-Nordstrom-dark energy black hole. We find that the space-time geometry of Reissner-Nordstrom-dark energy solution is Petrov type $D$ in the classification of space-times. It is also shown that the embedded space-time possesses an energy-momentum tensor of the electromagnetic field interacting with the dark energy having negative pressure. We find the energy-momentum tensor for dark energy violates the the strong energy condition due to the negative pressure, whereas that of the electromagnetic field obeys the strong energy condition. It is shown that the time-like vector field for an observer in the Reissner-Nordstrom-dark energy space is expanding, accelerating, shearing and non-rotating. We investigate the surface gravity of the horizons for the embedded dark energy black hole. The characteristic properties of relativistic dark energy based on the de Sitter solution is discussed in an appendix.
Strong gravitational lensing and dark energy complementarity
Linder, Eric V.
2004-01-21
In the search for the nature of dark energy most cosmological probes measure simple functions of the expansion rate. While powerful, these all involve roughly the same dependence on the dark energy equation of state parameters, with anticorrelation between its present value w{sub 0} and time variation w{sub a}. Quantities that have instead positive correlation and so a sensitivity direction largely orthogonal to, e.g., distance probes offer the hope of achieving tight constraints through complementarity. Such quantities are found in strong gravitational lensing observations of image separations and time delays. While degeneracy between cosmological parameters prevents full complementarity, strong lensing measurements to 1 percent accuracy can improve equation of state characterization by 15-50 percent. Next generation surveys should provide data on roughly 105 lens systems, though systematic errors will remain challenging.
Dark energy and formation of classical scalar fields
Houri Ziaeepour
2006-04-03
We present a quintessence model for the dark energy in which the quintessence scalar field is produced by the decay of a super heavy dark matter and gradually condensate to a classical scalar field. This model can explain both the smallness and the latest observations by WMAP for the equation of state of the dark energy which has $w \\sim -1.06$. We review both classical and field theoretical treatment of this model and briefly explain the most important parameters for obtaining the observed characteristic of the dark energy.
UTILIZING DARK SILICON TO SAVE ENERGY WITH
Wenisch, Thomas F.
DARK SILICON TO SAVE ENERGY WITH COMPUTATIONAL SPRINTING ................................................................................................................................................................................................................... COMPUTATIONAL SPRINTING ACTIVATES DARK SILICON TO IMPROVE RESPONSIVENESS BY BRIEFLY BUT INTENSELY EXCEEDING AT THE PLATFORM'S THERMAL LIMIT. ......Researchers predict increasingly underutilized chip area (dark silicon
Cold Fusion Dark Matter and Dark Energy
NASA Astrophysics Data System (ADS)
Levi, Mark
2009-05-01
Explanation of Cold Fusion [1] ``It is k-capture forming dineutrons followed by absorption by palladium.'' with excess heat energy no more than about .15 MeV per nucleon. Experimentally [1], ^1H and electrons are at high pressure at the center of a palladium wire sample, ``After hours of loading with ^1H, bubbles were present on the wire surface and the wire's resistance had stopped increasing, there was a fizz of hydrogen from the wire within a few seconds after loading current and large bubbles were stopped.'' a repeatable cycle. K-capture rate is affected by environment at the 1/10000 level has has been known since 1946 ( ref. [6]in [1]); and recently has been seen at the 0.35% level for 7Be in C60 [2]. Neutron halos have been seen recently in 8He [3], 6He [4] and others long ago. Conclusions: 1) the evidence for dineutrons is fairly good and as in all K-captures is accompanied by a neutrino emission. collapse of a star to a neutron star has a succession of K-captures in conditions like cold fusion i.e. high pressure. 2)Dark matter is dineutrons from formation of neutron stars and black holes, and dark energy of neutrinos generated in neutron stars, ordinary stars and black holes. If in the latter, then their mass must be zero for an infinite horizon. References: [1] M. Levi, DAMOP Meeting poster paper, session WP, 16-19 May,1995 [2]T. Ohtsuku et al., Phys. Rev. Lett. 98, 252501 (2007) [3] V. I. Ryjkov et al., Phys. Rev. Lett. 101, 01901 (2008) [4] L. B. Wang et al., Phys. Rev. Lett. 93 ,142501 (2004).
Interaction between DBI-essence and other Dark Energies
Surajit Chattopadhyay; Ujjal Debnath
2010-06-11
The present work considers interaction between DBI-essence and other candidates of dark energies like modified Chaplygin gas, hessence, tachyonic field, and new agegraphic dark energy. The potentials of the fields have been reconstructed under interaction and their evolutions have been viewed against cosmic time $t$ and scalar field $\\phi$. Equation of state parameters have also been obtained. The nature of potentials and the equation of state parameters of the dark energies have been found graphically in presence of interaction (both small and large interaction).
Cosmological constraints on a dark matter -- dark energy interaction
Mark B. Hoffman
2003-08-26
It is generally assumed that the two dark components of the energy density of the universe, a smooth component called dark energy and a fluid of nonrelativistic weakly interacting particles called dark matter, are independent of each other and interact only through gravity. In this paper, we consider a class of models in which the dark matter and dark energy interact directly. The dark matter particle mass is proportional to the value of a scalar field, and the energy density of this scalar field comprises the dark energy. We study the phenomenology of these models and calculate the luminosity distance as a function of redshift and the CMB anisotropy spectrum for several cases. We find that the phenomenology of these models can differ significantly from the standard case, and current observations can already rule out the simplest models.
NASA Astrophysics Data System (ADS)
Greyber, Howard
2009-11-01
By careful analysis of the data from the WMAP satellite, scientists were surprised to determine that about 70% of the matter in our universe is in some unknown form, and labeled it Dark Energy. Earlier, in 1998, two separate international groups of astronomers studying Ia supernovae were even more surprised to be forced to conclude that an amazing smooth transition occurred, from the expected slowing down of the expansion of our universe (due to normal positive gravitation) to an accelerating expansion of the universe that began at at a big bang age of the universe of about nine billion years. In 1918 Albert Einstein stated that his Lambda term in his theory of general relativity was ees,``the energy of empty space,'' and represented a negative pressure and thus a negative gravity force. However my 2004 ``Strong'' Magnetic Field model (SMF) for the origin of magnetic fields at Combination Time (Astro-ph0509223 and 0509222) in our big bang universe produces a unique topology for Superclusters, having almost all the mass, visible and invisible, i.e. from clusters of galaxies down to particles with mass, on the surface of an ellipsoid surrounding a growing very high vacuum. If I hypothesize, with Einstein, that there exists a constant ees force per unit volume, then, gradually, as the universe expands from Combination Time, two effects occur (a) the volume of the central high vacuum region increases, and (b) the density of positive gravity particles in the central region of each Supercluster in our universe decreases dramatically. Thus eventually Einstein's general relativity theory's repulsive gravity of the central very high vacuum region becomes larger than the positive gravitational attraction of all the clusters of galaxies, galaxies, quasars, stars and plasma on the Supercluster shell, and the observed accelerating expansion of our universe occurs. This assumes that our universe is made up mostly of such Superclusters. It is conceivable that the high vacuum region between Superclusters also plays a role in adding extra repulsive gravity force. Note that cosmologist Stephen Hawking comments on his website that ``There is no reason to rule out negative pressure. This is just tension.''
Cosmological dark energy effects from entanglement
S. Capozziello; O. Luongo; S. Mancini
2013-02-24
The thorny issue of relating information theory to cosmology is here addressed by assuming a possible connection between quantum entanglement measures and observable universe. In particular, we propose a cosmological toy model, where the equation of state of the cosmological fluid, which drives the today observed cosmic acceleration, can be inferred from quantum entanglement between different cosmological epochs. In such a way the dynamical dark energy results as byproduct of quantum entanglement.
Dynamics of dark energy with a coupling to dark matter
Christian G. Boehmer; Gabriela Caldera-Cabral; Ruth Lazkoz; Roy Maartens
2008-01-23
Dark energy and dark matter are the dominant sources in the evolution of the late universe. They are currently only indirectly detected via their gravitational effects, and there could be a coupling between them without violating observational constraints. We investigate the background dynamics when dark energy is modelled as exponential quintessence, and is coupled to dark matter via simple models of energy exchange. We introduce a new form of dark sector coupling, which leads to a more complicated dynamical phase space and has a better physical motivation than previous mathematically similar couplings.
Dark Energy vs. Dark Matter: Towards a Unifying Scalar Field?
A. Arbey
2008-12-18
The standard model of cosmology suggests the existence of two components, "dark matter" and "dark energy", which determine the fate of the Universe. Their nature is still under investigation, and no direct proof of their existences has emerged yet. There exist alternative models which reinterpret the cosmological observations, for example by replacing the dark energy/dark matter hypothesis by the existence of a unique dark component, the dark fluid, which is able to mimic the behaviour of both components. After a quick review of the cosmological constraints on this unifying dark fluid, we will present a model of dark fluid based on a complex scalar field and discuss the problem of the choice of the potential.
Structure formation in inhomogeneous Early Dark Energy models
Batista, R.C. [Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte, Caixa Postal 1524, 59072-970, Natal, Rio Grande do Norte (Brazil); Pace, F., E-mail: rbatista@ect.ufrn.br, E-mail: francesco.pace@port.ac.uk [Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth, PO1 3FX (United Kingdom)
2013-06-01
We study the impact of Early Dark Energy fluctuations in the linear and non-linear regimes of structure formation. In these models the energy density of dark energy is non-negligible at high redshifts and the fluctuations in the dark energy component can have the same order of magnitude of dark matter fluctuations. Since two basic approximations usually taken in the standard scenario of quintessence models, that both dark energy density during the matter dominated period and dark energy fluctuations on small scales are negligible, are not valid in such models, we first study approximate analytical solutions for dark matter and dark energy perturbations in the linear regime. This study is helpful to find consistent initial conditions for the system of equations and to analytically understand the effects of Early Dark Energy and its fluctuations, which are also verified numerically. In the linear regime we compute the matter growth and variation of the gravitational potential associated with the Integrated Sachs-Wolf effect, showing that these observables present important modifications due to Early Dark Energy fluctuations, though making them more similar to the ?CDM model. We also make use of the Spherical Collapse model to study the influence of Early Dark Energy fluctuations in the nonlinear regime of structure formation, especially on ?{sub c} parameter, and their contribution to the halo mass, which we show can be of the order of 10%. We finally compute how the number density of halos is modified in comparison to the ?CDM model and address the problem of how to correct the mass function in order to take into account the contribution of clustered dark energy. We conclude that the inhomogeneous Early Dark Energy models are more similar to the ?CDM model than its homogeneous counterparts.
Dark Matter, Dark Energy and the Chaplygin Gas
Neven Bilic; Gary B. Tupper; Raoul D. Viollier
2002-01-01
We formulate a Zel'dovich-like approximation for the Chaplygin gas equation\\u000aof state P = -A\\/rho, and sketch how this model unifies dark matter with dark\\u000aenergy in a geometric setting reminiscent of M-theory.
On the determination of dark energy
Clarkson, Chris [Cosmology and Gravity Group, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch 7701 (South Africa)
2010-06-23
I consider some of the issues we face in trying to understand dark energy. Huge fluctuations in the unknown dark energy equation of state can be hidden in distance data, so I argue that model-independent tests which signal if the cosmological constant is wrong are valuable. These can be constructed to remove degeneracies with the cosmological parameters. Gravitational effects can play an important role. Even small inhomogeneity clouds our ability to say something definite about dark energy. I discuss how the averaging problem confuses our potential understanding of dark energy by considering the backreaction from density perturbations to second-order in the concordance model: this effect leads to at least a 10% increase in the dynamical value of the deceleration parameter, and could be significantly higher. Large Hubble-scale inhomogeneity has not been investigated in detail, and could conceivably be the cause of apparent cosmic acceleration. I discuss void models which defy the Copernican principle in our Hubble patch, and describe how we can potentially rule out these models.This article is a summary of two talks given at the Invisible Universe Conference, Paris, 2009.
Dynamical dark energy: Current constraints and forecasts
NASA Astrophysics Data System (ADS)
Upadhye, Amol; Ishak, Mustapha; Steinhardt, Paul J.
2005-09-01
We consider how well the dark energy equation of state w as a function of redshift z will be measured using current and anticipated experiments. We use a procedure which takes fair account of the uncertainties in the functional dependence of w on z, as well as the parameter degeneracies, and avoids the use of strong prior constraints. We apply the procedure to current data from the Wilkinson Microwave Anisotropy Probe, Sloan Digital Sky Survey, and the supernova searches, and obtain results that are consistent with other analyses using different combinations of data sets. The effects of systematic experimental errors and variations in the analysis technique are discussed. Next, we use the same procedure to forecast the dark energy constraints achievable by the end of the decade, assuming 8 years of Wilkinson Microwave Anisotropy Probe data and realistic projections for ground-based measurements of supernovae and weak lensing. We find the 2? constraints on the current value of w to be ?w0(2?)=0.20, and on dw/dz (between z=0 and z=1) to be ?w1(2?)=0.37. Finally, we compare these limits to other projections in the literature. Most show only a modest improvement; others show a more substantial improvement, but there are serious concerns about systematics. The remaining uncertainty still allows a significant span of competing dark energy models. Most likely, new kinds of measurements, or experiments more sophisticated than those currently planned, are needed to reveal the true nature of dark energy.
Cluster Survey Studies of the Dark Energy
Joseph J Mohr
2004-08-25
Galaxy cluster surveys are power tools for studying the dark energy. In principle, the equation of state parameter w of the dark energy and its time evolution can be extracted from large solid angle, high yield surveys that deliver tens of thousands of clusters. Robust constraints require accurate knowledge of the survey selection, and crude cluster redshift estimates must be available. A simple survey observable like the cluster flux is connected to the underlying cluster halo mass through a so--called mass--observable relation. The calibration of this mass--observable relation and its redshift evolution is a key challenge in extracting precise cosmological constraints. Cluster survey self--calibration is a technique for meeting this challenge, and it can be applied to large solid angle surveys. In essence, the cluster redshift distribution, the cluster power spectrum, and a limited number of mass measurements can be brought together to calibrate the survey and study the dark energy simultaneously. Additional survey information like the shape of the mass function and its evolution with redshift can then be used to test the robustness of the dark energy constraints.
QCD modified ghost scalar field dark energy models
K. Karami; S. Asadzadeh; M. Mousivand; Z. Safari
2013-04-28
Within the framework of FRW cosmology, we study the QCD modified ghost scalar field models of dark energy in the presence of both interaction and viscosity. For a spatially non-flat FRW universe containing modified ghost dark energy (MGDE) and dark matter, we obtain the equation of state of MGDE, the deceleration parameter as well as a differential equation governing the MGDE density parameter. We also investigate the growth of structure formation for our model in a linear perturbation regime. Furthermore, we reconstruct both the dynamics and potentials of the quintessence, tachyon, K-essence and dilaton scalar field DE models according to the evolution of the MGDE density.
Complementarity of future dark energy probes
NASA Astrophysics Data System (ADS)
Tang, Jiayu; Abdalla, Filipe B.; Weller, Jochen
2011-09-01
In recent years a plethora of future surveys has been suggested to constrain the nature of dark energy. In this paper we adapt a binning approach to the equation of state factor 'w' and discuss how future weak lensing, galaxy cluster counts, supernovae and baryon acoustic oscillation surveys constrain the equation of state at different redshifts. We analyse a few representative future surveys, namely Dark Energy Survey (DES), Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1), Wide Field MultiObject Spectrograph (WFMOS), Pan-STARRS4, EUCLID, SuperNovae Acceleration Probe (SNAP) and Square Kilometer Array (SKA), and perform a principal component analysis for the 'w' bins. We also employ a prior from Planck cosmic microwave background measurements on the remaining cosmological parameters. We study at which redshifts a particular survey constrains the equation of state best and how many principal components are significantly determined. We then point out which surveys would be sufficiently complementary. We find that weak lensing surveys, like EUCLID, would constrain the equation of state best and would be able to constrain the order of three significant modes. Baryon acoustic oscillation surveys on the other hand provide a unique opportunity to probe the equation of state at relatively high redshifts.
Cosmology with Coupled Gravity and Dark Energy
Ti-Pei Li
2015-01-13
Dark energy is a fundamental constituent of our universe, its status in the cosmological field equation should be equivalent to that of gravity. Here we construct a dark energy and matter gravity coupling (DEMC) model of cosmology in a way that dark energy and gravity are introduced into the cosmological field equation in parallel with each other from the beginning. The DEMC universe possesses a composite symmetry from global Galileo invariance and local Lorentz invariance. The observed evolution of the universe expansion rate at redshift z>1 is in tension with the standard LCDM model, but can be well predicted by the DEMC model from measurements of only nearby epochs. The so far most precise measured expansion rate at high z is quite a bit slower than the expectations from LCDM, but remarkably consistent with that from DEMC. It is hoped that the DEMC scenario can also help to solve other existing challenges to cosmology: large scale anomalies in CMB maps and large structures up to about 10^3 Mpc of a quasar group. The DEMC universe is a well defined mechanical system. From measurements we can quantitatively evaluate its total rest energy, present absolute radius and expanding speed.
Dark energy interacting with neutrinos and dark matter: a phenomenological theory
G. M. Kremer
2007-04-03
A model for a flat homogeneous and isotropic Universe composed of dark energy, dark matter, neutrinos, radiation and baryons is analyzed. The fields of dark matter and neutrinos are supposed to interact with the dark energy. The dark energy is considered to obey either the van der Waals or the Chaplygin equations of state. The ratio between the pressure and the energy density of the neutrinos varies with the red-shift simulating massive and non-relativistic neutrinos at small red-shifts and non-massive relativistic neutrinos at high red-shifts. The model can reproduce the expected red-shift behaviors of the deceleration parameter and of the density parameters of each constituent.
The Hubble constant and dark energy from cosmological distance measures
Ichikawa, Kazuhide; Takahashi, Tomo E-mail: tomot@cc.saga-u.ac.jp
2008-04-15
We study how the determination of the Hubble constant from cosmological distance measures is affected by models of dark energy and vice versa. For this purpose, constraints on the Hubble constant and dark energy are investigated using the cosmological observations of cosmic microwave background, baryon acoustic oscillations and type Ia supernovae. When one investigates dark energy, the Hubble constant is often a nuisance parameter; thus it is usually marginalized over. On the other hand, when one focuses on the Hubble constant, simple dark energy models such as a cosmological constant and a constant equation of state are usually assumed. Since we do not know the nature of dark energy yet, it is interesting to investigate the Hubble constant assuming some types of dark energy and see to what extent the constraint on the Hubble constant is affected by the assumption concerning dark energy. We show that the constraint on the Hubble constant is not affected much by the assumption for dark energy. We furthermore show that this holds true even if we remove the assumption that the universe is flat. We also discuss how the prior on the Hubble constant affects the constraints on dark energy and/or the curvature of the universe.
The Hubble constant and dark energy from cosmological distance measures
Kazuhide Ichikawa; Tomo Takahashi
2008-05-03
We study how the determination of the Hubble constant from cosmological distance measures is affected by models of dark energy and vice versa. For this purpose, constraints on the Hubble constant and dark energy are investigated using the cosmological observations of cosmic microwave background, baryon acoustic oscillations and type Ia suprenovae. When one investigates dark energy, the Hubble constant is often a nuisance parameter, thus it is usually marginalized over. On the other hand, when one focuses on the Hubble constant, simple dark energy models such as a cosmological constant and a constant equation of state are usually assumed. Since we do not know the nature of dark energy yet, it is interesting to investigate the Hubble constant assuming some types of dark energy and see to what extent the constraint on the Hubble constant is affected by the assumption concerning dark energy. We show that the constraint on the Hubble constant is not affected much by the assumption for dark energy. We furthermore show that this holds true even if we remove the assumption that the universe is flat. We also discuss how the prior on the Hubble constant affects the constraints on dark energy and/or the curvature of the universe.
Gravitation and regular Universe without dark energy and dark matter
A. V. Minkevich
2011-02-03
It is shown that isotropic cosmology in the Riemann-Cartan spacetime allows to solve the problem of cosmological singularity as well as the problems of invisible matter components - dark energy and dark matter. All cosmological models filled with usual gravitating matter satisfying energy dominance conditions are regular with respect to energy density, spacetime metrics and the Hubble parameter. At asymptotics cosmological solutions of spatially flat models describe accelerating Universe without dark energy and dark matter, and quantitatively their behaviour is identical to that of standard cosmological \\Lambda CDM-model.
The integrated Sachs-Wolfe effect in cosmologies with coupled dark matter and dark energy
Bjoern Malte Schaefer
2008-03-14
The subject of this paper is the derivation of the integrated Sachs-Wolfe (iSW) effect in cosmologies with coupled dark matter and dark energy fluids. These couplings influence the iSW-effect in three ways: The Hubble function assumes a different scaling, the structure growth rate shows a different time evolution, and in addition, the Poisson equation, which relates the density perturbations to fluctuations in the gravitational potential, is changed, due to the violation of the scaling rho ~ a^{-3} of the matter density rho with scale factor a. Exemplarily, I derive the iSW-spectra for a model in which dark matter decays into dark energy, investigate the influence of the dark matter decay rate and the dark energy equation of state on the iSW-signal, and discuss the analogies for gravitational lensing. Quite generally iSW-measurements should reach similar accuracy in determining the dark energy equation of state parameter and the coupling constant.
