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1

Unified Field Equations Coupling Four Forces and Theory of Dark Matter and Dark Energy

Unified Field Equations Coupling Four Forces and Theory of Dark Matter and Dark Energy Tian Ma. Electroweak Theory VI. Unified Theory of Dark Energy and Dark Matter VII. Concluding Remarks 2 #12;References: 1. Tian Ma & Shouhong Wang, Gravitational Field Equations and Theory of Dark Matter and Dark Energy

Wang, Shouhong

2

Effective dark energy equation of state in interacting dark energy models

In models where dark matter and dark energy interact non-minimally, the total amount of matter in a fixed comoving volume may vary from the time of recombination to the present time due to energy transfer between the two components. This implies that, in interacting dark energy models, the fractional matter density estimated using the cosmic microwave background assuming no interaction between dark matter and dark energy will in general be shifted with respect to its true value. This may result in an incorrect determination of the equation of state of dark energy if the interaction between dark matter and dark energy is not properly accounted for, even if the evolution of the Hubble parameter as a function of redshift is known with arbitrary precision. In this paper we find an exact expression, as well as a simple analytical approximation, for the evolution of the effective equation of state of dark energy, assuming that the energy transfer rate between dark matter and dark energy is described by a simple two-parameter model. We also provide analytical examples where non-phantom interacting dark energy models mimic the background evolution and primary cosmic microwave background anisotropies of phantom dark energy models.

P. P. Avelino; H. M. R. da Silva

2012-01-02

3

Entropic dark energy and sourced Friedmann equations

In this paper we show that a recent attempt to derive dark energy as an entropic force suffers from the same problems as earlier attempts motivated by holography. The possible remedy is again the introduction of source terms.

Ulf H. Danielsson

2010-03-03

4

Dark energy models with variable equation of state parameter

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.

Anil Kumar Yadav; Farook Rahaman; Saibal Ray

2010-06-28

5

Linear Dark Energy Equation of State Revealed by Supernovae?

NASA Astrophysics Data System (ADS)

In this paper, we propose a test to detect the linearity of the dark energy equation of state, and apply it to the SNLS3 Type Ia Supernova (SN Ia) data set. We find that: (a) current SN Ia data are well described by a dark energy equation of state linear in the cosmic scale factor a, at least up to a redshift z = 1, independent of the pivot points chosen for the linear relation; (b) there is no significant evidence of any deviation from linearity. This apparent linearity may reflect the limit of dark energy information extractable from current SN Ia data.

Salzano, Vincenzo; Wang, Yun; Sendra, Irene; Lazkoz, Ruth

2014-01-01

6

A new equation of state for dark energy

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.

Dragan Slavkov Hajdukovic

2007-11-03

7

NASA Astrophysics Data System (ADS)

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.

Kumar, Suresh; Xu, Lixin

2014-10-01

8

Reconstructing the dark energy equation of state with varying alpha

The possibility of reconstructing the dark energy equation of state from variations in the fine structure constant is investigated for a class of models where the quintessence field is non-minimally coupled to the electromagnetic field. For given classes of couplings and quintessence interaction potentials, it is typically found that variations in alpha would need to be measured to within an accuracy of at least $5 \\times 10^{-7}$ to obtain a reconstructed equation of state with less than a twenty per cent deviation from the true equation of state between redshifts 0 and 3. In this case, it is argued that the sign of the first derivative of the equation of state can be uncovered from the reconstruction, thus providing unique information on how the universe developed into its present dark energy dominated phase independent of high redshift surveys. Such information would complement future observations anticipated from the Supernova Acceleration Probe.

N. J. Nunes; James E. Lidsey

2003-10-30

9

Nonparametric reconstruction of the dark energy equation of state

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.

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

10

Gravitational Field Equations and Theory of Dark Matter and Dark Energy

The main objective of this article is to derive a new set of gravitational field equations and to establish a new unified theory for dark energy and dark matter. The new gravitational field equations with scalar potential $\\varphi$ are derived using the Einstein-Hilbert functional, and the scalar potential $\\varphi$ is a natural outcome of the divergence-free constraint of the variational elements. Gravitation is now described by the Riemannian metric $g_{ij}$, the scalar potential $\\varphi$ and their interactions, unified by the new gravitational field equations. Associated with the scalar potential $\\varphi$ is the scalar potential energy density $\\frac{c^4}{8\\pi G} \\Phi=\\frac{c^4}{8\\pi G} g^{ij}D_iD_j \\varphi$, which represents a new type of energy caused by the non-uniform distribution of matter in the universe. The negative part of this potential energy density produces attraction, and the positive part produces repelling force. This potential energy density is conserved with mean zero: $\\int_M \\Phi dM=0$. The sum of this new potential energy density $\\frac{c^4}{8\\pi G} \\Phi$ and the coupling energy between the energy-momentum tensor $T_{ij}$ and the scalar potential field $\\varphi$ gives rise to a new unified theory for dark matter and dark energy: The negative part of this sum represents the dark matter, which produces attraction, and the positive part represents the dark energy, which drives the acceleration of expanding galaxies. In addition, the scalar curvature of space-time obeys $R=\\frac{8\\pi G}{c^4} T + \\Phi$. Furthermore, the new field equations resolve a few difficulties encountered by the classical Einstein field equations.