Binder, Gary A.; /Caltech /SLAC
2010-08-25
In order to make accurate measurements of dark energy, a system is needed to monitor the focus and alignment of the Dark Energy Camera (DECam) to be located on the Blanco 4m Telescope for the upcoming Dark Energy Survey. One new approach under development is to fit out-of-focus star images to a point spread function from which information about the focus and tilt of the camera can be obtained. As a first test of a new algorithm using this idea, simulated star images produced from a model of DECam in the optics software Zemax were fitted. Then, real images from the Mosaic II imager currently installed on the Blanco telescope were used to investigate the algorithm's capabilities. A number of problems with the algorithm were found, and more work is needed to understand its limitations and improve its capabilities so it can reliably predict camera alignment and focus.
Unified dark energy-dark matter model with inverse quintessence
Ansoldi, Stefano; Guendelman, Eduardo I. E-mail: guendel@bgu.ac.il
2013-05-01
We consider a model where both dark energy and dark matter originate from the coupling of a scalar field with a non-canonical kinetic term to, both, a metric measure and a non-metric measure. An interacting dark energy/dark matter scenario can be obtained by introducing an additional scalar that can produce non constant vacuum energy and associated variations in dark matter. The phenomenology is most interesting when the kinetic term of the additional scalar field is ghost-type, since in this case the dark energy vanishes in the early universe and then grows with time. This constitutes an ''inverse quintessence scenario'', where the universe starts from a zero vacuum energy density state, instead of approaching it in the future.
Unified Dark Energy-Dark Matter model with Inverse Quintessence
Stefano Ansoldi; Eduardo I. Guendelman
2013-01-24
We consider a model where both dark energy and dark matter originate from the coupling of a scalar field with a non-conventional kinetic term to, both, a metric measure and a non-metric measure. An interacting dark energy/dark matter scenario can be obtained by introducing an additional scalar that can produce non constant vacuum energy and associated variations in dark matter. The phenomenology is most interesting when the kinetic term of the additional scalar field is ghost-type, since in this case the dark energy vanishes in the early universe and then grows with time. This constitutes an "inverse quintessence scenario", where the universe starts from a zero vacuum energy density state, instead of approaching it in the future.
Ni, G
2005-01-01
In order to clarify why the zero-point energy associated with the vacuum fluctuations cannot be a candidate for the dark energy in the universe, a comparison with the Casimir effect is analyzed in some detail. A principle of epistemology is stressed that it is meaningless to talk about an absolute (isolated) thing. A relative thing can only be observed when it is changing with respect to other things. Then a new conjecture of antigravity --the repulsive force between matter and antimatter derived from the modified Einstein field equation in general relativity-- is proposed. this is due to the particle-antiparticle symmetry based on a new understanding about the essence of special relativity. Its possible consequences in the theory of cosmology are discussed briefly, including a new explanation for the accelerating universe and gamma-ray-bursts.
Energy Conservation (Bernoulli's Equation)
NSDL National Science Digital Library
Shih, Chiang
Professor Chiang Shih's Thermal-Fluids I course at the FAMU-FSU College of Engineering combines "the traditional thermal disciplines in Thermodynamics, heat transfer and fluid mechanics." This PowerPoint presentation, from Shih's lecture notes, illustrates Benoulli's Equation and energy conservation. Along with mathematical equations, there are also a number of illustrations and examples which show the concepts discussed.
Dark energy rest frame and the CMB dipole
Antonio L. Maroto
2006-09-08
If dark energy can be described as a perfect fluid, then, apart from its equation of state relating energy density and pressure, we should also especify the corresponding rest frame. Since dark energy is typically decoupled from the rest of components of the universe, in principle such a frame could be different from that of matter and radiation. In this work we consider the potential observable effects of the motion of dark energy and the possibility to measure the dark energy velocity relative to matter. In particular we consider the modification of the usual interpretation of the CMB dipole and its implications for the determination of matter bulk flows on very large scales. We also comment on the possible origin of a dark energy flow and its evolution in different models.
Dark Energy from Quantum Matter
Claudio Dappiaggi; Thomas-Paul Hack; Jan Möller; Nicola Pinamonti
2010-07-28
We study the backreaction of free quantum fields on a flat Robertson-Walker spacetime. Apart from renormalization freedom, the vacuum energy receives contributions from both the trace anomaly and the thermal nature of the quantum state. The former represents a dynamical realisation of dark energy, while the latter mimics an effective dark matter component. The semiclassical dynamics yield two classes of asymptotically stable solutions. The first reproduces the concordance model in a suitable regime. The second lacks a classical counterpart, but is in excellent agreement with recent observations.
Cahn, Robert N.; de Putter, Roland; Linder, Eric V.
2008-07-08
Scalar field dark energy evolving from a long radiation- or matter-dominated epoch has characteristic dynamics. While slow-roll approximations are invalid, a well defined field expansion captures the key aspects of the dark energy evolution during much of the matter-dominated epoch. Since this behavior is determined, it is not faithfully represented if priors for dynamical quantities are chosen at random. We demonstrate these features for both thawing and freezing fields, and for some modified gravity models, and unify several special cases in the literature.
w and w' of Scalar Field Models of Dark Energy
Takeshi Chiba
2009-11-06
Important observables to reveal the nature of dark energy are the equation of state $w$ and its time derivative in units of the Hubble time $w'$. Recently, it is shown that the simplest scalar field models of dark energy (quintessence) occupy rather narrow regions in the $w-w'$ plane. We extend the $w-w'$ plane to $wdark energy. We also derive bounds on tracker k-essence.
G. Chapline
2005-04-13
Event horizons and closed time-like curves cannot exist in the real world for the simple reason that they are inconsistent with quantum mechanics. Following ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago, and the speaker it is now possible to describe in some detail what happens physically when one approaches and crosses a region of space-time where classical general relativity predicts there should be an infinite red shift surface. This quantum critical physics provides a new perspective on a variety of enigmatic astrophysical phenomena, including supernovae explosions, gamma ray bursts, positron emission, and dark matter.
The cosmological constant and dark energy
P. J. Peebles; Bharat Ratra
2003-01-01
Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, Lambda; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is
Dark energy and 3-manifold topology
Torsten Asselmeyer-Maluga; Helge Rose
2007-11-21
We show that the differential-geometric description of matter by differential structures of spacetime leads to a unifying model of the three types of energy in the cosmos: matter, dark matter and dark energy. Using this model we are able to calculate the ratio of dark energy to the total energy of the cosmos.
Agegraphic Chaplygin gas model of dark energy
Ahmad Sheykhi
2010-02-07
We establish a connection between the agegraphic models of dark energy and Chaplygin gas energy density in non-flat universe. We reconstruct the potential of the agegraphic scalar field as well as the dynamics of the scalar field according to the evolution of the agegraphic dark energy. We also extend our study to the interacting agegraphic generalized Chaplygin gas dark energy model.
Interacting Ghost Dark Energy in Brans-Dicke Theory
Esmaeil Ebrahimi; Ahmad Sheykhi
2011-06-09
We investigate the QCD ghost model of dark energy in the framework of Brans-Dicke cosmology. First, we study the non-interacting ghost dark energy in a flat Brans-Dicke theory. In this case we obtain the EoS and the deceleration parameters and a differential equation governing the evolution of ghost energy density. Interestingly enough, we find that the EoS parameter of the non-interacting ghost dark energy can cross the phantom line ($w_D=-1$) provided the parameters of the model are chosen suitably. Then, we generalize the study to the interacting ghost dark energy in both flat and non-flat Brans-Dicke framework and find out that the transition of $w_D$ to phantom regime can be more easily achieved for than when resort to the Einstein field equations is made.
Cosmological anisotropy from non-comoving dark matter and dark energy
Harko, Tiberiu; Lobo, Francisco S. N. E-mail: flobo@cii.fc.ul.pt
2013-07-01
We consider a cosmological model in which the two major fluid components of the Universe, dark energy and dark matter, flow with distinct four-velocities. This cosmological configuration is equivalent to a single anisotropic fluid, expanding with a four-velocity that is an appropriate combination of the two fluid four-velocities. The energy density of the single cosmological fluid is larger than the sum of the energy densities of the two perfect fluids, i.e., dark energy and dark matter, respectively, and contains a correction term due to the anisotropy generated by the differences in the four-velocities. Furthermore, the gravitational field equations of the two-fluid anisotropic cosmological model are obtained for a Bianchi type I geometry. By assuming that the non-comoving motion of the dark energy and dark matter induces small perturbations in the homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker type cosmological background, and that the anisotropy parameter is small, the equations of the cosmological perturbations due to the non-comoving nature of the two major components are obtained. The time evolution of the metric perturbations is explicitly obtained for the cases of the exponential and power law background cosmological expansion. The imprints of a non-comoving dark energy - dark matter on the Cosmic Microwave Background and on the luminosity distance are briefly discussed, and the temperature anisotropies and the quadrupole are explicitly obtained in terms of the metric perturbations of the flat background metric. Therefore, if there is a slight difference between the four-velocities of the dark energy and dark matter, the Universe would acquire some anisotropic characteristics, and its geometry will deviate from the standard FLRW one. In fact, the recent Planck results show that the presence of an intrinsic large scale anisotropy in the Universe cannot be excluded a priori, so that the model presented in this work can be considered as a plausible and viable working hypothesis.
Dark Matter, Dark Energy and Fundamental Physics
Michael S. Turner
1999-01-01
More than sixty years ago Zwicky made the case that the great clusters of\\u000agalaxies are held together by the gravitational force of unseen (dark) matter.\\u000aToday, the case is stronger and more precise: Dark, nonbaryonic matter accounts\\u000afor 30% +\\/- 7% of the critical mass density, with baryons (most of which are\\u000adark) contributing only 4.5% +\\/- 0.5% of
Validity of Thermodynamical Laws in Dark Energy Filled Universe
NASA Astrophysics Data System (ADS)
Bhattacharya, Samarpita; Debnath, Ujjal
2011-02-01
We have considered the flat FRW model of the universe which is filled with only dark energy. The general descriptions of first and second laws of thermodynamics are investigated on the apparent horizon and event horizon of the universe. We have assumed the equation of state of three different types of dark energy models. We have examined the validity of first and second laws of thermodynamics on apparent and event horizons for these dark energies. For these dark energy models, it has been found that on the apparent horizon, first and second laws are always valid. On the event horizon, the laws are break down for dark energy models 1 and 2. For model 3, first law cannot be satisfied on the event horizon, but second law may be satisfied at the late stage of the evolution of the universe and so the validity of second law on the event horizon depends on the values of the parameters only.
Dark energy as a kinematic effect
Jennen, H
2015-01-01
We present a generalization of teleparallel gravity that is consistent with local spacetime kinematics regulated by the de Sitter group $SO(1,4)$. The mathematical structure of teleparallel gravity is shown to be given by a nonlinear Riemann--Cartan geometry without curvature, which inspires us to build the generalization on top of a de Sitter--Cartan geometry with a cosmological function. The cosmological function is given its own dynamics and naturally emerges nonminimally coupled to the gravitational field in a manner akin to teleparallel dark energy models or scalar-tensor theories in general relativity. New in the theory here presented, the cosmological function gives rise to a kinematic contribution in the deviation equation for the world lines of adjacent free-falling particles. While having its own dynamics, dark energy manifests itself in the local kinematics of spacetime.
Dark energy as a kinematic effect
H. Jennen; J. G. Pereira
2015-06-05
We present a generalization of teleparallel gravity that is consistent with local spacetime kinematics regulated by the de Sitter group $SO(1,4)$. The mathematical structure of teleparallel gravity is shown to be given by a nonlinear Riemann--Cartan geometry without curvature, which inspires us to build the generalization on top of a de Sitter--Cartan geometry with a cosmological function. The cosmological function is given its own dynamics and naturally emerges nonminimally coupled to the gravitational field in a manner akin to teleparallel dark energy models or scalar-tensor theories in general relativity. New in the theory here presented, the cosmological function gives rise to a kinematic contribution in the deviation equation for the world lines of adjacent free-falling particles. While having its own dynamics, dark energy manifests itself in the local kinematics of spacetime.
Simple implementation of general dark energy models
Bloomfield, Jolyon K.; Pearson, Jonathan A. E-mail: jonathan.pearson@durham.ac.uk
2014-03-01
We present a formalism for the numerical implementation of general theories of dark energy, combining the computational simplicity of the equation of state for perturbations approach with the generality of the effective field theory approach. An effective fluid description is employed, based on a general action describing single-scalar field models. The formalism is developed from first principles, and constructed keeping the goal of a simple implementation into CAMB in mind. Benefits of this approach include its straightforward implementation, the generality of the underlying theory, the fact that the evolved variables are physical quantities, and that model-independent phenomenological descriptions may be straightforwardly investigated. We hope this formulation will provide a powerful tool for the comparison of theoretical models of dark energy with observational data.
Scale Dependence of Dark Energy Antigravity
L. Perivolaropoulos
2001-06-25
We investigate the effects of negative pressure induced by dark energy (cosmological constant or quintessence) on the dynamics at various astrophysical scales. Negative pressure induces a repulsive term (antigravity) in Newton's law which dominates on large scales. Assuming a value of the cosmological constant consistent with the recent SnIa data we determine the critical scale $r_c$ beyond which antigravity dominates the dynamics ($r_c \\sim 1Mpc $) and discuss some of the dynamical effects implied. We show that dynamically induced mass estimates on the scale of the Local Group and beyond are significantly modified due to negative pressure. We also briefly discuss possible dynamical tests (eg effects on local Hubble flow) that can be applied on relatively small scales (a few $Mpc$) to determine the density and equation of state of dark energy.
Scale Dependence of Dark Energy Antigravity
NASA Astrophysics Data System (ADS)
Perivolaropoulos, L.
2002-09-01
We investigate the effects of negative pressure induced by dark energy (cosmological constant or quintessence) on the dynamics at various astrophysical scales. Negative pressure induces a repulsive term (antigravity) in Newton's law which dominates on large scales. Assuming a value of the cosmological constant consistent with the recent SnIa data we determine the critical scale $r_c$ beyond which antigravity dominates the dynamics ($r_c \\sim 1Mpc $) and discuss some of the dynamical effects implied. We show that dynamically induced mass estimates on the scale of the Local Group and beyond are significantly modified due to negative pressure. We also briefly discuss possible dynamical tests (eg effects on local Hubble flow) that can be applied on relatively small scales (a few $Mpc$) to determine the density and equation of state of dark energy.
Scale Dependence of Dark Energy Antigravity
Perivolaropoulos, L
2001-01-01
We investigate the effects of negative pressure induced by dark energy (cosmological constant or quintessence) on the dynamics at various astrophysical scales. Negative pressure induces a repulsive term (antigravity) in Newton's law which dominates on large scales. Assuming a value of the cosmological constant consistent with the recent SnIa data we determine the critical scale $r_c$ beyond which antigravity dominates the dynamics ($r_c \\sim 1Mpc $) and discuss some of the dynamical effects implied. We show that dynamically induced mass estimates on the scale of the Local Group and beyond are significantly modified due to negative pressure. We also briefly discuss possible dynamical tests (eg effects on local Hubble flow) that can be applied on relatively small scales (a few $Mpc$) to determine the density and equation of state of dark energy.
Dark Energy: Is It of Torsion Origin?
M. I. Wanas
2010-06-10
{\\it "Dark Energy"} is a term recently used to interpret supernovae type Ia observation. In the present work we give two arguments on a possible relation between dark energy and torsion of space-time.
Pavón, Diego
2013-01-01
It is argued that dark energy -or something dynamically equivalent at the background level- is necessary if the expanding universe is to behave as an ordinary macroscopic system; that is, if it is to tend to some thermodynamic equilibrium state in the long run.
Diego Pavón; Ninfa Radicella
2012-12-31
It is argued that dark energy -or something dynamically equivalent at the background level- is necessary if the expanding universe is to behave as an ordinary macroscopic system; that is, if it is to tend to some thermodynamic equilibrium state in the long run.
Cosmological dark energy and entanglement
Ghosh, Sanjay K.; Raha, Sibaji
2011-09-23
Astrophysical observations of recent years indicate that the universe is composed of a large amount of dark energy (DE) responsible for an accelerated expansion of the universe. Here, we argue that DE can arise entirely from the standard principles of strong interaction physics and quantum entanglement. Good quantitative agreement of our estimate with the observation, without the need for any adjustable parameters, is encouraging.
Vector Fields Resembling Dark Energy
NASA Astrophysics Data System (ADS)
Bretón, Nora
We review how vector fields have been introduced to produce inflationary scenarios in early universes and recently they have been invoked to mimick dark energy. These last approaches have been mostly qualitatives, requiring then to be tested with cosmological probes, in order to seriously be considered as one of the possible causes of the present accelerated expansion of the universe.
Interacting agegraphic tachyon model of dark energy
A. Sheykhi
2009-11-16
Scalar-field dark energy models like tachyon are often regarded as an effective description of an underlying theory of dark energy. In this Letter, we implement the interacting agegraphic dark energy models with tachyon field. We demonstrate that the interacting agegraphic evolution of the universe can be described completely by a single tachyon scalar field. We thus reconstruct the potential as well as the dynamics of the tachyon field according to the evolutionary behavior of interacting agegraphic dark energy.
Holographic dark energy from minimal supergravity
Landim, Ricardo C G
2015-01-01
We embed models of holographic dark energy coupled to dark matter in minimal supergravity plus matter, with one chiral superfield. We analyze two cases. The first one has the Hubble radius as the infrared cutoff and the interaction between the two fluids is proportional to the energy density of the dark energy. The second case has the future event horizon as infrared cutoff while the interaction is proportional to the energy density of both components of the dark sector.
Fitting Type Ia supernovae with coupled dark energy
Luca Amendola; Maurizio Gasperini; Federico Piazza
2004-09-13
We discuss the possible consistency of the recently discovered Type Ia supernovae at z>1 with models in which dark energy is strongly coupled to a significant fraction of dark matter, and in which an (asymptotic) accelerated phase exists where dark matter and dark energy scale in the same way. Such a coupling has been suggested for a possible solution of the coincidence problem, and is also motivated by string cosmology models of "late time" dilaton interactions. Our analysis shows that, for coupled dark energy models, the recent data are still consistent with acceleration starting as early as at $z=3$ (to within 90% c.l.), although at the price of a large "non-universality" of the dark energy coupling to different matter fields. Also, as opposed to uncoupled models which seem to prefer a ``phantom'' dark energy, we find that a large amount of coupled dark matter is compatible with present data only if the dark energy field has a conventional equation of state w>-1.
Dark Matter Equation of Motion and Density Profiles
Antonio R. Mondragon; Roland E. Allen
2001-06-17
Cold dark matter simulations appear to disagree with the observations on small distance scales. Here we consider a modified description of CDM particles which is implied by a new fundamental theory: In this picture, the dark matter is composed of supersymmetric WIMPs, but they are scalar bosons with an unconventional equation of motion. The modified dynamics leads to much weaker gravitational binding, and therefore to a reduced tendency to form small clumps and central cusps.
Dark energy and dark matter as curvature effects
S. Capozziello; V. F. Cardone; A. Troisi
2006-03-20
Astrophysical observations are pointing out huge amounts of dark matter and dark energy needed to explain the observed large scale structures and cosmic accelerating expansion. Up to now, no experimental evidence has been found, at fundamental level, to explain such mysterious components. The problem could be completely reversed considering dark matter and dark energy as shortcomings of General Relativity and claiming for the correct theory of gravity as that derived by matching the largest number of observational data. As a result, accelerating behavior of cosmic fluid and rotation curves of spiral galaxies are reproduced by means of curvature effects.
A two measure model of dark energy and dark matter
Guendelman, Eduardo; Yongram, Nattapong
2012-01-01
In this work we consider a gravitating scalar field, $\\phi$ with a non-conventional kinetic term as in the string theory tachyon, an arbitrary potential, $V(\\phi)$, and two measures -- a geometric measure ($\\sqrt{-g}$) and a non-metric measure ($\\Phi$). This model gives a unified picture of dark energy and dark matter. The model has two interesting features: (i) For potentials which are unstable and would give rise to tachyonic scalar field, this model can stabilize the scalar field. (ii) The form of the dark energy and dark matter that results from this model is fairly insensitive to the exact form of the scalar field potential.
On the Ricci dark energy model
Kyoung Yee Kim; Hyung Won Lee; Yun Soo Myung
2008-12-22
We study the Ricci dark energy model (RDE) which was introduced as an alternative to the holographic dark energy model. We point out that an accelerating phase of the RDE is that of a constant dark energy model. This implies that the RDE may not be a new model of explaining the present accelerating universe.
Power Spectra Tests of the Dark Energy
Hu, Wayne
Power Spectra Tests of the Dark Energy Wayne Hu NASA/Goddard Space Flight Center k (Mpc1) 0.05 2 4 6 8 0.1 Power Growth Distance #12;Outline Power spectrum tests of the dark energy in light · Marcos Lima #12;Dark Energy Sensitivity · Growth: G=(growth rate)/a; normalized to high z · Distances: D
Cosmic structure growth and dark energy
Eric V. Linder; Adrian Jenkins
2003-01-01
Dark energy has a dramatic effect on the dynamics of the Universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behaviour of the growth of large-scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time-varying
Unravelling the Dark Matter - Dark Energy Paradigm
Reginald T. Cahill
2009-01-01
The standard LambdaCDM model of cosmology is usually understood to arise from\\u000ademanding that the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric satisfy\\u000athe General Relativity dynamics for spacetime metrics. The FLRW data-based\\u000adominant parameter values, Omega_Lambda=0.73 and Omega_m=0.27 for the dark\\u000aenergy and dark matter+matter, respectively, are then determined by fitting the\\u000asupernova red-shift data. However in the pressure-less flat-space case the\\u000aLambdaCDM
New holographic dark energy model with non-linear interaction
NASA Astrophysics Data System (ADS)
Oliveros, A.; Acero, Mario A.
2015-05-01
In this paper the cosmological evolution of a holographic dark energy model with a non-linear interaction between the dark energy and dark matter components in a FRW type flat universe is analysed. In this context, the deceleration parameter q and the equation state w ? are obtained. We found that, as the square of the speed of sound remains positive, the model is stable under perturbations since early times; it also shows that the evolution of the matter and dark energy densities are of the same order for a long period of time, avoiding the so-called coincidence problem. We have also made the correspondence of the model with the dark energy densities and pressures for the quintessence and tachyon fields. From this correspondence we have reconstructed the potential of scalar fields and their dynamics.
New holographic dark energy model with non-linear interaction
A. Oliveros; Mario A. Acero
2014-12-23
In this paper the cosmological evolution of a holographic dark energy model with a non-linear interaction between the dark energy and dark matter components in a FRW type flat universe is analysed. In this context, the deceleration parameter $q$ and the equation state $w_{\\Lambda}$ are obtained. We found that, as the square of the speed of sound remains positive, the model is stable under perturbations since early times; it also shows that the evolution of the matter and dark energy densities are of the same order for a long period of time, avoiding the so--called coincidence problem. We have also made the correspondence of the model with the dark energy densities and pressures for the quintessence and tachyon fields. From this correspondence we have reconstructed the potential of scalar fields and their dynamics.
Figures of merit for present and future dark energy probes
Mortonson, Michael J. [Center for Cosmology and AstroParticle Physics, Ohio State University, Columbus, Ohio 43210 (United States); Huterer, Dragan [Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040 (United States); Hu, Wayne [Kavli Institute for Cosmological Physics, Department of Astronomy and Astrophysics, and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States)
2010-09-15
We compare current and forecasted constraints on dynamical dark energy models from Type Ia supernovae and the cosmic microwave background using figures of merit based on the volume of the allowed dark energy parameter space. For a two-parameter dark energy equation of state that varies linearly with the scale factor, and assuming a flat universe, the area of the error ellipse can be reduced by a factor of {approx}10 relative to current constraints by future space-based supernova data and CMB measurements from the Planck satellite. If the dark energy equation of state is described by a more general basis of principal components, the expected improvement in volume-based figures of merit is much greater. While the forecasted precision for any single parameter is only a factor of 2-5 smaller than current uncertainties, the constraints on dark energy models bounded by -1{<=}w{<=}1 improve for approximately 6 independent dark energy parameters resulting in a reduction of the total allowed volume of principal component parameter space by a factor of {approx}100. Typical quintessence models can be adequately described by just 2-3 of these parameters even given the precision of future data, leading to a more modest but still significant improvement. In addition to advances in supernova and CMB data, percent-level measurement of absolute distance and/or the expansion rate is required to ensure that dark energy constraints remain robust to variations in spatial curvature.
Studies of dark energy with X-ray observatories.
Vikhlinin, Alexey
2010-04-20
I review the contribution of Chandra X-ray Observatory to studies of dark energy. There are two broad classes of observable effects of dark energy: evolution of the expansion rate of the Universe, and slow down in the rate of growth of cosmic structures. Chandra has detected and measured both of these effects through observations of galaxy clusters. A combination of the Chandra results with other cosmological datasets leads to 5% constraints on the dark energy equation-of-state parameter, and limits possible deviations of gravity on large scales from general relativity. PMID:20404207
Fine-structure constant constraints on dark energy
Martins, C J A P
2015-01-01
We use astrophysical and atomic clock tests of the stability of the fine-structure constant $\\alpha$, together with Type Ia supernova and Hubble parameter data, to constrain the simplest class of dynamical dark energy models where the same degree of freedom is assumed to provide both the dark energy and (through a dimensionless coupling, $\\zeta$, to the electromagnetic sector) the $\\alpha$ variation. We show how current data tightly constrains a combination of $\\zeta$ and the dark energy equation of state $w_0$. At the $95\\%$ confidence level and marginalizing over $w_0$ we find $|\\zeta|stable spectrographs will enable significantly tighter constraints.
Holographic Dark Energy with Generalized Chaplygin Gas in Higher Dimensions
NASA Astrophysics Data System (ADS)
Ghose, S.; Saha, A.; Paul, B. C.
2014-11-01
We investigate holographic dark energy (HDE) correspondence of interacting Generalized Chaplygin Gas (GCG) in the framework of compact Kaluza-Klein (KK) cosmology. The evolution of the modified HDE with corresponding equation of state is obtained here. Considering the present value of the density parameter a stable configuration is found which accommodates Dark Energy (DE). We note a connection between DE and Phantom fields. It reveals that the DE might have evolved from a Phantom state in the past.
Cosmological evolution of a quintom model of dark energy
Zong-Kuan Guo; Yun-Song Piao; Xinmin Zhang; Yuan-Zhong Zhang
2005-01-01
We investigate in this Letter the cosmological evolution of a dark energy model with two scalar fields where one of the scalar has canonical kinetic energy and another scalar has negative kinetic energy term. For such a system with exponential potentials we find that during the evolution of the universe the equation of state w changes from w??1 to w?1,
Eddington-Born-Infeld action for dark matter and dark energy
Máximo Bañados
2008-01-01
We argue that Einstein gravity coupled to a Born-Infeld theory provides an attractive candidate to represent dark matter and dark energy. For cosmological models, the Born-Infeld field has an equation of state which interpolates between matter, p=0 (small times), and a cosmological constant p=-rho (large times). On galactic scales, the Born-Infeld theory predicts asymptotically flat rotation curves.
Dark Energy from Discrete Spacetime
Trout, Aaron D.
2013-01-01
Dark energy accounts for most of the matter-energy content of our universe, yet current theories of its origin rely on radical physical assumptions such as the holographic principle or controversial anthropic arguments. We give a better motivated explanation for dark energy, claiming that it arises from a small negative scalar-curvature present even in empty spacetime. The vacuum has this curvature because spacetime is fundamentally discrete and there are more ways for a discrete geometry to have negative curvature than positive. We explicitly compute this effect using a variant of the well known dynamical-triangulations (DT) model for quantum gravity. Our model predicts a time-varying non-zero cosmological constant with a current value, in natural units, in agreement with observation. This calculation is made possible by a novel characterization of the possible DT action values combined with numerical evidence concerning their degeneracies. PMID:24312502
Dark energy from discrete spacetime.
Trout, Aaron D
2013-01-01
Dark energy accounts for most of the matter-energy content of our universe, yet current theories of its origin rely on radical physical assumptions such as the holographic principle or controversial anthropic arguments. We give a better motivated explanation for dark energy, claiming that it arises from a small negative scalar-curvature present even in empty spacetime. The vacuum has this curvature because spacetime is fundamentally discrete and there are more ways for a discrete geometry to have negative curvature than positive. We explicitly compute this effect using a variant of the well known dynamical-triangulations (DT) model for quantum gravity. Our model predicts a time-varying non-zero cosmological constant with a current value, [Formula: see text] in natural units, in agreement with observation. This calculation is made possible by a novel characterization of the possible DT action values combined with numerical evidence concerning their degeneracies. PMID:24312502
From confinement to dark energy
B. Holdom
2011-02-24
The infrared divergence of the self-energy of a color charge is due to an enhancement of the long wavelength modes of the color Coulomb potential field. There are also long wavelength contributions to the QCD vacuum energy that are similarly enhanced. Vacuum modes of Hubble scale wavelengths may be affected in a cosmological setting and this can lead to a residual positive energy density of the form $H^d\\Lambda_{\\rm QCD}^{4-d}$. Lattice studies constrain $d$. If the dark energy takes this form then the universe is driven towards de Sitter expansion, and we briefly study this cosmology when $d$ is just slightly above unity.
Dark energy and the quietness of the local Hubble flow
NASA Astrophysics Data System (ADS)
Axenides, M.; Perivolaropoulos, L.
2002-06-01
The linearity and quietness of the local (<10 Mpc) Hubble flow (LHF) in view of the very clumpy local universe is a long standing puzzle in standard and in open CDM (cold dark matter) cosmogony. The question addressed in this paper is whether the antigravity component of the recently discovered dark energy can cool the velocity flow enough to provide a solution to this puzzle. We calculate the growth of matter fluctuations in a flat universe containing a fraction ?X(t0) of dark energy obeying the time independent equation of state pX=w?X. We find that dark energy can indeed cool the LHF. However the dark energy parameter values required to make the predicted velocity dispersion consistent with the observed value vrms~=40 km/s have been ruled out by other observational tests constraining the dark energy parameters w and ?X. Therefore despite the claims of recent qualitative studies, dark energy with time independent equation of state cannot by itself explain the quietness and linearity of the local Hubble flow.
Reconstructing Quintom from Ricci Dark Energy
Chao-Jun Feng
2009-02-11
The holographic dark energy with Ricci scalar as IR cutoff called Ricci dark energy(RDE) probes the nature of dark energy with respect to the holographic principle of quantum gravity theory. The scalar field dark energy models like quintom are often viewed as effective description of the underlying field theory of dark energy. In this letter, we assume RDE model as the underlying field theory to find how the generalized ghost condensate model(GGC) that can easily realize quintom behavior can be used to effectively describe it and reconstruct the function $h(\\phi)$ of the generalized ghost condensate model.
Jackson, Brendan Marc
2011-11-23
One of the most pressing, modern cosmological mysteries is the cause of the accelerated expansion of the universe. The energy density required to cause this large scale opposition to gravity is known to be both far in ...
A Modified Generalized Chaplygin Gas as the Unified Dark Matter-Dark Energy Revisited
Xue-Mei Deng
2011-10-10
A modified generalized Chaplygin gas (MGCG) is considered as the unified dark matter-dark energy revisited. The character of MGCG is endued with the dual role, which behaves as matter at early times and as an quiessence dark energy at late times. The equation of state for MGCG is $p=-\\alpha\\rho/(1+\\alpha)-\\vartheta(z)\\rho^{-\\alpha}/(1+\\alpha) $, where $\\vartheta(z)=-[\\rho_{0c}(1+z)^{3}]^{(1+\\alpha)}(1-\\Omega_{0B})^{\\alpha}\\{\\alpha\\Omega_{0DM}+ \\Omega_{0DE}[\\omega_{DE}+\\alpha(1+\\omega_{DE})](1+z)^{3\\omega_{DE}(1+\\alpha)}\\}$. Some cosmological quantities, such as the densities of different components of the universe $\\Omega_{i}$ ($i$ respectively denotes baryons, dark matter and dark energy) and the deceleration parameter $q$, are obtained. The present deceleration parameter $q_{0}$, the transition redshift $z_{T}$ and the redshift $z_{eq}$, which describes the epoch when the densities in dark matter and dark energy are equal, are also calculated. To distinguish MGCG from others, we then apply the Statefinder diagnostic. Later on, the parameters ($\\alpha$ and $\\omega_{DE}$) of MGCG are constrained by combination of the sound speed $c^{2}_{s}$, the age of the universe $t_{0}$, the growth factor $m$ and the bias parameter $b$. It yields $\\alpha=-3.07^{+5.66}_{-4.98}\\times10^{-2}$ and $\\omega_{DE}=-1.05^{+0.06}_{-0.11}$. Through the analysis of the growth of density perturbations for MGCG, it is found that the energy will transfer from dark matter to dark energy which reach equal at $z_{eq}\\sim 0.48$ and the density fluctuations start deviating from the linear behavior at $z\\sim 0.25$ caused by the dominance of dark energy.
Dark matter and dark energy: summary and future directions
Jonathan Richard Ellis
2003-01-01
This paper reviews the progress reported at the Discussion Meeting and advertises some possible future directions in our drive to understand dark matter and dark energy. Additionally, a first attempt is made to place in context the exciting new results from the Wilkinson Microwave Anisotropy Probe satellite, which were published shortly after this meeting. In the first part of this
Tachyon dark energy models: Dynamics and constraints
Calcagni, Gianluca; Liddle, Andrew R. [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom)
2006-08-15
We explore the dynamics of dark energy models based on a Dirac-Born-Infeld (DBI) tachyonic action, studying a range of potentials. We numerically investigate the existence of tracking behavior and determine the present-day value of the equation of state parameter and its running, which are compared with observational bounds. We find that tachyon models have quite similar phenomenology to canonical quintessence models. While some potentials can be selected amongst many possibilities and fine-tuned to give viable scenarios, there is no apparent advantage in choosing a DBI scalar field instead of a Klein-Gordon one.
Fingerprinting Dark Energy III: distinctive marks of viscosity
Domenico Sapone; Elisabetta Majerotto
2012-03-09
The characterisation of dark energy is one of the primary goals in cosmology especially now that many new experiments are being planned with the aim of reaching a high sensitivity on cosmological parameters. It is known that if we move away from the simple cosmological constant model then we need to consider perturbations in the dark energy fluid. This means that dark energy has two extra degrees of freedom: the sound speed $\\cs$ and the anisotropic stress $\\sigma$. If dark energy is inhomogenous at the scales of interest then the gravitational potentials are modified and the evolution of the dark matter perturbations is also directly affected. In this paper we add an anisotropic component to the dark energy perturbations. Following the idea introduced in \\cite{Sapone:2009mb}, we solve analytically the equations of perturbations in the dark sector, finding simple and accurate approximated solutions. We also find that the evolution of the density perturbations is governed by an effective sound speed which depends on both the sound speed and the anisotropic stress parameter. We then use these solutions to look at the impact of the dark energy perturbations on the matter power spectrum and on the Integrated Sachs-Wolfe effect in the Cosmic Microwave Background.
Genesis of Dark Energy: Dark Energy as a Consequence of Cosmological Nuclear Energy
R. C. Gupta
2004-12-07
Recent observations on Type-Ia supernovae and low density measurement of matter (including dark matter) suggest that the present day universe consists mainly of repulsive-gravity type exotic-matter with negative-pressure often referred as dark-energy. But the mystery is about the nature of dark-energy and its puzzling questions such as why, how, where & when about the dark- energy are intriguing. In the present paper the author attempts to answer these questions while making an effort to reveal the genesis of dark-energy, and suggests that the cosmological nuclear-binding-energy liberated during primordial nucleo-synthesis remains trapped for long time and then is released free which manifests itself as dark-energy in the universe. It is also explained why for dark energy the parameter w = -2/3. Noting that w=+1for stiff matter and w=+1/3 for radiation; w = - 2/3 is for dark energy, because -1 is due to deficiency of stiff-nuclear-matter and that this binding energy is ultimately released as radiation contributing +1/3, making w = -1 + 1/3 = -2/3. This thus almost solves the dark-energy mystery of negative-pressure & repulsive-gravity. It is concluded that dark-energy is a consequence of released-free nuclear-energy of cosmos. The proposed theory makes several estimates / predictions, which agree reasonably well with the astrophysical constraints & observations.
Designing dark energy afterglow experiments
Amol Upadhye; Jason H. Steffen; Aaron S. Chou
2012-04-24
Chameleon fields, which are scalar field dark energy candidates, can evade fifth force constraints by becoming massive in high-density regions. However, this property allows chameleon particles to be trapped inside a vacuum chamber with dense walls. Afterglow experiments constrain photon-coupled chameleon fields by attempting to produce and trap chameleon particles inside such a vacuum chamber, from which they will emit an afterglow as they regenerate photons. Here we discuss several theoretical and systematic effects underlying the design and analysis of the GammeV and CHASE afterglow experiments. We consider chameleon particle interactions with photons, Fermions, and other chameleon particles, as well as with macroscopic magnetic fields and matter. The afterglow signal in each experiment is predicted, and its sensitivity to various properties of the experimental apparatus is studied. Finally, we use CHASE data to exclude a wide range of photon-coupled chameleon dark energy models.
S. Dodelson; L. Knox
2000-03-24
We find that current Cosmic Microwave Background (CMB) anisotropy data strongly constrain the mean spatial curvature of the Universe to be near zero, or, equivalently, the total energy density to be near critical-as predicted by inflation. This result is robust to editing of data sets, and variation of other cosmological parameters (totaling seven, including a cosmological constant). Other lines of argument indicate that the energy density of non-relativistic matter is much less than critical. Together, these results are evidence, independent of supernovae data, for dark energy in the Universe.
Inferring "Dark Matter" and "Dark Energy" from Videos Dan Xie , Sinisa Todorovic
Zhu, Song Chun
Inferring "Dark Matter" and "Dark Energy" from Videos Dan Xie , Sinisa Todorovic , and Song.g., grass lawns). Therefore, functional objects can be viewed as "dark matter", emanating "dark energy, we regard these unrecognizable functional objects as sources of "dark energy", i.e., "dark matter
The possible nature of dark energy and dark matter Nathalie Olivi-Tran1,2
Paris-Sud XI, Université de
The possible nature of dark energy and dark matter Nathalie Olivi-Tran1,2 1 Laboratoire Charles the nature of time. Keywords: dark matter; dark energy; nature of time 1 hal-00719998,version1-23Jul2012 is dark energy? what is dark matter? A simple analysis regarding the fluctuations of the hyper
Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios T. Jenke,1,*
Rotter, Stefan
Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios T. Jenke,1,* G this work, we link these new measurements to dark matter and dark energy searches. Observational cosmology has determined the dark matter and dark energy density parameters to an accuracy of 2 significant
Dark Energy and Dark Matter from the same Vacuum Condensate
NASA Astrophysics Data System (ADS)
Sarfatti, Jack
2003-04-01
The micro-quantum Dirac negative energy electron Fermi sphere with Planck scale cutoff is unstable to the formation of off-mass-shell Cooper pairs of virtual electrons and positrons from their static Coulomb attraction. The resulting virtual BEC complex macro-quantum coherent local order parameter (0|e+e-|0) gives rise to both spin 2 gravity guv and spin 0 quintessence / from the Goldstone and Higgs oscillations respectively, Susskind's "world hologram" conjecture replaces the Planck scale Lp with Lp^2/3L^1/3 at scale L. Hagen Kleinert's strain tensor for the "world crystal" is Einstein's geometrodynamic field: guv = nuv + Lp^4/3L^2/3Du,Dvarg(0|e+e-|0)/2 nuv = Minkowski metric, = anti-commutator Du = ,u + TaAu^a is the spin 1 gauge covariant derivative for Lie group P with Lie algebra [Ta,Tb] = Cab^cTc / = Lp-4/3L-2/3[1 - Lp^2L|(0|e+e-|0)|^2] When L = size of visible universe 10^28 cm, Lp^2/3L^1/3 1 fermi / > 0 is anti-gravitating zero point vacuum dark energy, i.e. Kip Thorne's "exotic matter" for traversable wormhole time machines. / < 0 is gravitating zero point vacuum dark matter The non-perturbative BCS energy gap equation for a basic vacuum polarization closed loop with one virtual photon Feynman diagram is: z^2 = ge^-(1/gz) z = (Lp/L)^1/3 and the dimensionless coupling vertex is g^1/2 http://stardrive.org/Jack/nambu.pdf http://stardrive.org/Jack/Lambda1.pdf
Inhomogeneous alternative to dark energy?
Alnes, Haavard; Amarzguioui, Morad; Groen, Oyvind
2006-04-15
Recently, there have been suggestions that the apparent accelerated expansion of the universe is not caused by repulsive gravitation due to dark energy, but is rather a result of inhomogeneities in the distribution of matter. In this work, we investigate the behavior of a dust-dominated inhomogeneous Lemaitre-Tolman-Bondi universe model, and confront it with various astrophysical observations. We find that such a model can easily explain the observed luminosity distance-redshift relation of supernovae without the need for dark energy, when the inhomogeneity is in the form of an underdense bubble centered near the observer. With the additional assumption that the universe outside the bubble is approximately described by a homogeneous Einstein-de Sitter model, we find that the position of the first peak in the cosmic microwave background (CMB) power spectrum can be made to match the WMAP observations. Whether or not it is possible to reproduce the entire CMB angular power spectrum in an inhomogeneous model without dark energy is still an open question.
Cuscuton cosmology: Dark energy meets modified gravity
Afshordi, Niayesh; Chung, Daniel J. H.; Geshnizjani, Ghazal; Doran, Michael
2007-06-15
In a companion paper, we have introduced a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a k-essence fluid. In this paper, we study how Cuscuton modifies the constraint sector of Einstein gravity. In particular, we study Cuscuton cosmology and show that even though Cuscuton can have an arbitrary equation of state, or time dependence, and is thus inhomogeneous, its perturbations do not introduce any additional dynamical degree of freedom and only satisfy a constraint equation, amounting to an effective modification of gravity on large scales. Therefore, Cuscuton can be considered to be a minimal theory of evolving dark energy, or a minimal modification of a cosmological constant, as it has no internal dynamics. Moreover, this is the only modification of Einstein gravity to our knowledge, that does not introduce any additional degrees of freedom (and is not conformally equivalent to the Einstein gravity). We then study two simple Cuscuton models, with quadratic and exponential potentials. The quadratic model has the exact same expansion history as {lambda}CDM, and yet contains an early dark energy component with constant energy fraction, which is constrained to {omega}{sub Q} < or approx. 2%, mainly from WMAP Cosmic Microwave Background and Sloan Digital Sky Survey Lyman-{alpha} forest observations. The exponential model has the same expansion history as the Dvali-Gabadadze-Poratti self-accelerating brane-world model, but generates a much smaller Integrated Sachs-Wolfe effect, and is thus consistent with the Cosmic Microwave Background observations. Finally, we show that the evolution is local on superhorizon scales, implying that there is no gross violation of causality, despite Cuscuton's infinite speed of sound.
Cuscuton Cosmology: Dark Energy meets Modified Gravity
Niayesh Afshordi; Daniel J. H. Chung; Michael Doran; Ghazal Geshnizjani
2007-05-11
In a companion paper (hep-th/0609150), we have introduced a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a k-essence fluid. In this paper, we study how Cuscuton modifies the constraint sector of Einstein gravity. In particular, we study Cuscuton cosmology and show that even though Cuscuton can have an arbitrary equation of state, or time dependence, and is thus inhomogeneous; its perturbations do not introduce any additional dynamical degree of freedom and only satisfy a constraint equation, amounting to an effective modification of gravity on large scales. Therefore, Cuscuton can be considered to be a minimal theory of evolving dark energy, or a minimal modification of a cosmological constant, as it has no internal dynamics. Moreover, this is the only modification of Einstein gravity to our knowledge that does not introduce any additional degrees freedom (and is not conformally equivalent to the Einstein gravity). We then study two simple Cuscuton models, with quadratic and exponential potentials. The quadratic model has the exact same expansion history as LCDM, and yet contains an early dark energy component with constant energy fraction, which is constrained to < 2%, mainly from WMAP Cosmic Microwave Background (CMB) and SDSS Lyman-alpha forest observations. The exponential model has the same expansion history as the DGP self-accelerating braneworld model, but generates a much smaller Integrated Sachs-Wolfe (ISW) effect, and is thus consistent with the CMB observations. Finally, we show that the evolution is local on super-horizon scales, implying that there is no gross violation of causality, despite Cuscuton's infinite speed of sound.
Bulk-Brane Interaction and Holographic Dark Energy
M R Setare
2006-12-14
In this paper we consider the bulk-brane interaction to obtain the equation of state for the holographic energy density in non-flat universe enclosed by the event horizon measured from the sphere of horizon named $L$. We assumes that the cold dark matter energy density on the brane is conserved, but the holographic dark energy density on the brane is not conserved due to brane-bulk energy exchange. Our calculation show, taking $\\Omega_{\\Lambda}=0.73$ for the present time, the lower bound of $w_{\\rm \\Lambda}^{eff}$ is -0.9. This implies that one can not generate phantom-like equation of state from an interacting holographic dark energy model in non-flat universe.
Spherical collapse of dark energy with an arbitrary sound speed
Basse, Tobias; Bjælde, Ole Eggers [Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C (Denmark); Wong, Yvonne Y.Y., E-mail: tb06@phys.au.dk, E-mail: oeb@phys.au.dk, E-mail: yvonne.wong@physik.rwth-aachen.de [Institut für Theoretische Teilchenphysik und Kosmologie, RWTH Aachen, D-52056 Aachen (Germany)
2011-10-01
We consider a generic type of dark energy fluid, characterised by a constant equation of state parameter w and sound speed c{sub s}, and investigate the impact of dark energy clustering on cosmic structure formation using the spherical collapse model. Along the way, we also discuss in detail the evolution of dark energy perturbations in the linear regime. We find that the introduction of a finite sound speed into the picture necessarily induces a scale-dependence in the dark energy clustering, which in turn affects the dynamics of the spherical collapse in a scale-dependent way. As with other, more conventional fluids, we can define a Jeans scale for the dark energy clustering, and hence a Jeans mass M{sub J} for the dark matter which feels the effect of dark energy clustering via gravitational interactions. For bound objects (halos) with masses M >> M{sub J}, the effect of dark energy clustering is maximal. For those with M << M{sub J}, the dark energy component is effectively homogeneous, and its role in the formation of these structures is reduced to its effects on the Hubble expansion rate. To compute quantitatively the virial density and the linearly extrapolated threshold density, we use a quasi-linear approach which is expected to be valid up to around the Jeans mass. We find an interesting dependence of these quantities on the halo mass M, given some w and c{sub s}. The dependence is the strongest for masses lying in the vicinity of M ? M{sub J}. Observing this M-dependence will be a tell-tale sign that dark energy is dynamic, and a great leap towards pinning down its clustering properties.
Dark energy and fundamental physics
NASA Astrophysics Data System (ADS)
Binétruy, P.
2013-10-01
The acceleration of the expansion of the Universe which has been identified in recent years has deep connections with some of the most central issues in fundamental physics. At present, the most plausible explanation is some form of vacuum energy. The puzzle of vacuum energy is a central question which lies at the interface between quantum theory and general relativity. Solving it will presumably require to construct a quantum theory of gravity and a correspondingly consistent picture of spacetime. To account for the acceleration of the expansion, one may also think of more dynamical forms of energy, what is known as dark energy, or modifications of gravity. In what follows, we review the vacuum energy problem as well as the basic models for dark energy or modification of gravity. We emphasize the conceptual aspects rather than the techniques involved. We also discuss the difficulties encountered in each approach. This review is intended for astrophysicists or physicists not specialized in particle physics, who are interested in apprehending the issues at stake in fundamental physics.
A new class of parametrization for dark energy without divergence
Feng, Chao-Jun; Shen, Xian-Yong; Li, Ping; Li, Xin-Zhou, E-mail: fengcj@shnu.edu.cn, E-mail: 1000304237@smail.shnu.edu.cn, E-mail: li57120@126.com, E-mail: kychz@shnu.edu.cn [Shanghai United Center for Astrophysics (SUCA), Shanghai Normal University, 100 Guilin Road, Shanghai 200234, P.R.China (China)
2012-09-01
A new class of parametrization of the equation of state of dark energy is proposed in this paper. In contrast with the famous CPL parametrization, the equation of state with this new kind of parametrization does not divergent during the evolution of the Universe even in the future. By using the Markov Chain Monte Carlo (MCMC) method, we perform an observational constraint on two simplest dark energy models belonging to this new class of parametrization with the combined latest observational data from the type Ia supernova compilations including Union2(557), cosmic microwave background, and baryon acoustic oscillation.
Dark energy constraints from galaxy cluster peculiar velocities
Bhattacharya, Suman; Kosowsky, Arthur [Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, Pennsylvania 15260 (United States)
2008-04-15
Future multifrequency microwave background experiments with arcminute resolution and micro-Kelvin temperature sensitivity will be able to detect the kinetic Sunyaev-Zeldovich effect, providing a way to measure radial peculiar velocities of massive galaxy clusters. We show that cluster peculiar velocities have the potential to constrain several dark energy parameters. We compare three velocity statistics (the distribution of radial velocities, the mean pairwise streaming velocity, and the velocity correlation function) and analyze the relative merits of these statistics in constraining dark energy parameters. Of the three statistics, mean pairwise streaming velocity provides constraints that are least sensitive to velocity errors: the constraints on parameters degrade only by a factor of 2 when the random error is increased from 100 to 500 km/s. We also compare cluster velocities with other dark energy probes proposed in the Dark Energy Task Force report. For cluster velocity measurements with realistic priors, the eventual constraints on the dark energy density, the dark energy equation of state and its evolution are comparable to constraints from supernovae measurements, and better than cluster counts and baryon acoustic oscillations; adding velocity to other dark energy probes improves constraints on the figure of merit by more than a factor of 2. For upcoming Sunyaev-Zeldovich galaxy cluster surveys, even velocity measurements with errors as large as 1000 km/s will substantially improve the cosmological constraints compared to using the cluster number density alone.
Constraining dark energy with clusters: Complementarity with other probes
Cunha, Carlos; Huterer, Dragan; Frieman, Joshua A.
2009-09-15
The Figure of Merit Science Working Group recently forecast the constraints on dark energy that will be achieved prior to the Joint Dark Energy Mission by ground-based experiments that exploit baryon acoustic oscillations, type Ia supernovae, and weak gravitational lensing. We show that cluster counts from ongoing and near-future surveys should provide robust, complementary dark energy constraints. In particular, we find that optimally combined optical and Sunyaev-Zel'dovich effect cluster surveys should improve the Dark Energy Task Force figure of merit for pre-Joint Dark Energy Mission projects by a factor of 2 even without prior knowledge of the nuisance parameters in the cluster mass-observable relation. Comparable improvements are achieved in the forecast precision of parameters specifying the principal component description of the dark energy equation of state parameter, as well as in the growth index {gamma}. These results indicate that cluster counts can play an important complementary role in constraining dark energy and modified gravity even if the associated systematic errors are not strongly controlled.
Dark Energy Constraints from Galaxy Cluster Peculiar Velocities
Suman Bhattacharya; Arthur Kosowsky
2008-04-25
Future multifrequency microwave background experiments with arcminute resolution and micro-Kelvin temperature sensitivity will be able to detect the kinetic Sunyaev-Zeldovich (kSZ) effect, providing a way to measure radial peculiar velocities of massive galaxy clusters. We show that cluster peculiar velocities have the potential to constrain several dark energy parameters. We compare three velocity statistics (the distribution of radial velocities, the mean pairwise streaming velocity, and the velocity correlation function) and analyze the relative merits of these statistics in constraining dark energy parameters. Of the three statistics, mean pairwise streaming velocity provides constraints that are least sensitive to velocity errors: the constraints on parameters degrades only by a factor of two when the random error is increased from 100 to 500 km/s. We also compare cluster velocities with other dark energy probes proposed in the Dark Energy Task Force report. For cluster velocity measurements with realistic priors, the eventual constraints on the dark energy density, the dark energy equation of state and its evolution are comparable to constraints from supernovae measurements, and better than cluster counts and baryon acoustic oscillations; adding velocity to other dark energy probes improves constraints on the figure of merit by more than a factor of two. For upcoming Sunyaev-Zeldovich galaxy cluster surveys, even velocity measurements with errors as large as 1000 km/s will substantially improve the cosmological constraints compared to using the cluster number density alone.
Reconstruction of dark energy and expansion dynamics using Gaussian processes
Seikel, Marina; Clarkson, Chris; Smith, Mathew, E-mail: marina.seikel@uct.ac.za, E-mail: chris.clarkson@uct.ac.za, E-mail: mathew.smith@uct.ac.za [Astrophysics, Cosmology and Gravity Centre (ACGC), and Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, 7701 (South Africa)
2012-06-01
An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space, as the errors found depend strongly on the parametrisation considered. We present a new non-parametric approach to reconstructing the history of the expansion rate and dark energy using Gaussian Processes, which is a fully Bayesian approach for smoothing data. We present a pedagogical introduction to Gaussian Processes, and discuss how it can be used to robustly differentiate data in a suitable way. Using this method we show that the Dark Energy Survey - Supernova Survey (DES) can accurately recover a slowly evolving equation of state to ?{sub w} = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimum error of ±0.025 at the sweet-spot at z ? 0.16, provided the other parameters of the model are known. Errors on the expansion history are an order of magnitude smaller, yet make no assumptions about dark energy whatsoever. A code for calculating functions and their first three derivatives using Gaussian processes has been developed and is available for download.
Dark Energy and the quietness of the Local Hubble Flow
M. Axenides; L. Perivolaropoulos
2002-03-28
The linearity and quietness of the Local ($Hubble Flow (LHF) in view of the very clumpy local universe is a long standing puzzle in standard and in open CDM cosmogony. The question addressed in this paper is whether the antigravity component of the recently discovered dark energy can cool the velocity flow enough to provide a solution to this puzzle. We calculate the growth of matter fluctuations in a flat universe containing a fraction $\\Omega_X(t_0)$ of dark energy obeying the time independent equation of state $p_X = w \\rho_X$. We find that dark energy can indeed cool the LHF. However the dark energy parameter values required to make the predicted velocity dispersion consistent with the observed value $v_{rms}\\simeq 40km/sec$ have been ruled out by other observational tests constraining the dark energy parameters $w$ and $\\Omega_X$. Therefore despite the claims of recent qualitative studies dark energy with time independent equation of state can not by itself explain the quietness and linearity of the Local Hubble Flow.
New agegraphic dark energy in Ho?ava-Lifshitz cosmology
Jamil, Mubasher [Center for Advanced Mathematics and Physics, National University of Sciences and Technology, H-12, Islamabad (Pakistan); Saridakis, Emmanuel N., E-mail: mjamil@camp.edu.pk, E-mail: msaridak@phys.uoa.gr [College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing, 400065 (China)
2010-07-01
We investigate the new agegraphic dark energy scenario in a universe governed by Ho?ava-Lifshitz gravity. We consider both the detailed and non-detailed balanced version of the theory, we impose an arbitrary curvature, and we allow for an interaction between the matter and dark energy sectors. Extracting the differential equation for the evolution of the dark energy density parameter and performing an expansion of the dark energy equation-of-state parameter, we calculate its present and its low-redshift value as functions of the dark energy and curvature density parameters at present, of the Ho?ava-Lifshitz running parameter ?, of the new agegraphic dark energy parameter n, and of the interaction coupling b. We find that w{sub 0} = ?0.82{sup +0.08}{sub ?0.08} and w{sub 1} = 0.08{sup +0.09}{sub ?0.07}. Although this analysis indicates that the scenario can be compatible with observations, it does not enlighten the discussion about the possible conceptual and theoretical problems of Ho?ava-Lifshitz gravity.
Reconstruction of dark energy and expansion dynamics using Gaussian processes
NASA Astrophysics Data System (ADS)
Seikel, Marina; Clarkson, Chris; Smith, Mathew
2012-06-01
An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space, as the errors found depend strongly on the parametrisation considered. We present a new non-parametric approach to reconstructing the history of the expansion rate and dark energy using Gaussian Processes, which is a fully Bayesian approach for smoothing data. We present a pedagogical introduction to Gaussian Processes, and discuss how it can be used to robustly differentiate data in a suitable way. Using this method we show that the Dark Energy Survey - Supernova Survey (DES) can accurately recover a slowly evolving equation of state to ?w = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimum error of ±0.025 at the sweet-spot at z ~ 0.16, provided the other parameters of the model are known. Errors on the expansion history are an order of magnitude smaller, yet make no assumptions about dark energy whatsoever. A code for calculating functions and their first three derivatives using Gaussian processes has been developed and is available for download.
A Kinematical Approach to Dark Energy Studies
Rapetti, David; Allen, Steven W.; Amin, Mustafa A.; Blandford, Roger D.; /KIPAC, Menlo Park
2006-06-06
We present and employ a new kinematical approach to cosmological ''dark energy'' studies. We construct models in terms of the dimensionless second and third derivatives of the scale factor a(t) with respect to cosmic time t, namely the present-day value of the deceleration parameter q{sub 0} and the cosmic jerk parameter, j(t). An elegant feature of this parameterization is that all {Lambda}CDM models have j(t) = 1 (constant), which facilitates simple tests for departures from the {Lambda}CDM paradigm. Applying our model to the three best available sets of redshift-independent distance measurements, from type Ia supernovae and X-ray cluster gas mass fraction measurements, we obtain clear statistical evidence for a late time transition from a decelerating to an accelerating phase. For a flat model with constant jerk, j(t) = j, we measure q{sub 0} = -0.81 {+-} 0.14 and j = 2.16{sub -0.75}{sup +0.81}, results that are consistent with {Lambda}CDM at about the 1{sigma} confidence level. A standard ''dynamical'' analysis of the same data, employing the Friedmann equations and modeling the dark energy as a fluid with an equation of state parameter, w (constant), gives {Omega}{sub m} = 0.306{sub -0.040}{sup +0.042} and w = -1.15{sub -0.18}{sup +0.14}, also consistent with {Lambda}CDM at about the 1{sigma} level. In comparison to dynamical analyses, the kinematical approach uses a different model set and employs a minimum of prior information, being independent of any particular gravity theory. The results obtained with this new approach therefore provide important additional information and we argue that both kinematical and dynamical techniques should be employed in future dark energy studies, where possible. Our results provide further interesting support for the concordance {Lambda}CDM paradigm.
No evidence for Dark Energy Metamorphosis ?
Jonsson, J; Amanullah, R; Bergström, L
2004-01-01
Recent attempts to fit Type Ia supernova data by modeling the dark energy density as a truncated Taylor series have suggested the possibility of metamorphosis, i.e., a rapidly evolving equation of state parameter, w_DE(z). However, we show that fits using that parametrization have significant problems: evolution of w_DE(z) is both favoured and in some sense forced, and the equation of state parameter blows up or diverges in large regions of the parameter space used for the truncated series. To further elucidate these problems we have simulated sets of supernova data in a Lambda-universe to show that the suggested ``evidence'' for metamorphosis is also common for w_DE=-1.
No evidence for Dark Energy Metamorphosis ?
J. Jonsson; A. Goobar; R. Amanullah; L. Bergstrom
2004-10-11
Recent attempts to fit Type Ia supernova data by modeling the dark energy density as a truncated Taylor series have suggested the possibility of metamorphosis, i.e., a rapidly evolving equation of state parameter, w_DE(z). However, we show that fits using that parametrization have significant problems: evolution of w_DE(z) is both favoured and in some sense forced, and the equation of state parameter blows up or diverges in large regions of the parameter space. To further elucidate these problems we have simulated sets of supernova data in a Lambda-universe to show that the suggested ``evidence'' for metamorphosis is also common for w_DE=-1.
Dark Energy and the Hubble Age
Lawrence M. Krauss
2003-01-09
I point out that an effective upper limit of approximately 20 Gyr (for a Hubble constant of 72 km/s/Mpc) or alternatively on the $H_0$-independent quantity $H_0t_0 age of the Universe, essentially independent of the unknown equation of state of the dominant dark energy component in the Universe. Unless astrophysical constraints on the age of the Universe can convincingly reduce the upper limit to below this value no useful lower limit on the equation of state parameter $w$ for this component can be obtained. Direct dating by stars does not provide a useful constraint, but model-dependent cosmological limits from supernovae and the CMB observations may. For a constant value of $w$, a bound $H_0t_0 -1.5$
Stability of Ghost Dark Energy in CBD Model of Gravity
Khaled Saaidi
2012-02-18
We study the stability of the ghost dark energy model versus perturbation. Since this kind of dark energy is instable in Einsteinian general relativity theory, then we study a new type of Brans-Dicke theory which has non-minimal coupling with matter which is called chameleon Brans-Dicke (CBD) model of gravity. Due to this coupling the equation of conservation energy is modified. For considering the stability of the model we use the adiabatic squared sound speed, $c_s^2$, whose sign of it determines the stability of the model in which for $c_s^2 >0 $ the model is stable and for $c_s^2 ghost dark energy (CBDGDE) with cold dark matter in non flat FLRW metric. We show that in all cases of investigation the model is stable with a suitable choice of parameters.
Evolution of entropic dark energy and its phantom nature
Titus K. Mathew; Chinthak Murali; Shejeelammal J
2015-06-23
Assuming the form of the entropic dark energy as arises form the surface term in the Einstein-Hilbert's action, it's evolution were analyzed in an expanding flat universe. The model parameters were evaluated by constraining model using the Union data on Type Ia supernovae. We found that the model predicts an early decelerated phase and a later accelerated phase at the background level. The evolution of the Hubble parameter, dark energy density, equation of state parameter and deceleration parameter were obtained. The model is diagnosed with $Om$ parameter. The model is hardly seems to be supporting the linear perturbation growth for the structure formation. We also found that the entropic dark energy shows phantom nature for redshifts $zbig-rip effect at which both the scale factor of expansion and the dark energy density become infinitely large and the big rip time is found to be around 36 Giga Years from now.
Some FRW Models of Accelerating Universe with Dark Energy
Suresh Kumar
2010-10-01
The paper deals with a spatially homogeneous and isotropic FRW space-time filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally, and therefore their energy momentum tensors are conserved separately. A special law of variation for the Hubble parameter proposed by Berman (1983) has been utilized to solve the field equations. The Berman's law yields two explicit forms of the scale factor governing the FRW space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of FRW universe. It has been found that dark energy dominates the universe at the present epoch, which is consistent with the observations. The physical behavior of the universe is discussed in detail.
Dark energy in some integrable and nonintegrable FRW cosmological models
Kuralay Esmakhanova; Nurgissa Myrzakulov; Gulgasyl Nugmanova; Yerlan Myrzakulov; Leonid Chechin; Ratbay Myrzakulov
2011-09-14
One of the greatest challenges in cosmology today is to determine the nature of dark energy, the sourse of the observed present acceleration of the Universe. Besides the vacuum energy, various dark energy models have been suggested. The Friedmann - Robertson - Walker (FRW) spacetime plays an important role in modern cosmology. In particular, the most popular models of dark energy work in the FRW spacetime. In this work, a new class of integrable FRW cosmological models is presented. These models induced by the well-known Painlev$\\acute{e}$ equations. Some nonintegrable FRW models are also considered. These last models are constructed with the help of Pinney, Schr$\\ddot{o}$dinger and hypergeometric equations. Scalar field description and two-dimensional generalizations of some cosmological models are presented. Finally some integrable and nonintegrable $F(R)$ and $F(G)$ gravity models are constructed.
A User-Friendly Dark Energy Model Generator
Hinton, Kyle A; Huterer, Dragan
2015-01-01
We provide software with a graphical user interface to calculate the phenomenology of a wide class of dark energy models featuring multiple scalar fields. The user chooses a subclass of models and, if desired, initial conditions, or else a range of initial parameters for Monte Carlo. The code calculates the energy density of components in the universe, the equation of state of dark energy, and the linear growth of density perturbations, all as a function of redshift and scale factor. The output also includes an approximate conversion into the average equation of state, as well as the common $(w_0, w_a)$ parametrization. The code is available here: http://github.com/kahinton/Dark-Energy-UI-and-MC
Cluster number counts dependence on dark energy inhomogeneities and coupling to dark matter
M. Manera; D. F. Mota
2006-07-04
Cluster number counts can be used to test dark energy models. We investigate dark energy candidates which are coupled to dark matter. We analyze the cluster number counts dependence on the amount of dark matter coupled to dark energy. Further more, we study how dark energy inhomogeneities affect cluster abundances. It is shown that increasing the coupling reduces significantly the cluster number counts, and that dark energy inhomogeneities increases cluster abundances. Wiggles in cluster number counts are shown to be a specific signature of coupled dark energy models. Future observations will possibly detect such oscillations and discriminate among the different dark energy models.
Interacting Dark matter and Holographic dark energy in Bianchi type-V universe
NASA Astrophysics Data System (ADS)
Adhav, K. S.; Munde, S. L.; Tayade, G. B.; Bokey, V. D.
2015-09-01
The spatially homogeneous and anisotropic Bianchi type-V universe filled with interacting Dark matter and Holographic dark energy has been studied. The exact solutions of Einstein's field equations are obtained by (i) applying the special law of variation of Hubble parameter that yields constant values of the deceleration parameter and (ii) using a special form of deceleration parameter. It has been observed that for suitable choice of interaction between dark matter and holographic dark energy there is no coincidence problem (unlike CDM). Also, in all the resulting models the anisotropy of expansion dies out very quickly and attains isotropy after some finite time. The physical and geometrical aspects of the models are also discussed.
Dark Energy From Fifth Dimensional Brans-Dicke Theory
Bahrehbakhsh, Amir F; Vakili, Hajar
2013-01-01
Following the approach of the induced-matter theory, we investigate the cosmological implications of a five-dimensional Brans-Dicke theory, and propose to explain the acceleration of the universe. After inducing in a four-dimensional hypersurface, we classify the energy-momentum tensor into two parts in a way that, one part represents all kind of the matter (the baryonic and dark) and the other one contains every extra terms emerging from the scale factor of the fifth dimension and the scalar field, which we consider as the energy-momentum tensor of dark energy. We also separate the energy-momentum conservation equation into two conservation equations, one for matter and the other for dark energy. We perform this procedure for different cases, without interacting term and with two particular (suitable) interacting terms between the two parts. By assuming the parameter of the state equation for dark energy to be constant, the equations of the model admit the power-law solutions. Though, the non-interacting cas...
Isotropic turbulence in the dark fluid universe with inhomogeneous equation of state
R. D. Boko; M. E. Rodrigues; M. J. S. Houndjo; J. B. Chabi-Orou; R. Myrzakulov
2015-03-26
We investigate the turbulence effect in dark fluid universe with linear inhomogeneous equation of state. Attention is attached to two physical situations. First, we perform the perturbative analysis of turbulence and check its effects around the Big Rip. Later, treating the turbulence energy density as a part of total dark fluid, we study the stability of the system. The result shows that the stability is achieving as the energy density of turbulence decreases, changing into heat (the radiation), in perfect agreement with the avoidance of the Big Rip.
Statefinder Parameters for Tachyon Dark Energy Model
Ying Shao; Yuanxing Gui
2007-03-22
In this paper we study the statefinder parameters for the tachyon dark energy model. There are two kinds of stable attractor solutions in this model. The statefinder diagrams characterize the properties of the tachyon dark energy model. Our results show that the evolving trajectories of the attractor solutions lie in the total region and pass through the LCDM fixed point, which is different from other dark energy model.
On the Chemical Potential of Dark Energy
S. H. Pereira
2008-06-23
It is widely assumed that the observed universe is accelerating due to the existence of a new fluid component called dark energy. In this article, the thermodynamics consequences of a nonzero chemical potential on the dark energy component is discussed with special emphasis to the phantom fluid case. It is found that if the dark energy fluid is endowed with a negative chemical potential, the phantom field hypothesis becomes thermodynamically consistent with no need of negative temperatures as recently assumed in the literature.
Eisenstein, Daniel
2007-02-14
I present galaxy clustering results from the Sloan Digital Sky Survey that reveal the signature of acoustic oscillations of the photon-baryon fluid in the first million years of the Universe. The scale of this feature can be computed and hence the detection in the galaxy clustering serves as a standard ruler, giving a geometric distance to a redshift of 0.35. I will discuss the implications of this measurement for the composition of the universe, including dark energy and spatial curvature. I will close with a discussion of the prospects for future redshift surveys to use the acoustic peak to map the expansion history of the universe.
Zibin, James P; Moss, Adam; Scott, Douglas
2008-12-19
The idea that we live near the center of a large, nonlinear void has attracted attention recently as an alternative to dark energy or modified gravity. We show that an appropriate void profile can fit both the latest cosmic microwave background and supernova data. However, this requires either a fine-tuned primordial spectrum or a Hubble rate so low as to rule these models out. We also show that measurements of the radial baryon acoustic scale can provide very strong constraints. Our results present a serious challenge to void models of acceleration. PMID:19113691
Bistable dark solitons of a cubic-quintic Helmholtz equation
Christian, J. M.; McDonald, G. S.; Chamorro-Posada, P. [Joule Physics Laboratory, School of Computing, Science and Engineering, Materials and Physics Research Centre, University of Salford, Salford M5 4WT (United Kingdom); Departamento de Teoria de la Senal y Comunicaciones e Ingenieria Telematica, Universidad de Valladolid, ETSI Telecomunicacion, Campus Miguel Delibes s/n, 47011 Valladolid (Spain)
2010-05-15
We provide a report on exact analytical bistable dark spatial solitons of a nonlinear Helmholtz equation with a cubic-quintic refractive-index model. Our analysis begins with an investigation of the modulational instability characteristics of Helmholtz plane waves. We then derive a dark soliton by mapping the desired asymptotic form onto a uniform background field and obtain a more general solution by deploying rotational invariance laws in the laboratory frame. The geometry of the new soliton is explored in detail, and a range of new physical predictions is uncovered. Particular attention is paid to the unified phenomena of arbitrary-angle off-axis propagation and nondegenerate bistability. Crucially, the corresponding solution of paraxial theory emerges in a simultaneous multiple limit. We conclude with a set of computer simulations that examine the role of Helmholtz dark solitons as robust attractors.
Leptogenesis, Dark Energy, Dark Matter and the neutrinos
NASA Astrophysics Data System (ADS)
Sarkar, Utpal
2007-10-01
In this review we discuss how the models of neutrino masses can accommodate solutions to the problem of matter-antimatter asymmetry in the universe, dark energy or cosmological constant problem and dark matter candidates. The matter-antimatter asymmetry is explained by leptogenesis, originating from the lepton number violation associated with the neutrino masses. The dark energy problem is correlated with a mass varying neutrinos, which could originate from a pseudo-Nambu-Goldstone boson. In some radiative models of neutrino masses, there exists a Higgs doublet that does not acquire any vacuum expectation value. This field could be inert and the lightest inert particle could then be a dark matter candidate. We reviewed these scenarios in connection with models of neutrino masses with right-handed neutrinos and with triplet Higgs scalars.
A two measure model of dark energy and dark matter
Guendelman, Eduardo [Department of Physics, Ben-Gurion University, Beer-Sheva (Israel); Singleton, Douglas; Yongram, N., E-mail: guendel@bgu.ac.il, E-mail: dougs@csufresno.edu, E-mail: nattapongy@nu.ac.th [Physics Department, California State University Fresno, Fresno, CA 93740 (United States)
2012-11-01
In this work we construct a unified model of dark energy and dark matter. This is done with the following three elements: a gravitating scalar field, ? with a non-conventional kinetic term, as in the string theory tachyon; an arbitrary potential, V(?); two measures — a metric measure ((?g){sup 1/2}) and a non-metric measure (?). The model has two interesting features: (i) For potentials which are unstable and would give rise to tachyonic scalar field, this model can stabilize the scalar field. (ii) The form of the dark energy and dark matter that results from this model is fairly insensitive to the exact form of the scalar field potential.
Dark Energy and Neutrino CPT Violation
Pei-Hong Gu; Xiao-Jun Bi; Xinmin Zhang
2007-02-05
In this paper we study the dynamical CPT violation in the neutrino sector induced by the dark energy of the Universe. Specifically we consider a dark energy model where the dark energy scalar derivatively interacts with the right-handed neutrinos. This type of derivative coupling leads to a cosmological CPT violation during the evolution of the background field of the dark energy. We calculate the induced CPT violation of left-handed neutrinos and find the CPT violation produced in this way is consistent with the present experimental limit and sensitive to the future neutrino oscillation experiments, such as the neutrino factory.
Statefinder diagnostic for holographic dark energy model
Xin Zhang
2005-09-29
In this paper we study the holographic dark energy model proposed by Li from the statefinder viewpoint. We plot the evolutionary trajectories of the model with $c=1$ in the statefinder parameter-planes. The statefinder diagrams characterize the properties of the holographic dark energy and show the discrimination between this scenario and other dark energy models. We also perform a statefinder diagnostic to the holographic dark energy model in cases of different $c$ which given by three fits to observational data. The result indicates that from the statefinder viewpoint $c$ plays a significant role in this model and should thus be determined seriously by future high precision experiments.
$?$CDM coupled to radiation. Dark energy and Universe acceleration
Renat R. Abbyazov; Sergey V. Chervon; Volker Müller
2014-09-02
Recently the Chiral Cosmological Model (CCM) coupled to cold dark matter (CDM) has been investigated as $\\sigma$CDM model to study the observed accelerated expansion of the Universe. Dark sector fields (as Dark Energy content) coupled to cosmic dust were considered as the source of Einstein gravity in Friedmann-Robertson-Walker (FRW) cosmology. Such model had a beginning at the matter-dominated era. The purposes of our present investigation are two folds: to extend > of the $\\sigma$CDM for earlier times to radiation-dominated era and to take into account variation of the exponential potential via variation of the interaction parameter $\\lambda $. We use Markov Chain Monte Carlo (MCMC) procedure to investigate possible values of initial conditions constrained by the measured amount of the dark matter, dark energy and radiation component today. Our analysis includes dark energy contribution to critical density, the ratio of the kinetic and potential energies, deceleration parameter, effective equation of state and evolution of DE equation of state with variation of coupling constant $\\lambda $. A comparison with the $\\Lambda$CDM model was performed. A new feature of the model is the existence of some values of potential coupling constant, leading to a $\\sigma$CDM solution without transit into accelerated expansion epoch.
Dark energy interacting with dark matter and unparticle
Songbai Chen; Jiliang Jing
2009-07-16
We study dynamical behaviors of the dark energy models interacting with dark matter and unparticle in the standard flat FRW cosmology. We considered four different interacting models and examined the stability of the critical points. We find that there exist late-time scaling attractors corresponding to an accelerating Universe and the alleviation of the coincidence problem depends on the choice of parameters in the models.
Dark matter and dark energy from quark bag model
Brilenkov, Maxim [Department of Theoretical Physics, Odessa National University, Dvoryanskaya st. 2, Odessa 65082 (Ukraine); Eingorn, Maxim [Physics Department, North Carolina Central University, Fayetteville st. 1801, Durham, North Carolina 27707 (United States); Jenkovszky, Laszlo [Bogolyubov Institute for Theoretical Physics, Kiev 03680 (Ukraine); Zhuk, Alexander, E-mail: maxim.brilenkov@gmail.com, E-mail: maxim.eingorn@gmail.com, E-mail: jenk@bitp.kiev.ua, E-mail: ai.zhuk2@gmail.com [Astronomical Observatory, Odessa National University, Dvoryanskaya st. 2, Odessa 65082 (Ukraine)
2013-08-01
We calculate the present expansion of our Universe endowed with relict colored objects — quarks and gluons — that survived hadronization either as isolated islands of quark-gluon ''nuggets'' or spread uniformly in the Universe. In the first scenario, the QNs can play the role of dark matter. In the second scenario, we demonstrate that uniform colored objects can play the role of dark energy providing the late-time accelerating expansion of the Universe.
Spintessence! New models for dark matter and dark energy
Latham A. Boyle; Robert R. Caldwell; Marc Kamionkowski
2002-01-01
We investigate a class of models for dark matter and\\/or negative-pressure, dynamical dark energy consisting of “spintessence”, a complex scalar field ? spinning in a U(1)-symmetric potential V(?)=V(|?|). As the Universe expands, the field spirals slowly toward the origin. The internal angular momentum plays an important role in the cosmic evolution and fluctuation dynamics. We outline the constraints on a cosmic
Nonparametric dark energy reconstruction from supernova data.
Holsclaw, Tracy; Alam, Ujjaini; Sansó, Bruno; Lee, Herbert; Heitmann, Katrin; Habib, Salman; Higdon, David
2010-12-10
Understanding the origin of the accelerated expansion of the Universe poses one of the greatest challenges in physics today. Lacking a compelling fundamental theory to test, observational efforts are targeted at a better characterization of the underlying cause. If a new form of mass-energy, dark energy, is driving the acceleration, the redshift evolution of the equation of state parameter w(z) will hold essential clues as to its origin. To best exploit data from observations it is necessary to develop a robust and accurate reconstruction approach, with controlled errors, for w(z). We introduce a new, nonparametric method for solving the associated statistical inverse problem based on Gaussian process modeling and Markov chain Monte Carlo sampling. Applying this method to recent supernova measurements, we reconstruct the continuous history of w out to redshift z=1.5. PMID:21231517
Stable Gravastars of Anisotropic Dark Energy
R. Chan; M. F. A. da Silva; P. Rocha; Anzhong Wang
2009-02-11
Dynamical models of prototype gravastars made of phantom energy are constructed, in which an infinitely thin spherical shell of a perfect fluid with the equation of state $p = (1-\\gamma)\\sigma$ divides the whole spacetime into two regions, the internal region filled with a dark energy (or phantom) fluid, and the external Schwarzschild region. It is found that in some cases the models represent the "bounded excursion" stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes or normal stars. In the phase space, the region for the "bounded excursion" gravastars is very small in comparison to that of black holes, but not empty, as found in our previous papers. Therefore, although the existence of gravastars can not be completely excluded from current analysis, the opposite is not possible either, that is, even if gravastars exist, they do not exclude the existence of black holes.
Dark energy and supermassive black holes
Gonzalez-Diaz, Pedro F. [Colina de los Chopos, Centro de Fisica 'Miguel A. Catalan', Instituto de Matematicas y Fisica Fundamental, Consejo Superior de Investigaciones Cientificas, Serrano 121, 28006 Madrid (Spain)
2004-09-15
This paper deals with a cosmological model in which the universe is filled with tachyon dark energy in order to describe current and future accelerating expansion. We obtain that the simplest condition for the regime of phantom energy to occur in this scenario is that the scalar field be Wick rotated to imaginary values which correspond to an axionic field classically. By introducing analytical expressions for the scale factor or the Hubble parameter that satisfy all constraint equations of the used models we show that such models describe universes which may develop a big rip singularity in the finite future. It is argued that, contrary to a recent claim, the entropy for a universe filled with dark energy is definite positive even on the phantom regime where the universe would instead acquire a negative temperature. It is also seen that, whichever the fate of the tachyonic accelerating universe, it will be stable to any fluctuations of the scalar field, and that since the considered models have all an imaginary sound speed, any overdense regions will undergo an accelerated collapse leading rapidly to formation of giant black holes. Finally the conjecture is advanced that these black holes may be the supermassive black holes that most galaxies harbor at their center.
Topology and dark energy: testing gravity in voids.
Spolyar, Douglas; Sahlén, Martin; Silk, Joe
2013-12-13
Modified gravity has garnered interest as a backstop against dark matter and dark energy (DE). As one possible modification, the graviton can become massive, which introduces a new scalar field--here with a Galileon-type symmetry. The field can lead to a nontrivial equation of state of DE which is density and scale dependent. Tension between type Ia supernovae and Planck could be reduced. In voids, the scalar field dramatically alters the equation of state of DE, induces a soon-observable gravitational slip between the two metric potentials, and develops a topological defect (domain wall) due to a nontrivial vacuum structure for the field. PMID:24483641
The Dark Energy Camera (DECam)
K. Honscheid; D. L. DePoy; for the DES Collaboration
2008-10-20
In this paper we describe the Dark Energy Camera (DECam), which will be the primary instrument used in the Dark Energy Survey. DECam will be a 3 sq. deg. mosaic camera mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo International Observatory (CTIO). It consists of a large mosaic CCD focal plane, a five element optical corrector, five filters (g,r,i,z,Y), a modern data acquisition and control system and the associated infrastructure for operation in the prime focus cage. The focal plane includes of 62 2K x 4K CCD modules (0.27"/pixel) arranged in a hexagon inscribed within the roughly 2.2 degree diameter field of view and 12 smaller 2K x 2K CCDs for guiding, focus and alignment. The CCDs will be 250 micron thick fully-depleted CCDs that have been developed at the Lawrence Berkeley National Laboratory (LBNL). Production of the CCDs and fabrication of the optics, mechanical structure, mechanisms, and control system for DECam are underway; delivery of the instrument to CTIO is scheduled for 2010.
New holographic dark energy model inspired by the DGP braneworld
A. Sheykhi; M. H. Dehghani; S. Ghaffari
2015-06-05
The energy density of the holographic dark energy is based on the area law of entropy, and thus any modification of the area law leads to a modified holographic energy density. Inspired by the entropy expression associated with the apparent horizon of a Friedmann-Robertson-Walker (FRW) Universe in DGP braneworld, we propose a new model for the holographic dark energy in the framework of DGP brane cosmology. We investigate the cosmological consequences of this new model and calculate the equation of state parameter by choosing the Hubble radius, $L = H^{-1}$, as the system's IR cutoff. Our study show that, due to the effects of the extra dimension (bulk), the identification of IR-cutoff with Hubble radius, can reproduce the present acceleration of the Universe expansion. This is in contrast to the ordinary holographic dark energy in standard cosmology which leads to the zero equation of state parameter in the case of choosing the Hubble radius as system's IR cutoff in the absence of interaction between dark matter and dark energy.
A Quantum Cosmology: No Dark Matter, Dark Energy nor Accelerating Universe
Reginald T Cahill
2007-09-18
We show that modelling the universe as a pre-geometric system with emergent quantum modes, and then constructing the classical limit, we obtain a new account of space and gravity that goes beyond Newtonian gravity even in the non-relativistic limit. This account does not require dark matter to explain the spiral galaxy rotation curves, and explains as well the observed systematics of black hole masses in spherical star systems, the bore hole $g$ anomalies, gravitational lensing and so on. As well the dynamics has a Hubble expanding universe solution that gives an excellent parameter-free account of the supernovae and gamma-ray-burst red-shift data, without dark energy or dark matter. The Friedmann-Lema\\^{i}tre-Robertson-Walker (FLRW) metric is derived from this dynamics, but is shown not satisfy the General Relativity based Friedmann equations. It is noted that General Relativity dynamics only permits an expanding flat 3-space solution if the energy density in the pressure-less dust approximation is non-zero. As a consequence dark energy and dark matter are required in this cosmological model, and as well the prediction of a future exponential accelerating Hubble expansion. The FLRW $\\Lambda$CDM model data-based parameter values, $\\Omega_\\Lambda=0.73$, $\\Omega_{DM}=0.27$, are derived within the quantum cosmology model, but are shown to be merely artifacts of using the Friedmann equations in fitting the red-shift data.
Nonlocal String Tachyon as a Model for Cosmological Dark Energy
I. Ya
There are many different phenomenological models describing the cosmological dark energy and accelerating Universe by choosing adjustable functions. In this paper we consider a specific model of scalar tachyon field which is derived from the NSR string field theory and study its cosmological applications. We find that in the effective field theory approximation the equation of state parameter w <
Dark energy: Vacuum fluctuations, the effective phantom phase, and holography
Emilio Elizalde; Shin'ichi Nojiri; S. D. Odintsov; Peng Wang
2005-01-01
We aim at the construction of dark energy models without exotic matter but with a phantomlike equation of state (an effective phantom phase). The first model we consider is decaying vacuum cosmology where the fluctuations of the vacuum are taken into account. In this case, the phantom cosmology (with an effective, observational omega being less than -1 ) emerges even
Cosmological Surprises from Braneworld models of Dark Energy
Varun Sahni
2005-01-01
Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w -1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future `Big Rip' singularity which is usually encountered by Phantom models.
Hessence: a new view of quintom dark energy
Hao Wei; Rong-Gen Cai; Ding-Fang Zeng
2005-01-01
Recently a lot of attention has been given to building a dark energy model in which the equation-of-state parameter w can cross the phantom divide w = -1. One of the models to realize crossing the phantom divide is called the quintom model, in which two real scalar fields appear, one is a normal scalar field and the other is
Dark Energy From Vacuum Fluctuations
Djorgovski, S G
2006-01-01
We describe briefly a novel interpretation of the physical nature of dark energy (DE), based on the vacuum fluctuations model by Gurzadyan & Xue, and describe an internally consistent solution for the behavor of DE as a function of redshift. A key choice is the nature of the upper bound used for the computation of energy density contributions by vacuum modes. We show that use of the comoving horizon radius produces a viable model, whereas use of the proper horizon radius is inconsistent with the observations. After introduction of a single phenomenological parameter, the model is consistent with all of the curently available data, and fits them as well as the standard cosmological constant model, while making testable predictions. While some substantial interpretative uncertainties remain, future developments of this model may lead to significant new insights into the physical nature of DE.
Dark Energy From Vacuum Fluctuations
S. G. Djorgovski; V. G. Gurzadyan
2006-10-06
We describe briefly a novel interpretation of the physical nature of dark energy (DE), based on the vacuum fluctuations model by Gurzadyan & Xue, and describe an internally consistent solution for the behavor of DE as a function of redshift. A key choice is the nature of the upper bound used for the computation of energy density contributions by vacuum modes. We show that use of the comoving horizon radius produces a viable model, whereas use of the proper horizon radius is inconsistent with the observations. After introduction of a single phenomenological parameter, the model is consistent with all of the curently available data, and fits them as well as the standard cosmological constant model, while making testable predictions. While some substantial interpretative uncertainties remain, future developments of this model may lead to significant new insights into the physical nature of DE.
Disformal dark energy at colliders
Philippe Brax; Clare Burrage; Christoph Englert
2015-06-11
Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static non-relativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalisation group flow.
Disformal dark energy at colliders
Brax, Philippe; Englert, Christoph
2015-01-01
Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static non-relativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalisation group flow.
R. Chan; M. F. A. da Silva; J. F. Villas da Rocha
2008-12-30
We have constructed star models consisting of four parts: (i) a homogeneous inner core with anisotropic pressure (ii) an infinitesimal thin shell separating the core and the envelope; (iii) an envelope of inhomogeneous density and isotropic pressure; (iv) an infinitesimal thin shell matching the envelope boundary and the exterior Schwarzschild spacetime. We have analyzed all the energy conditions for the core, envelope and the two thin shells. We have found that, in order to have static solutions, at least one of the regions must be constituted by dark energy. The results show that there is no physical reason to have a superior limit for the mass of these objects but for the ratio of mass and radius.
Disformal dark energy at colliders
NASA Astrophysics Data System (ADS)
Brax, Philippe; Burrage, Clare; Englert, Christoph
2015-08-01
Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the Universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static nonrelativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high-energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalization group flow.
Dynamical and Gravitational Instability of Oscillating-Field Dark Energy and Dark Matter
Matthew C. Johnson; Marc Kamionkowski
2008-05-12
Coherent oscillations of a scalar field can mimic the behavior of a perfect fluid with an equation-of-state parameter determined by the properties of the potential, possibly driving accelerated expansion in the early Universe (inflation) and/or in the Universe today (dark energy) or behaving as dark matter. We consider the growth of inhomogeneities in such a field, mapping the problem to that of two coupled anharmonic oscillators. We provide a simple physical argument that oscillating fields with a negative equation-of-state parameter possess a large-scale dynamical instability to growth of inhomogeneities. This instability renders these models unsuitable for explaining cosmic acceleration. We then consider the gravitational instability of oscillating fields in potentials that are close to, but not precisely, harmonic. We use these results to show that if axions make up the dark matter, then the small-scale cutoff in the matter power spectrum is around $10^{-15} M_\\oplus$.
Dark Energy and Modified Gravity
Ruth Durrer; Roy Maartens
2008-11-25
Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy, and a failure to solve the so-called coincidence problem. Two broad alternatives to dark energy have emerged as candidate models: these typically address only the coincidence problem and not the vacuum energy problem. The first is based on general relativity and attempts to describe the acceleration as an effect of inhomogeneity in the universe. If this alternative could be shown to work, then it would provide a dramatic resolution of the coincidence problem; however, a convincing demonstration of viability has not yet emerged. The second alternative is based on infra-red modifications to general relativity, leading to a weakening of gravity on the largest scales and thus to acceleration. Most examples investigated so far are scalar-tensor or brane-world models, and we focus on the simplest candidates of each type: $f(R)$ models and DGP models respectively. Both of these provide a new angle on the problem, but they also face serious difficulties. However, investigation of these models does lead to valuable insights into the properties of gravity and structure formation, and it also leads to new strategies for testing the validity of General Relativity itself on cosmological scales.
GALAXY CLUSTERS AS A PROBE OF EARLY DARK ENERGY
Alam, Ujjaini [ISR-1, ISR Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Lukic, Zarija; Bhattacharya, Suman, E-mail: ujjaini@lanl.gov, E-mail: zarija@lanl.gov, E-mail: sumanb@lanl.gov [T-2, T Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)
2011-02-01
We study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and show that these models may be successfully constrained using future observations of galaxy clusters, in particular the redshift abundance, and the Sunyaev-Zel'dovich (SZ) power spectrum. We make predictions for EDE models, as well as {Lambda}CDM for incoming X-ray (eROSITA) and microwave (South Pole Telescope) observations. We show that galaxy clusters' mass function and the SZ power spectrum will put strong constraints both on the equation of state of dark energy today and the redshift at which EDE transits to present-day {Lambda}CDM-like behavior for these models, thus providing complementary information to the geometric probes of dark energy. Not including perturbations in EDE models leads to those models being practically indistinguishable from {Lambda}CDM. An MCMC analysis of future galaxy cluster surveys provides constraints for EDE parameters that are competitive with and complementary to background expansion observations such as supernovae.
Is there supernova evidence for dark energy metamorphosis?
Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky
2004-01-01
We reconstruct the equation of state w(z) of dark energy (DE) using a recently released data set containing 172 Type Ia supernovae (SNe) without assuming the prior w(z) >=-1 (in contrast to previous studies). We find that DE evolves rapidly and metamorphoses from dust-like behaviour at high z (w~= 0 at z~ 1) to a strongly negative equation of state
Constraining dark energy through the stability of cosmic structures
Pavlidou, V. [Department of Physics, University of Crete, and Fo.R.T.H., 711 10 Heraklion (Greece); Tetradis, N. [Department of Physics, University of Athens, Zographou 157 84 (Greece); Tomaras, T.N., E-mail: pavlidou@physics.uoc.gr, E-mail: ntetrad@phys.uoa.gr, E-mail: tomaras@physics.uoc.gr [Department of Physics and CCTP, University of Crete, Heraklion 711 10 (Greece)
2014-05-01
For a general dark-energy equation of state, we estimate the maximum possible radius of massive structures that are not destabilized by the acceleration of the cosmological expansion. A comparison with known stable structures constrains the equation of state. The robustness of the constraint can be enhanced through the accumulation of additional astrophysical data and a better understanding of the dynamics of bound cosmic structures.
Describing Dark Matter and Dark Energy through a non-minimal gravitational coupling
NASA Astrophysics Data System (ADS)
Páramos, Jorge; Bertolami, Orfeu
A model exhibiting a non-minimal coupling between matter and the scalar curvature is pre-sented, generalizing the framework of the so-called f(R) theories; its main fundamental impli-cations are discussed, namely the non-conservation of the energy-momentum tensor and the deviation from geodesic motion. The flexibility of this model is first presented through a suitable application to the description of known dark matter profiles, thus accounting for the reported flattening of the galaxy rotation curves and encompassing the phenomenological Tully-Fisher law. In a cosmological context, the non-minimal gravitational coupling is further explored, and a description of a dynamic negative pressure perfect fluid (i.e. Dark Energy) is obtained; its main features are discussed, with a focus on the evolution of the deceleration parameter and the dark energy equation of state parameter.
Interacting polytropic gas model of phantom dark energy in non-flat universe
K. Karami; S. Ghaffari; J. Fehri
2009-11-25
By introducing the polytropic gas model of interacting dark energy, we obtain the equation of state for the polytropic gas energy density in a non-flat universe. We show that for even polytropic index by choosing $K>Ba^{\\frac{3}{n}}$, one can obtain $\\omega^{\\rm eff}_{\\Lambda}universe dominated by phantom dark energy.
A Dust Universe Solution to the Dark Energy Problem
James G. Gilson
2010-01-14
Astronomical measurements of the Omegas for mass density, cosmological constant lambda and curvature k are shown to be sufficient to produce a unique and detailed cosmological model describing dark energy influences based on the Friedman equations. The equation of state Pressure turns out to be identically zero at all epochs as a result of the theory. The partial omega, for dark energy, has the exact value, minus unity, as a result of the theory and is in exact agreement with the astronomer's measured value. Thus this measurement is redundant as it does not contribute to the construction of the theory for this model. Rather, the value of this omega is predicted from the theory. The model has the characteristic of changing from deceleration to acceleration at exactly half the epoch time at which the input measurements are taken. This is a mysterious feature of the model for which no explanation has so far been found. An attractive feature of the model is that the acceleration change time occurs at a red shift of approximately 0.8 as predicted by the dark energy workers. Using a new definition of dark energy density it is shown that the contribution of this density to the acceleration process is via a negative value for the gravitational constant, -G, exactly on a par with gravitational mass which occurs via the usual positive value for G. This paper also contains an appendix on dark energy dynamics with its own abstract.
Cosmological perturbations in elastic dark energy models
NASA Astrophysics Data System (ADS)
Battye, Richard A.; Moss, Adam
2007-07-01
We discuss the general framework for a perfect continuum medium in cosmology and show that an interesting generalization of the fluids normally used is for the medium to have rigidity and, hence, be analogous to an elastic solid. Such models can provide perfect, adiabatic fluids which are stable even when the pressure is negative, if the rigidity is sufficiently large, making them natural candidates to describe the dark energy. In fact, if the medium is adiabatic and isotropic, they provide the most general description of linearized perturbations. We derive the equations of motion and wave propagation speeds in the isotropic case. We point out that anisotropic models can also be incorporated within the same formalism and that they are classified by the standard Bravais lattices. We identify the adiabatic and isocurvature modes allowed in both the scalar and vector sectors and discuss the predictions they make for cosmic microwave background and matter power spectra. We comment on the relationship between these models and other fluid-based approaches to dark energy, and discuss a possible microphysical manifestation of this class of models as a continuum description of defect-dominated scenarios.
Stable gravastars of anisotropic dark energy
Chan, R. [Coordenacao de Astronomia e Astrofisica, Observatorio Nacional, Rua General Jose Cristino 77, Sao Cristovao 20921-400, Rio de Janeiro, RJ (Brazil)] [Coordenacao de Astronomia e Astrofisica, Observatorio Nacional, Rua General Jose Cristino 77, Sao Cristovao 20921-400, Rio de Janeiro, RJ (Brazil); Silva, M.F.A. da; Rocha, P. [Departamento de Fisica Teorica, Instituto de Fisica, Universidade do Estado do Rio de Janeiro, Rua Sao Francisco Xavier 524, Maracana 20550-900, Rio de Janeiro, RJ (Brazil)] [Departamento de Fisica Teorica, Instituto de Fisica, Universidade do Estado do Rio de Janeiro, Rua Sao Francisco Xavier 524, Maracana 20550-900, Rio de Janeiro, RJ (Brazil); Wang, Anzhong, E-mail: chan@on.br, E-mail: mfasnic@gmail.com, E-mail: pedrosennarocha@gmail.com, E-mail: anzhong_wang@baylor.edu [GCAP-CASPER, Department of Physics, Baylor University, Waco, TX 76798 (United States)] [GCAP-CASPER, Department of Physics, Baylor University, Waco, TX 76798 (United States)
2009-03-15
Dynamical models of prototype gravastars made of anisotropic dark energy fluid are constructed, in which an infinitely thin spherical shell of a perfect fluid with the equation of state p = (1-{gamma}){sigma} divides the whole spacetime into two regions, the internal region is filled with an anisotropic dark energy fluid, and the external region is the Schwarzschild. It is found that in some cases the models represent the ''bounded excursion'' stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes or normal stars. In the phase space, the region for the ''bounded excursion'' gravastars is very small in comparison to that of black holes, but not empty, as found in our previous papers. Therefore, although the existence of gravastars can not be completely excluded from current analysis, the opposite is not possible either, that is, even if gravastars exist, they do not exclude the existence of black holes.
The Dark Energy Survey instrument design
Flaugher, B.; /Fermilab
2006-05-01
We describe a new project, the Dark Energy Survey (DES), aimed at measuring the dark energy equation of state parameter, w, to a statistical precision of {approx}5%, with four complementary techniques. The survey will use a new 3 sq. deg. mosaic camera (DECam) mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo International Observatory (CTIO). DECam includes a large mosaic camera, a five element optical corrector, four filters (g,r,i,z), and the associated infrastructure for operation in the prime focus cage. The focal plane consists of 62 2K x 4K CCD modules (0.27''/pixel) arranged in a hexagon inscribed within the 2.2 deg. diameter field of view. We plan to use the 250 micron thick fully-depleted CCDs that have been developed at the Lawrence Berkeley National Laboratory (LBNL). At Fermilab, we will establish a packaging factory to produce four-side buttable modules for the LBNL devices, as well as to test and grade the CCDs. R&D is underway and delivery of DECam to CTIO is scheduled for 2009.
Examining the evidence for dynamical dark energy.
Zhao, Gong-Bo; Crittenden, Robert G; Pogosian, Levon; Zhang, Xinmin
2012-10-26
We apply a new nonparametric Bayesian method for reconstructing the evolution history of the equation of state w of dark energy, based on applying a correlated prior for w(z), to a collection of cosmological data. We combine the latest supernova (SNLS 3 year or Union 2.1), cosmic microwave background, redshift space distortion, and the baryonic acoustic oscillation measurements (including BOSS, WiggleZ, and 6dF) and find that the cosmological constant appears consistent with current data, but that a dynamical dark energy model which evolves from w<-1 at z~0.25 to w>-1 at higher redshift is mildly favored. Estimates of the Bayesian evidence show little preference between the cosmological constant model and the dynamical model for a range of correlated prior choices. Looking towards future data, we find that the best fit models for current data could be well distinguished from the ?CDM model by observations such as Planck and Euclid-like surveys. PMID:23215174
The case for dynamical dark energy revisited
Ujjaini Alam; Varun Sahni; A. A. Starobinsky
2004-01-01
We investigate the behaviour of dark energy using the recently released supernova data of Riess et al 2004 and a model independent parametrization for dark energy (DE). We find that, if no priors are imposed on Omega0m and h, DE which evolves with time provides a better fit to the SNe data than LambdaCDM. This is also true if we
Probing dark energy: Methods and strategies
Dragan Huterer; Michael S. Turner
2001-01-01
The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and, indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about two-thirds of the critical density, is smoothly distributed, has large negative pressure, and is very mysterious. For now, all of its discernible cosmological consequences follow from its effect
Astrophysikalisches Institut Potsdam Probes of Dark Energy
Matter Power Spectrum, Baryonic Acoustic Oscillations, and Clusters of Galaxies Dissertation zur energy is one of the greatest unsolved questions in science and the understanding of dark energy (and dark matter) was rated at the top of 125 open questions of science by Science in 2005. Probing
Singularity-free dark energy star
Farook Rahaman; Anil Kumar Yadav; Saibal Ray; Raju Maulick; Ranjan Sharma
2011-08-25
We propose a model for an anisotropic dark energy star where we assume that the radial pressure exerted on the system due to the presence of dark energy is proportional to the isotropic perfect fluid matter density. We discuss various physical features of our model and show that the model satisfies all the regularity conditions and stable as well as singularity-free.
Vacuum quantum fluctuation energy in expanding universe and dark energy
Shun-Jin Wang
2014-10-27
This article is based on the Planckon densely piled vacuum model and the principle of cosmology. With the Planck era as initial conditions and including the early inflation, we have solved the Einstein-Friedmann equations to describe the evolution of the universe. The results are: 1) the ratio of the dark energy density to the vacuum quantum fluctuation energy density is $\\frac{{{\\rho }_{de}}}{{{\\rho }_{vac}}}\\sim{{(\\frac{{{t}_{P}}}{{{T}_{0}}})}^{2}}\\sim{{10}^{-122}} $; 2) at the inflation time ${{t}_{\\inf }}={{10}^{-35}}s$, the calculated universe radiation energy density is $\\rho ({{t}_{\\inf }})\\sim{{10}^{-16}}{{\\rho }_{vac}}$ and the corresponding temperature is ${{E}_{c}}\\sim{{10}^{15}}GeV$ consistent with the GUT phase transition temperature; 3) the expanding universe with vacuum as its environment is a non-equilibrium open system constantly exchanging energy with vacuum; during its expansion, the Planckons in the universe lose quantum fluctuation energy and create the cosmic expansion quanta-cosmons, the energy of cosmons is the lost part of the vacuum quantum fluctuation energy and contributes to the universe energy with the calculated value ${{E}_{\\cos mos}}={{10}^{22}}{{M}_{\\otimes }}{{c}^{2}}$ (where ${{M}_{\\otimes }}$ is solar mass); 4) the total energy of the universe, namely the negative gravity energy plus the positive universe energy is zero; 5) the negative gravity potential and the gravity acceleration related to the creation of cosmons are derived with the nature of outward repulsive force, indicating that the cosmon may be the candidate of the dark energy quantum; 6) both the initial Planck era solution and the infinite asymptotic solution of the Einstein-Friedman equations are unstable: the former tends to expand and the latter tends to shrink, so that the Einstein-Friedman universe will undergo a cyclic evolution of successive expansion and shrinking.
Remarks on Dynamical Dark Energy Measured by the Conformal Age of the Universe
Ishwaree P. Neupane
2007-12-03
We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R.G. Cai, arXiv:0708.0884] where the present-day dark energy density was taken to be $\\rho_q \\equiv 3 \\alpha^2 m_P^2/\\eta^2$, where $\\eta$ is the conformal time and $\\alpha$ is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field $q$, the model may be adjusted to the present values of the dark energy density fraction $\\Omega\\Z{q} \\simeq 0.73$ and the equation of state parameter $w\\Z{q} dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.
[Dark matter and dark energy of the universe].
Aguilar Peris, José
2005-01-01
At the turn of the 20th Century, the Universe was thought to consist of our solar system, the Sun, planets, satellites and comets, floating under the Milky Way. The astronomers were ignorant of the existence of galaxies, clusters, quasars and black holes. Over the last ten years the Cosmology has made remarkable progress in our understanding of the composition of the Universe: 23 per cent is in an unknown form called dark matter; 73 per cent in another form called dark energy; 3 per cent is made of free hydrogen and helium atoms; 0.5 per cent makes up all the light we see in the night including the stars, clusters and superclusters; 0.3 per cent is in free neutrino particles; and finally, 0.03 per cent is in the heavier nuclei of which the Sun, the Earth and ourselves are made. In this work we study specially the dark matter and the dark energy. The first one appears to be attached to galaxies, and astronomers agree that it is cold, meaning that the particles that make up that matter are not moving fast. Very recently astronomers discovered that a tremendous amount of the so-cahled dark energy exists and that it is pushing and accelerating the expansion of the Universe. Should this expansion continue for another 14,000 million years, the sky will darken with only a handful of galaxies remaining visible. PMID:16463572
Sussman, Roberto A. [Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico (ICN-UNAM), A. P. 70--543, 04510 Mexico D. F. (Mexico)
2009-01-15
A numerical approach is considered for spherically symmetric spacetimes that generalize Lemaitre-Tolman-Bondi dust solutions to nonzero pressure ('LTB spacetimes'). We introduce quasilocal (QL) variables that are covariant LTB objects satisfying evolution equations of Friedman-Lemaitre-Robertson-Walker (FLRW) cosmologies. We prove rigorously that relative deviations of the local covariant scalars from the QL scalars are nonlinear, gauge invariant and covariant perturbations on a FLRW formal background given by the QL scalars. The dynamics of LTB spacetimes is completely determined by the QL scalars and these exact perturbations. Since LTB spacetimes are compatible with a wide variety of ''equations of state,'' either single fluids or mixtures, a large number of known solutions with dark matter and dark energy sources in a FLRW framework (or with linear perturbations) can be readily examined under idealized but nontrivial inhomogeneous conditions. Coordinate choices and initial conditions are derived for a numerical treatment of the perturbation equations, allowing us to study nonlinear effects in a variety of phenomena, such as gravitational collapse, nonlocal effects, void formation, dark matter and dark energy couplings, and particle creation. In particular, the embedding of inhomogeneous regions can be performed by a smooth matching with a suitable FLRW solution, thus generalizing the Newtonian 'top hat' models that are widely used in astrophysical literature. As examples of the application of the formalism, we examine numerically the formation of a black hole in an expanding Chaplygin gas FLRW universe, as well as the evolution of density clumps and voids in an interactive mixture of cold dark matter and dark energy.
Roberto A. Sussman
2008-12-23
A numerical approach is considered for spherically symmetric spacetimes that generalize Lemaitre-Tolman-Bondi dust solutions to nonzero pressure ("LTB spacetimes"). We introduce quasi-local (QL) variables that are covariant LTB objects satisfying evolution equations of Friedman-Lemaitre-Robertson-Walker (FLRW) cosmologies. We prove rigorously that relative deviations of the local covariant scalars from the QL scalars are non-linear, gauge invariant and covariant perturbations on a FLRW formal "background" given by the QL scalars. The dynamics of LTB spacetimes is completely determined by the QL scalars and these exact perturbations. Since LTB spacetimes are compatible with a wide variety of "equations of state", either single fluids or mixtures, a large number of known solutions with dark matter and dark energy sources in a FLRW framework (or with linear perturbations) can be readily examined under idealized but non-trivial inhomogeneous conditions. Coordinate choices and initial conditions are derived for a numerical treatment of the perturbation equations, allowing us to study non-linear effects in a variety of phenomena, such as gravitational collapse, non-local effects, void formation, dark matter and dark energy couplings and particle creation. In particular, the embedding of inhomogeneous regions can be performed by a smooth matching with a suitable FLRW solution, thus generalizing the Newtonian "top hat" models that are widely used in astrophysical literature. As examples of the application of the formalism, we examine numerically the formation of a black hole in an expanding Chaplygin gas FLRW universe, as well as the evolution of density clumps and voids in an interactive mixture of cold dark matter and dark energy.
Universe Filled with Dark Energy (DE) from a Wet Dark Fluid (WDF) in f( R, T) Gravity
NASA Astrophysics Data System (ADS)
Samanta, G. C.
2013-07-01
We studied the Bianchi type-V universe filled with dark energy (DE) from a wet dark fluid (WDF) in the framework of f( R, T) gravity (Harko in Phys. Rev. D 84:024020, 2011). A new equation of state for the dark energy (DE) component of the universe has been used. It is modeled on the equation of state p= w( ?- ? ?) which can be describing a liquid, for example water. The exact solutions to the corresponding field equations are obtained for exponential and power-law volumetric expansion. It is observed that the universe can approach to isotropy monotonically even in the presence of wet dark fluid. Also we have discussed the well-known astrophysical phenomena, namely the look-back time, proper distance, the luminosity distance and angular diameter distance with redshift.
On cosmic acceleration without dark energy
Kolb, E.W.; /Fermilab /Chicago U., Astron. Astrophys. Ctr. /Chicago U., EFI; Matarrese, S.; /Padua U. /INFN, Padua; Riotto, A.; /INFN, Padua; ,
2005-06-01
We elaborate on the proposal that the observed acceleration of the Universe is the result of the backreaction of cosmological perturbations, rather than the effect of a negative-pressure dark energy fluid or a modification of general relativity. Through the effective Friedmann equations describing an inhomogeneous Universe after smoothing, we demonstrate that acceleration in our local Hubble patch is possible even if fluid elements do not individually undergo accelerated expansion. This invalidates the no-go theorem that there can be no acceleration in our local Hubble patch if the Universe only contains irrotational dust. We then study perturbatively the time behavior of general-relativistic cosmological perturbations, applying, where possible, the renormalization group to regularize the dynamics. We show that an instability occurs in the perturbative expansion involving sub-Hubble modes, which indicates that acceleration in our Hubble patch may originate from the backreaction of cosmological perturbations on observable scales.
Holographic tachyon model of dark energy
M R Setare
2007-09-11
In this paper we consider a correspondence between the holographic dark energy density and tachyon energy density in FRW universe. Then we reconstruct the potential and the dynamics of the tachyon field which describe tachyon cosmology.
DBI models for the unification of dark matter and dark energy
NASA Astrophysics Data System (ADS)
Chimento, Luis P.; Lazkoz, Ruth; Sendra, Irene
2010-06-01
We propose a model based on a DBI action for the unification of dark matter and dark energy. This is supported by the results of the study of its background behavior at early and late times, and reinforced by the analysis of the evolution of perturbations. We also perform a Bayesian analysis to set observational constraints on the parameters of the model using type Ia SN, CMB shift and BAO data. Finally, to complete the study we investigate its kinematics aspects, such as the effective equation of state parameter, acceleration parameter and transition redshift. Particularizing them for the best fit one appreciates that an effective phantom behavior is preferred.
Can we test Dark Energy with Running Fundamental Constants ?
Michael Doran
2005-04-26
We investigate a link between the running of the fine structure constant $\\alpha$ and a time evolving scalar dark energy field. Employing a versatile parameterization for the equation of state, we exhaustively cover the space of dark energy models. Under the assumption that the change in $\\alpha$ is to first order given by the evolution of the Quintessence field, we show that current Oklo, Quasi Stellar Objects and Equivalence Principle observations restrict the model parameters considerably stronger than observations of the Cosmic Microwave Background, Large Scale Structure and Supernovae Ia combined.
Precision Studies of Dark Energy with LSST
J. Anthony Tyson; for the LSST Collaboration
2006-09-19
Starting around 2013, data from the Large Synoptic Survey Telescope (LSST) will be analyzed for a wide range of phenomena. By separately tracing the development of mass structure and rate of expansion of the universe, these data will address the physics of dark matter and dark energy, the possible existence of modified gravity on large scales, large extra dimensions, the neutrino mass, and possible self interaction of dark matter particles.
Revealing the Nature of Dark Energy Using Bayesian Evidence
T. D. Saini; J. Weller; S. L. Bridle
2003-05-28
We apply the Bayesian concept of `evidence' to reveal systematically the nature of dark energy from present and future supernova luminosity distance measurements. We express the unknown dark energy equation of state w(z) as a low order polynomial in redshift and use evidence to find the polynomial order, thereby establishing the minimum order required by the data. We apply this method to the current supernova data, and with a prior -1 evidence for future supernova data sets such as distances obtainable from surveys like the Supernova Acceleration Probe (SNAP). Given a low uncertainty on the present day matter density we find that, if the underlying dark energy model is only modestly evolving, then a constant $w(z)$ fit is sufficient. However, if the evolution of the dark energy equation of state to linear order is larger than |w_1| ~ 0.5, then the evolution can be established with statistical significance. For models where we can assume the prior -1 < w(z) < 1, the correct polynomial order can be established even for modestly evolving equations of state.
?CDM coupled to radiation: Dark energy and Universe acceleration
NASA Astrophysics Data System (ADS)
Abbyazov, Renat R.; Chervon, Sergey V.; Müller, Volker
2015-07-01
Recently, the Chiral Cosmological Model (CCM) coupled to cold dark matter (CDM) has been investigated as ?CDM model to study the observed accelerated expansion of the Universe. Dark sector fields (as Dark Energy content) coupled to cosmic dust were considered as the source of Einstein gravity in Friedmann-Robertson-Walker (FRW) cosmology. Such model had a beginning at the matter-dominated era. The purposes of our present investigation are two-fold: To extend “life” of the ?CDM for earlier times to radiation-dominated era and to take into account variation of the exponential potential V = V0exp ?? ? MP + V0exp ?? ? MP via variation of the interaction parameter ?. We use Markov Chain Monte Carlo (MCMC) procedure to investigate possible values of initial conditions constrained by the measured amount of the dark matter, dark energy and radiation component today. Our analysis includes dark energy contribution to critical density, the ratio of the kinetic and potential energies, deceleration parameter, effective equation of state (EoS) and evolution of DE EoS with variation of coupling constant ?. A comparison with the ?CDM model was performed. A new feature of the model is the existence of some values of potential coupling constant, leading to a ?CDM solution without transition into accelerated expansion epoch.
Is dark energy an effect of averaging?
Nan Li; Marina Seikel; Dominik J. Schwarz
2008-01-22
The present standard model of cosmology states that the known particles carry only a tiny fraction of total mass and energy of the Universe. Rather, unknown dark matter and dark energy are the dominant contributions to the cosmic energy budget. We review the logic that leads to the postulated dark energy and present an alternative point of view, in which the puzzle may be solved by properly taking into account the influence of cosmic structures on global observables. We illustrate the effect of averaging on the measurement of the Hubble constant.
A quantum model of dark energy
Chang-Yu Zhu; Heng Fan
2009-11-20
We propose a quantum model of dark energy. The proposed candidate for dark energy is gluon field, as is well-known, gluons are the elementary particles. We assume that gluons may not be completely massless but have tiny masses, thus the gluon field can provide a non-zero energy-momentum tensor. This model corresponds to Einstein's cosmological constant which is one of the generally accepted models for dark energy. Besides the gluon field, we also discuss the properties of electroweak boson field and compare our results with previous known results.
Anisotropic cosmological models with perfect fluid and dark energy revisited
Bijan Saha
2005-01-21
We consider a self-consistent system of Bianchi type-I (BI) gravitational field and a binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be the one obeying the usual equation of state, i.e., $p = \\zeta \\ve$, with $\\zeta \\in [0, 1]$ whereas, the dark energy is considered to be obeying a quintessence-like equation of state. Exact solutions to the corresponding Einstein equations are obtained. The model in consideration gives rise to a Universe which is spatially finite. Depending on the choice of problem parameters the Universe is either close with a space-time singularity, or an open one which is oscillatory, regular and infinite in time.
Reconstructing f(R) modified gravity with dark energy parametrization
NASA Astrophysics Data System (ADS)
Morita, Masaaki; Takahashi, Hirotaka
2014-03-01
We demonstrate the reconstruction of f(R) modified gravity theory with late-time accelerated cosmic expansion. A second-order differential equation for Lagrangian density is obtained from the field equation, and is solved as a function of the cosmic scale factor in two cases. First we begin with the case of a wCDM cosmological model, in which a dark-energy equation-of-state parameter w is constant, for simplicity. Next we extend the method to a case in which the parameter w is epoch-dependent and is expressed as the Chevallier-Polarski-Linder parametrization. Thus we can represent Lagrangian density of f(R) modified gravity theory in terms of dark energy parameters.
From the Dark Matter Universe to the Dark Energy Universe
Burra G. Sidharth
2008-03-30
Till the late nineties the accepted cosmological model was that of a Universe that had originated in the Big Bang and was now decelerating under the influence of as yet undetected dark matter, so that it would come to a halt and eventually collapse. In 1997 however, the author had put forward a contra model wherein the Universe was driven by dark energy, essentially the quantum zero point field, and was accelerating with a small cosmological constant. There were other deductions too, all in total agreement with observation. All this got confirmation in 1998 and subsequent observations have reconfirmed the findings.
Dark matter and dark energy from Bose-Einstein condensate
Saurya Das; Rajat K. Bhaduri
2015-03-25
We show that Dark Matter consisting of bosons of mass of about 1eV or less has critical temperature exceeding the temperature of the universe at all times, and hence would have formed a Bose-Einstein condensate at very early epochs. We also show that the wavefunction of this condensate, via the quantum potential it produces, gives rise to a cosmological constant which may account for the correct dark energy content of our universe. We argue that massive gravitons or axions are viable candidates for these constituents. In the far future this condensate is all that remains of our universe.
Dark Energy: Observational Evidence and Theoretical Models
B. Novosyadlyj; V. Pelykh; Yu. Shtanov; A. Zhuk
2015-02-14
The book elucidates the current state of the dark energy problem and presents the results of the authors, who work in this area. It describes the observational evidence for the existence of dark energy, the methods and results of constraining of its parameters, modeling of dark energy by scalar fields, the space-times with extra spatial dimensions, especially Kaluza---Klein models, the braneworld models with a single extra dimension as well as the problems of positive definition of gravitational energy in General Relativity, energy conditions and consequences of their violation in the presence of dark energy. This monograph is intended for science professionals, educators and graduate students, specializing in general relativity, cosmology, field theory and particle physics.
Dark Energy: Observational Evidence and Theoretical Models
Novosyadlyj, B; Shtanov, Yu; Zhuk, A
2015-01-01
The book elucidates the current state of the dark energy problem and presents the results of the authors, who work in this area. It describes the observational evidence for the existence of dark energy, the methods and results of constraining of its parameters, modeling of dark energy by scalar fields, the space-times with extra spatial dimensions, especially Kaluza---Klein models, the braneworld models with a single extra dimension as well as the problems of positive definition of gravitational energy in General Relativity, energy conditions and consequences of their violation in the presence of dark energy. This monograph is intended for science professionals, educators and graduate students, specializing in general relativity, cosmology, field theory and particle physics.
Gravity effects of the quantum vacuum. Dark energy and dark matter
Emilio Santos
2015-05-09
The stress-energy tensor of the quantum vacuum is studied for the particular case of quantum electrodynamics (QED), that is a fictituous universe where only the electromagnetic and the electron-positron fields exist. The integrals involved are ultraviolet divergent but it is suggested that a natural cut-off may exist. It is shown that, in spite of the fact that the stress-energy tensor of the electromagnetic field alone is traceless (i.e the pressure P equals 1/3 the energy density u), the total QED tensor is proportional to the metric tensor to a good approximation (i. e. P = -u). It is proposed that there is a cosmological constant in Einstein equation that exactly balances the stress-energy of the vacuum. It is shown that vacuum fluctuations give rise to a modified spacetime metric able to explain dark energy. Particular excitations of the vacuum are studied that might explain dark matter.
Gravity effects of the quantum vacuum. Dark energy and dark matter
Santos, Emilio
2015-01-01
The stress-energy tensor of the quantum vacuum is studied for the particular case of quantum electrodynamics (QED), that is a fictituous universe where only the electromagnetic and the electron-positron fields exist. The integrals involved are ultraviolet divergent but it is suggested that a natural cut-off may exist. It is shown that, in spite of the fact that the stress-energy tensor of the electromagnetic field alone is traceless (i.e the pressure P equals 1/3 the energy density u), the total QED tensor is proportional to the metric tensor to a good approximation (i. e. P = -u). It is proposed that there is a cosmological constant in Einstein equation that exactly balances the stress-energy of the vacuum. It is shown that vacuum fluctuations give rise to a modified spacetime metric able to explain dark energy. Particular excitations of the vacuum are studied that might explain dark matter.
Dark Matter and Dark Energy - Fact or Fantasy?
NASA Astrophysics Data System (ADS)
Mannheim, Philip
We show that the origin of the dark matter and dark energy problems originates in the assumption of standard Einstein gravity that Newton's constant is fundamental. We discuss an alternate, conformal invariant, metric theory of gravity in which Newton's constant is induced dynamically, with the global induced one which is effective for cosmology being altogether weaker than the local induced one needed for the solar system. We find that in the theory dark matter is no longer needed, and that the accelerating universe data can be fitted without fine-tuning using a cosmological constant as large as particle physics suggests. In the conformal theory then it is not the cosmological constant which is quenched but rather the amount of gravity that it produces.
Tachyon as a Dark Energy Source
S. K. Srivastava
2005-03-10
It is demonstrated that dark energy, driven by tachyons having non-minimal coupling with curvature and self-interacting inverse cubic potential, decays to cold dark matter in the late universe. It is found that this penomenon yields a solution to `` cosmic coincidence problem''.
Report of the Dark Energy Task Force
Andreas Albrecht; Gary Bernstein; Robert Cahn; Wendy L. Freedman; Jacqueline Hewitt; Wayne Hu; John Huth; Marc Kamionkowski; Edward W. Kolb; Lloyd Knox; John C. Mather; Suzanne Staggs; Nicholas B. Suntzeff
2006-01-01
Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short
Dark Energy and Tachyon Field in Bianchi Type-V Space-time
Sadeghi, J
2014-01-01
In this paper, we consider Bianchi type-V space-time and study a cosmological model of dark energy based on Tachyon scalar field. We assumed three different kinds of matter without possibility of interaction with scalar dark energy. Assuming power law Hubble parameter in terms of scale factor we obtain evolution of scalar field, scalar potential and equation of state parameter.
Tachyon field as a dark energy in Bianchi type-V spacetime
NASA Astrophysics Data System (ADS)
Sadeghi, J.; Farahani, H.
2015-01-01
In this paper, we consider Bianchi type-V spacetime and study a cosmological model of dark energy based on tachyon scalar field. We assumed three different kinds of matter without possibility of interaction with scalar dark energy. Assuming power law Hubble parameter in terms of scale factor we obtain evolution of scalar field, scalar potential and equation of state parameter.
Dark Energy and Tachyon Field in Bianchi Type-V Space-time
J. Sadeghi; H. Farahani
2014-04-15
In this paper, we consider Bianchi type-V space-time and study a cosmological model of dark energy based on Tachyon scalar field. We assumed three different kinds of matter without possibility of interaction with scalar dark energy. Assuming power law Hubble parameter in terms of scale factor we obtain evolution of scalar field, scalar potential and equation of state parameter.
Dark energy appears to have caused the expansion of our universe.
NASA Astrophysics Data System (ADS)
Gehrels, Tom
A new paradigm based on an equation of Chandrasekhar points directly to a specific multi-verse(http://arXiv.org/abs/0912.5367 and references there in). Dark energy accelerating the expansion is an essential part of the model -in turn, it shows that dark energy may have caused the expansion to begin with. At the present time, 72
Bianchi Type III Anisotropic Dark Energy Models with Constant Deceleration Parameter
Anil Kumar Yadav; Lallan Yadav
2010-10-02
The Bianchi type III dark energy models with constant deceleration parameter are investigated. The equation of state parameter $\\omega$ is found to be time dependent and its existing range for this model is consistent with the recent observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy clustering statistics. The physical aspect of the dark energy models are discussed.
NASA Astrophysics Data System (ADS)
Katore, S. D.; Hatkar, S. P.
2015-01-01
In this paper, the authors have investigated the Kaluza Klein universe with magnetized anisotropic dark energy in the context of Lyra manifold. Exponential and power law volumetric expansion is assumed to obtain the solution of the field equations. It is observed that magnetic field plays significant role in isotropization of the dark energy. The physical parameters of the models have been discussed in detail.
Evolution of entropic dark energy and its phantom nature
Mathew, Titus K; J, Shejeelammal
2015-01-01
Assuming the form of the entropic dark energy as arises form the surface term in the Einstein-Hilbert's action, it's evolution were analyzed in an expanding flat universe. The model parameters were evaluated by constraining model using the Union data on Type Ia supernovae. We found that the model predicts an early decelerated phase and a later accelerated phase at the background level. The evolution of the Hubble parameter, dark energy density, equation of state parameter and deceleration parameter were obtained. The model is diagnosed with $Om$ parameter. The model is hardly seems to be supporting the linear perturbation growth for the structure formation. We also found that the entropic dark energy shows phantom nature for redshifts $zinfinitely large and the big rip time is found to be around 36 Giga Years from now.
Gravastars and Black Holes of Anisotropic Dark Energy
R. Chan; M. F. A. da Silva; P. Rocha
2011-03-31
Dynamical models of prototype gravastars made of anisotropic dark energy are constructed, in which an infinitely thin spherical shell of a perfect fluid with the equation of state $p = (1-\\gamma)\\sigma$ divides the whole spacetime into two regions, the internal region filled with a dark energy fluid, and the external Schwarzschild region. The models represent "bounded excursion" stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes. Here we show, for the first time in the literature, a model of gravastar and formation of black hole with both interior and thin shell constituted exclusively of dark energy. Besides, the sign of the parameter of anisotropy ($p_t - p_r$) seems to be relevant to the gravastar formation. The formation is favored when the tangential pressure is greater than the radial pressure, at least in the neighborhood of the isotropic case ($\\omega=-1$).
Fine-structure constant constraints on dark energy
C. J. A. P. Martins; A. M. M. Pinho
2015-05-08
We use astrophysical and atomic clock tests of the stability of the fine-structure constant $\\alpha$, together with Type Ia supernova and Hubble parameter data, to constrain the simplest class of dynamical dark energy models where the same degree of freedom is assumed to provide both the dark energy and (through a dimensionless coupling, $\\zeta$, to the electromagnetic sector) the $\\alpha$ variation. We show how current data tightly constrains a combination of $\\zeta$ and the dark energy equation of state $w_0$. At the $95\\%$ confidence level and marginalizing over $w_0$ we find $|\\zeta|atomic clock tests dominating the constraints. The forthcoming generation of high-resolution ultra-stable spectrographs will enable significantly tighter constraints.
Dark Energy, A Cosmological Constant, and Type Ia Supernovae
Lawrence M. Krauss; Katherine Jones-Smith; Dragan Huterer
2007-01-24
We focus on uncertainties in supernova measurements, in particular of individual magnitudes and redshifts, to review to what extent supernovae measurements of the expansion history of the universe are likely to allow us to constrain a possibly redshift-dependent equation of state of dark energy, $w(z)$. focus in particular on the central question of how well one might rule out the possibility of a cosmological constant $w=-1$. We argue that it is unlikely that we will be able to significantly reduce the uncertainty in the determination of $w$ beyond its present bounds, without significant improvements in our ability to measure the cosmic distance scale as a function of redshift. Thus, unless the dark energy significantly deviates from $w(z)=-1$ at some redshift, very stringent control of the statistical and systematic errors will be necessary to have a realistic hope of empirically distinguishing exotic dark energy from a cosmological constant.
What We Know About Dark Energy From Supernovae
Filippenko, Alex [University of California, Berkeley, California, United States
2010-01-08
The measured distances of type Ia (white dwarf) supernovae as a function of redshift (z) have shown that the expansion of the Universe is currently accelerating, probably due to the presence of dark energy (X) having a negative pressure. Combining all of the data with existing results from large-scale structure surveys, we find a best fit for Omega M and Omega X of 0.28 and 0.72 (respectively), in excellent agreement with the values derived independently from WMAP measurements of the cosmic microwave background radiation. Thus far, the best-fit value for the dark energy equation-of-state parameter is -1, and its first derivative is consistent with zero, suggesting that the dark energy may indeed be Einstein's cosmological constant.
Comparison of Supernovae datasets Constraints on Dark Energy
Chengwu Zhang; Lixin Xu; Baorong Chang; Hongya Liu
2007-04-23
Cosmological measurements suggest that our universe contains a dark energy component. In order to study the dark energy evolution, we constrain a parameterized dark energy equation of state $w(z)=w_0 + w_1 \\frac{z}{1+z}$ using the recent observational datasets: 157 Gold type Ia supernovae and the newly released 182 Gold type Ia supernovae by maximum likelihood method. It is found that the best fit $w(z)$ crosses -1 in the past and the present best fit value of $w(0)<-1$ obtained from 157 Gold type Ia supernovae. The crossing of -1 is not realized and $w_0=-1$ is not ruled out in $1\\sigma$ confidence level for the 182 Gold type Ia supernovae. We also find that the range of parameter $w_0$ is wide even in $1\\sigma$ confidence level and the best fit $w(z)$ is sensitive to the prior of $\\Omega_m$.
Dark Matter and Dark Energy in the Universe
Michael S. Turner
2000-01-01
For the first time, we have a plausible and complete accounting of matter and energy in the Universe. Expressed a fraction of the critical density it goes like this: neutrinos, between 0.3% and 15%; stars, between 0.3% and 0.6%; baryons (total), 5% ± 0.5%; matter (total), 40% ± 10%; smooth, dark energy, 80% ± 20%; totaling to the critical density
a Comprehensive Model of Dark Energy, Inflation and Black Holes
NASA Astrophysics Data System (ADS)
Biermann, Peter L.; Harms, Benjamin C.
2015-01-01
We derive two new equations of quantum gravity and combine them with reinterpretations of previously proposed concepts of dark energy, black holes, inflation, the arrow of time and the energy at which rest-mass first manifests itself into a theory which may be a first step toward a comprehensive description of all these phenomena. The resulting theory also predicts new tests which can be experimentally checked within a few years.
Is Hubble's Expansion due to Dark Energy
R. C. Gupta; Anirudh Pradhan
2010-10-19
{\\it The universe is expanding} is known (through Galaxy observations) since 1929 through Hubble's discovery ($V = H D$). Recently in 1999, it is found (through Supernovae observations) that the universe is not simply expanding but is accelerating too. We, however, hardly know only $4\\%$ of the universe. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite observational data suggest $73\\%$ content of the universe in the form of dark-energy, $23\\%$ in the form of non-baryonic dark-matter and the rest $4\\%$ in the form of the usual baryonic matter. The acceleration of the universe is ascribed to this dark-energy with bizarre properties (repulsive-gravity). The question is that whether Hubble's expansion is just due to the shock of big-bang & inflation or it is due to the repulsive-gravity of dark-energy? Now, it is believed to be due to dark-energy, say, by re-introducing the once-discarded cosmological-constant $\\Lambda$. In the present paper, it is shown that `the formula for acceleration due to dark-energy' is (almost) exactly of same-form as `the acceleration formula from the Hubble's law'. Hence, it is concluded that: yes, `indeed it is the dark-energy responsible for the Hubble's expansion too, in-addition to the current on-going acceleration of the universe'.
Spinor dark energy and cosmological coincidence problem
NASA Astrophysics Data System (ADS)
Wei, Hao
2011-01-01
Recently, the so-called Elko spinor field has been proposed to be a candidate of dark energy. It is a non-standard spinor and has unusual properties. When the Elko spinor field is used in cosmology, its unusual properties could bring some interesting consequences. In the present work, we discuss the cosmological coincidence problem in the spinor dark energy models by using the dynamical system method. Our results show that the cosmological coincidence problem should be taken to heart in the investigations of spinor dark energy models.
G-corrected holographic dark energy model
Malekjani, M
2013-01-01
Here we investigate the holographic dark energy model in the framework of FRW cosmology where the Newtonian gravitational constant,$G$, is varying with cosmic time. Using the complementary astronomical data which support the time dependency of $G$, the evolutionary treatment of EoS parameter and energy density of dark energy model are calculated in the presence of time variation of $G$. It has been shown that in this case, the phantom regime can be achieved at the present time. We also calculate the evolution of $G$- corrected deceleration parameter for holographic dark energy model and show that the dependency of $G$ on the comic time can influence on the transition epoch from decelerated expansion to the accelerated phase. Finally we perform the statefinder analysis for $G$- corrected holographic model and show that this model has a shorter distance from the observational point in $s-r$ plane compare with original holographic dark energy model.
G-corrected holographic dark energy model
NASA Astrophysics Data System (ADS)
Malekjani, M.; Honari-Jafarpour, M.
2013-08-01
Here we investigate the holographic dark energy model in the framework of FRW cosmology where the Newtonian gravitational constant, G, is varying with cosmic time. Using the complementary astronomical data which support the time dependency of G, the evolutionary treatment of EoS parameter and energy density of dark energy model are calculated in the presence of time variation of G. It has been shown that in this case, the phantom regime can be achieved at the present time. We also calculate the evolution of G-corrected deceleration parameter for holographic dark energy model and show that the dependency of G on the comic time can influence on the transition epoch from decelerated expansion to the accelerated phase. Finally we perform the statefinder analysis for G-corrected holographic model and show that this model has a shorter distance from the observational point in s- r plane compare with original holographic dark energy model.
G-corrected holographic dark energy model
M. Malekjani; M. Honari-Jafarpour
2013-05-01
Here we investigate the holographic dark energy model in the framework of FRW cosmology where the Newtonian gravitational constant,$G$, is varying with cosmic time. Using the complementary astronomical data which support the time dependency of $G$, the evolutionary treatment of EoS parameter and energy density of dark energy model are calculated in the presence of time variation of $G$. It has been shown that in this case, the phantom regime can be achieved at the present time. We also calculate the evolution of $G$- corrected deceleration parameter for holographic dark energy model and show that the dependency of $G$ on the comic time can influence on the transition epoch from decelerated expansion to the accelerated phase. Finally we perform the statefinder analysis for $G$- corrected holographic model and show that this model has a shorter distance from the observational point in $s-r$ plane compare with original holographic dark energy model.
Self-interacting Dark Matter Energy Density
Stiele, Rainer; Schaffner-Bielich, Jurgen
2011-01-01
We investigate cosmological implications of an energy density contribution arising by elastic dark matter self-interactions. Its scaling behaviour shows that it can be the dominant energy contribution in the early universe. Constraints from primordial nucleosynthesis give an upper limit on the self-interaction strength which allows for the same strength as standard model strong interactions. Furthermore we explore the cosmological consequences of an early self-interaction dominated universe. Chemical dark matter decoupling requires that self-interacting dark matter particles are rather light (keV range) but we find that super-weak inelastic interactions are predicted by strong elastic dark matter self-interactions. Assuming a second, collisionless cold dark matter component, its natural decoupling scale exceeds the weak scale and is in accord with the electron and positron excess observed by PAMELA and Fermi-LAT. Structure formation analysis reveals a linear growing solution during self-interaction domination...
Holographic Dark Energy in Brans-Dicke Theory
Lixin Xu; Wenbo Li; Jianbo Lu
2009-04-14
In this paper, the holographic dark energy model is considered in Brans-Dicke theory where the holographic dark energy density $\\rho_{\\Lambda} =3c^2 M^{2}_{pl} L^{-2}$ is replaced with $\\rho_{h}=3c^2 \\Phi(t)L^{-2}$. Here $\\Phi(t)=\\frac{1}{8\\pi G}$ is a time variable Newton constant. With this replacement, it is found that no accelerated expansion universe will be achieved when the Hubble horizon is taken as the role of IR cut-off. When the event horizon is adopted as the IR cut-off, an accelerated expansion universe is obtained. In this case, the equation of state of holographic dark energy $w_h$ takes a modified form $w_h=-{1/3}(1+\\alpha+\\frac{2}{c}\\sqrt{\\Omega_{h}})$. In the limit $\\alpha\\to 0$, the 'standard' holographic dark energy is recovered. In the holographic dark energy dominated epoch, power-law and de Sitter time-space solutions are obtained.
James G. Gilson
2012-06-23
A solution to Einstein's field equations via the Friedman equations is shown to produce a cosmological model that is in exact agreement with the measurements made by the dark energy astronomers. All the essential physical parameters are obtained as epoch dependent functions all in closed form. The equations of state are obtained for total density, non-dark energy density and dark energy density. An interpretation of the structure involving a dark energy mass distribution that is twice the usual value is shown to clarify greatly the physical significance of the mathematics. It is asserted that the astronomer's measurements together with the mathematical model proves that the universe is permeated uniformly with a positive mass density that caries a negative gravitational constant, -G, characteristic. This mass component is identified with the dark energy content of the universe that has been postulated to explain the observed acceleration. Another result implied by the model is that there is twice the amount of dark energy that is usually considered to be present. This last point is analysed in more detail in appendix 1 using Einstein's field equations. Five additional appendices, 2, 3, 4, 5 and 6 in which isothermal gravitational dark matter equilibrium and the galactic rotations curve flatness problem are examined in detail. Appendix 5 is concerned with mass clumping and expressing gravitational isothermal equilibrium constraints using a cosmological Schr\\"odinger equation to demonstrate the existence of a new quantum force involved with galactic stability. Appendix 6 is concerned with gravitational quantization. Each appendix has its own abstract.
On dark energy models of single scalar field
Li, Mingzhe; Qiu, Taotao; Cai, Yifu; Zhang, Xinmin E-mail: xsjqiu@gmail.com E-mail: xmzhang@ihep.ac.cn
2012-04-01
In this paper we revisit the dynamical dark energy model building based on single scalar field involving higher derivative terms. By imposing a degenerate condition on the higher derivatives in curved spacetime, one can select the models which are free from the ghost mode and the equation of state is able to cross the cosmological constant boundary smoothly, dynamically violate the null energy condition. Generally the Lagrangian of this type of dark energy models depends on the second derivatives linearly. It behaves like an imperfect fluid, thus its cosmological perturbation theory needs to be generalized. We also study such a model with explicit form of degenerate Lagrangian and show that its equation of state may cross -1 without any instability.
NASA Astrophysics Data System (ADS)
Balfagon, Alberto C.
2015-10-01
Particle creation has been considered as a possible justification for the accelerated expansion of the universe, obeying the second law of thermodynamics, together with the possible existence of dark energy. This paper introduces the possibility that the destruction of baryonic and/or dark matter particles also verifies the second law of thermodynamics thanks to a particle exchange with dark energy. General equations for the variation of the number of particles in accelerated universes have been obtained. Finally, a new model of the universe has been developed which predicts dark energy properties as well as particle exchange processes between dark energy and baryonic and/or dark matter.
Alberto C. Balfagon
2015-10-02
Particle creation has been considered as a possible justification for the accelerated expansion of the universe, obeying the second law of thermodynamics, together with the possible existence of Dark Energy. This paper introduces the possibility that the destruction of baryonic and/or dark matter particles also verifies the second law of thermodynamics thanks to a particle exchange with dark energy. General equations for the variation of the number of particles in accelerated universes have been obtained. Finally, a new model of the universe has been developed which predicts dark energy properties as well as particle exchange processes between dark energy and baryonic and/or dark matter.
Reconstruction of Hessence Dark Energy and the Latest Type Ia Supernovae Gold Dataset
Hao Wei; Ningning Tang; Shuang Nan Zhang
2007-02-28
Recently, many efforts have been made to build dark energy models whose equation-of-state parameter can cross the so-called phantom divide $w_{de}=-1$. One of them is the so-called hessence dark energy model in which the role of dark energy is played by a non-canonical complex scalar field. In this work, we develop a simple method based on Hubble parameter $H(z)$ to reconstruct the hessence dark energy. As examples, we use two familiar parameterizations for $H(z)$ and fit them to the latest 182 type Ia supernovae Gold dataset. In the reconstruction, measurement errors are fully considered.
Energy Conservation Equations of Motion
Vinokurov, Nikolay A
2015-01-01
A conventional derivation of motion equations in mechanics and field equations in field theory is based on the principle of least action with a proper Lagrangian. With a time-independent Lagrangian, a function of coordinates and velocities that is called energy is constant. This paper presents an alternative approach, namely derivation of a general form of equations of motion that keep the system energy, expressed as a function of generalized coordinates and corresponding velocities, constant. These are Lagrange equations with addition of gyroscopic forces. The important fact, that the energy is defined as the function on the tangent bundle of configuration manifold, is used explicitly for the derivation. The Lagrangian is derived from a known energy function. A development of generalized Hamilton and Lagrange equations without the use of variational principles is proposed. The use of new technique is applied to derivation of some equations.
Spiral Galaxies from a Dark Matter Solution of Einstein's Equations
NASA Astrophysics Data System (ADS)
Ringermacher, H. I.; Mead, L. R.
2005-12-01
We describe an exact 5-D solution of Einstein's equations based upon a hyperbolic axisymmetric Poincaré 3-space, (dx2+dy^2+dz^2)/ (1+B(x+y))2, that appears to represent the geometry due to dark matter alone underlying the structure of spiral galaxies. The triaxial 3-space, (dx2+dy^2+dz^2) /(1+B(x+y+z))2, is also a solution. The observed Euclidean 3-space and the hyperbolic space of the dark matter do not have a common center so that care must be taken in calculating proper length. The 4th and 5th dimensions are proper time and cosmological time (epoch), respectively. Geodesic motion yields observed flat rotation curves of spirals. From these, we obtain "isochrones": the locus of stellar positions at fixed proper time and epoch in their motion along geodesics. Isochrones are of the form r(? )=A/ln[B tan(? /2N)], where B(0.05-3.0) and N(2-64) are shape constants, while A scales. The isochrones yield the structures of the Hubble spiral galaxy classes including barred and grand designs. Polar ring solutions are found including angled rings, as well as warped "integral" and "banana" rings. Other galactic problems are also resolved. The similarity of rotation curves for grand spirals and bars, first pointed out by Rubin, is explained by a duality transformation of the parameters describing grands and bars in the rotation curves. The 75% abundance of spirals arises from the dimensional symmetry of the Poincaré 3-space: there are 3 axisymmetric coordinate permutations (x+y, x+z, y+z) and only 1 triaxial (x+y+z), amounting to geometric "equipartition". The solution correctly represents the known effects of dark matter in galaxies and thereby suggests that this geometry provides the skeletal stabilizing structure underlying the dynamics attributed to dark matter into which baryonic matter may be added.
Evolution of perturbations in distinct classes of canonical scalar field models of dark energy
Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)
2010-04-15
Dark energy must cluster in order to be consistent with the equivalence principle. The background evolution can be effectively modeled by either a scalar field or by a barotropic fluid. The fluid model can be used to emulate perturbations in a scalar field model of dark energy, though this model breaks down at large scales. In this paper we study evolution of dark energy perturbations in canonical scalar field models: the classes of thawing and freezing models. The dark energy equation of state evolves differently in these classes. In freezing models, the equation of state deviates from that of a cosmological constant at early times. For thawing models, the dark energy equation of state remains near that of the cosmological constant at early times and begins to deviate from it only at late times. Since the dark energy equation of state evolves differently in these classes, the dark energy perturbations too evolve differently. In freezing models, since the equation of state deviates from that of a cosmological constant at early times, there is a significant difference in evolution of matter perturbations from those in the cosmological constant model. In comparison, matter perturbations in thawing models differ from the cosmological constant only at late times. This difference provides an additional handle to distinguish between these classes of models and this difference should manifest itself in the integrated Sachs-Wolfe effect.
Essential building blocks of dark energy
Gleyzes, Jerome; Vernizzi, Filippo [CEA, IPhT, 91191 Gif-sur-Yvette cédex, France CNRS, URA-2306, 91191 Gif-sur-Yvette cédex (France); Langlois, David; Piazza, Federico, E-mail: jerome.gleyzes@cea.fr, E-mail: langlois@apc.univ-paris7.fr, E-mail: fpiazza@apc.univ-paris7.fr, E-mail: filippo.vernizzi@cea.fr [APC, (CNRS-Université Paris 7), 10 rue Alice Domon et Léonie Duquet, 75205 Paris (France)
2013-08-01
We propose a minimal description of single field dark energy/modified gravity within the effective field theory formalism for cosmological perturbations, which encompasses most existing models. We start from a generic Lagrangian given as an arbitrary function of the lapse and of the extrinsic and intrinsic curvature tensors of the time hypersurfaces in unitary gauge, i.e. choosing as time slicing the uniform scalar field hypersurfaces. Focusing on linear perturbations, we identify seven Lagrangian operators that lead to equations of motion containing at most two (space or time) derivatives, the background evolution being determined by the time-dependent coefficients of only three of these operators. We then establish a dictionary that translates any existing or future model whose Lagrangian can be written in the above form into our parametrized framework. As an illustration, we study Horndeski's — or generalized Galileon — theories and show that they can be described, up to linear order, by only six of the seven operators mentioned above. This implies, remarkably, that the dynamics of linear perturbations can be more general than that of Horndeski while remaining second order. Finally, in order to make the link with observations, we provide the entire set of linear perturbation equations in Newtonian gauge, the effective Newton constant in the quasi-static approximation and the ratio of the two gravitational potentials, in terms of the time-dependent coefficients of our Lagrangian.
Holographic dark energy in the DGP model
Norman Cruz; Samuel Lepe; Francisco Peña
2011-09-09
The braneworld model proposed by Dvali, Gabadadze and Porrati leads to an accelerated universe without cosmological constant or other form of dark energy. Nevertheless, we have investigated the consequences of this model when an holo- graphic dark energy is included, taken the Hubble scale as IR cutoff. We have found that the holographic dark energy leads to an accelerated universe flat (de Sitter like expansion) for the two branch: {\\ko} = \\pm1 of the DGP model. Nevertheless, in universes with no null curvature the dark energy presents an EoS corresponding to a phantom fluid during the present era and evolving to a de Sitter like phase for future cosmic time. In the special case in which the holographic parameter c is equal to one we have found a sudden singularity in closed universes. In this case the expansion is decelerating. Manuscript
Particle mixing, flavor condensate and dark energy
Massimo Blasone; Antonio Capolupo; Giuseppe Vitiello
2009-12-08
The mixing of neutrinos and quarks generate a vacuum condensate that, at the present epoch, behaves as a cosmological constant. The value of the dark energy is constrained today by the very small breaking of the Lorentz invariance.
Neutrino mixing, flavor states and dark energy
M. Blasone; A. Capolupo; S. Capozziello; G. Vitiello
2007-11-06
We shortly summarize the quantum field theory formalism for the neutrino mixing and report on recent results showing that the vacuum condensate induced by neutrino mixing can be interpreted as a dark energy component of the Universe.
Lectures on Dark Energy and Cosmic Acceleration
Frieman, Joshua A
2009-01-01
The discovery ten years ago that the expansion of the Universe is accelerating put in place the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. Yet the underlying cause of cosmic acceleration remains a mystery: it could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. In these lectures, I present the observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss a few of the theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that we hope will shed light on this enigma in the coming years.
Towards Dark Energy from String-Theory
Axel Krause
2008-03-12
We discuss vacuum energy in string and M-theory with a focus on heterotic M-theory. In the latter theory a mechanism is described for maintaining zero vacuum energy after supersymmetry breaking. Higher-order corrections can be expected to give a sufficiently small amount of vacuum energy to possibly account for dark energy.
Dark energy as double N-flation - observational predictions
NASA Astrophysics Data System (ADS)
Gott, J. Richard; Slepian, Zachary
2011-09-01
We propose a simple model for dark energy useful for comparison with observations. It is based on the idea that dark energy and inflation should be caused by the same physical process. As motivation, we note that Linde's simple chaotic inflation ? produces values of ns= 0.967 and r= 0.13, which are consistent with the Wilkinson Microwave Anisotropy Probe (WMAP) 1? error bars. We therefore propose ? with m1˜ 10-5 and m2? 10-60, where c= 1 =? and the reduced Planck mass is set to unity. The field ?1 drives inflation and has damped by now (?1, 0= 0), while ?2 is currently rolling down its potential to produce dark energy. Using this model, we derive the formula ?w(z) ?w(z) + 1 =?w0(H0/H(z))2 via the slow-roll approximation. Our numerical results from exact and self-consistent solution of the equations of motion for ?2 and the Friedmann equations support this formula, and it should hold for any slow-roll dark energy. Our potential can be easily realized in N-flation models with many fields, and is easily falsifiable by upcoming experiments - for example, if Linde's chaotic inflation is ruled out. But if r values consistent with Linde's chaotic inflation are detected then one should take this model seriously indeed.
GRAVITATION & DARK ENERGY Part One (of 3) : Einstein's Theory of General Relativity
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Search for dark energy potentials in quintessence
NASA Astrophysics Data System (ADS)
Muromachi, Yusuke; Okabayashi, Akira; Okada, Daiki; Hara, Tetsuya; Itoh, Yutaka
2015-09-01
The time evolution of the equation of state w for quintessence models with a scalar field as dark energy is studied up to the third derivative big (d^3w/da^3big ) with respect to the scale factor a, in order to predict future observations and specify the scalar potential parameters with the observables. The third derivative of w for general potential V is derived and applied to several types of potentials. They are the inverse power law big (V=M^{4+? }/Q^{? }big ), the exponential big (V=M^4exp {? M/Q}big ), the mixed big (V=M^{4+? }exp {? M/Q}/Q^{? }big ), the cosine big (V=M^4[cos (Q/f)+1]big ), and the Gaussian types big (V=M^4exp big {-Q^2/? ^2big }big ), which are prototypical potentials for the freezing and thawing models. If the parameter number for a potential form is n, it is necessary to find at least n+2 independent observations to identify the potential form and the evolution of the scalar field (Q and dot {Q}). Such observations would be the values of ? _Q, w, dw/da,ldots , dw^n/da^n. From these specific potentials, we can predict the n+1 and higher derivatives of w: dw^{n+1}/da^{n+1},ldots . Since four of the abovementioned potentials have two parameters, it is necessary to calculate the third derivative of w for them to estimate the predicted values. If they are tested observationally, it will be understood whether the dark energy can be described by a scalar field with this potential. At least it will satisfy the necessary conditions. Numerical analysis for d^3w/da^3 is performed with some specified parameters in the investigated potentials, except for the mixed one. It becomes possible to distinguish the potentials by accurately observing dw/da and d^2w/da^2 for some parameters.
A thermodynamic motivation for dark energy
Radicella, Ninfa
2010-01-01
A thermodynamic motivation for dark energyIt is argued that the discovery of cosmic acceleration could have been anticipated on thermodynamic grounds, namely, the generalized second law and the approach to equilibrium at large scale factor. Therefore, the existence of dark energy -or equivalently, some modified gravity theory- should have been expected. Constraints on cosmological models and modified gravity theories obtained from the said criteria show compatibility with constraints derived from observational data.
Thermodynamics of viscous dark energy in an RSII braneworld
M. R. Setare; A. Sheykhi
2011-03-05
We show that for an RSII braneworld filled with interacting viscous dark energy and dark matter, one can always rewrite the Friedmann equation in the form of the first law of thermodynamics, $dE=T_hdS_h+WdV$, at apparent horizon. In addition, the generalized second law of thermodynamics can fulfilled in a region enclosed by the apparent horizon on the brane for both constant and time variable 5-dynamical Newton's constant $G_5$. These results hold regardless of the specific form of the dark energy. Our study further support that in an accelerating universe with spatial curvature, the apparent horizon is a physical boundary from the thermodynamical point of view.