Tian Ma; Shouhong Wang

2012-06-22

11

The state of the dark energy equation of state

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

Alessandro Melchiorri; Laura Mersini; Carolina J. Ödman; Mark Trodden

2003-01-01

12

Nonparametric reconstruction of the dark energy equation of state

A basic aim of ongoing and upcoming cosmological surveys is to unravel the mystery of dark energy. In the absence of a compelling theory to test, a natural approach is to better characterize the properties of dark energy in search of clues that can lead to a more fundamental understanding. One way to view this characterization is the improved determination of the redshift-dependence of the dark energy equation of state parameter, w(z). To do this requires a robust and bias-free method for reconstructing w(z) from data that does not rely on restrictive expansion schemes or assumed functional forms for w(z). We present a new nonparametric reconstruction method that solves for w(z) as a statistical inverse problem, based on a Gaussian process representation. This method reliably captures nontrivial behavior of w(z) and provides controlled error bounds. We demonstrate the power of the method on different sets of simulated supernova data; the approach can be easily extended to include diverse cosmological probes.

Holsclaw, Tracy; Sanso, Bruno; Lee, Herbert [Department of Applied Mathematics and Statistics, University of California, Santa Cruz, California 95064 (United States); Alam, Ujjaini; Heitmann, Katrin [ISR-1, Mailstop D466, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Habib, Salman [T-2, Mailstop B285, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Higdon, David [CCS-6, Mailstop F600, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2010-11-15

13

New constraints on the dark energy equation of state

NASA Astrophysics Data System (ADS)

We combine recent measurements of cosmic microwave background anisotropies, supernovae luminosity distances, and baryonic acoustic oscillations to derive constraints on the dark energy equation of state w in the redshift range 0

Said, Najla; Baccigalupi, Carlo; Martinelli, Matteo; Melchiorri, Alessandro; Silvestri, Alessandra

2013-08-01

14

New Constraints On The Dark Energy Equation of State

We combine recent measurements of Cosmic Microwave Background Anisotropies, Supernovae luminosity distances and Baryonic Acoustic Oscillations to derive constraints on the dark energy equation of state w in the redshift range 0

Said, Najla; Martinelli, Matteo; Melchiorri, Alessandro; Silvestri, Alessandra

2013-01-01

15

Cardassian Expansion: Dark Energy Density from Modified Friedmann Equations

The Cardassian universe is a proposed modification to the Friedmann equation in which the universe is flat, matter dominated, and accelerating. In the ordinary Friedmann equation, the right hand side is a linear function of the energy density, $H^2 \\sim \\rho$. Here, instead, the right hand side of the Friedmann equation is a different function of the energy density, $H^2 \\sim g(\\rho)$. This function returns to ordinary Friedmann at early times, but drives acceleration of the universe at the current epoch. The only ingredients in this universe are matter and radiation: in particular, there is NO vacuum contribution. The new term required may arise, e.g., as a consequence of our observable universe living as a 3-dimensional brane in a higher dimensional universe. A second possible interpretation of Cardassian expansion is developed, in which we treat the modified Friedman equations as due to a fluid, in which the energy density has new contributions with negative pressure (possibly due to dark matter with self-interactions). Predictions are shown for observational tests of generalized Cardassian models in future supernova surveys.

Katherine Freese

2005-01-31

16

From Generalized Dirac Equations to a Candidate for Dark Energy

We consider extensions of the Dirac equation with mass terms m1+i*gamma5*m2 and i*m_1+gamma*m2. The corresponding Hamiltonians are Hermitian and pseudo-Hermitian ("gamma5 Hermitian"), respectively. The fundamental spinor solutions for all generalized Dirac equations are found in the helicity basis and brought into concise analytic form. We postulate that the time-ordered product of field operators should yield the Feynman propagator (i*epsilon prescription), and we also postulate that the tardyonic as well as tachyonic Dirac equations should have a smooth massless limit. These postulates lead to sum rules that connect the form of the fundamental field anticommutators with the tensor sums of the fundamental plane-wave eigenspinors and the projectors over positive-energy and negative-energy states. In the massless case, the sum rules are fulfilled by two egregiously simple, distinguished functional forms. The first sum rule remains valid in the case of a tardyonic theory and leads to the canonical massive Dirac field. The second sum rule is valid for a tachyonic mass term and leads to a natural suppression of the right-handed helicity states for tachyonic particles, and left-handed helicity states for tachyonic spin-1/2 antiparticles. When applied to neutrinos, the theory contains a free tachyonic mass parameter. Tachyons are known to be repulsed by gravity. We discuss a possible role of a tachyonic neutrino as a contribution to the accelerated expansion of the Universe ("dark energy").

U. D. Jentschura; B. J. Wundt

2012-05-02

17

Time variation of Equation of State for Dark Energy

The time variation of the equation of state ($w_Q$) for the dark energy are analyzed by the present values of parameters $\\Omega_Q $, $w_Q $ and its time derivatives. In the future the detailed feature of the dark energy could be observed, so we have considered the second derivatives of $w_Q$ for two typical potentials $V=M^{4+\\alpha}/Q^{\\alpha}$ and $V=M^4\\exp{(\\beta M/Q)}$. The first derivative $dw_Q/da$ and the second derivative $d^2 w_Q/da^2$ for both potentials are derived. The first derivative is estimated by the observed two parameters $\\Delta=w_Q+1$ and $\\Omega_Q$, with the assuming for $Q_0$. In the limit $\\Delta \\rightarrow 0$, the first derivative is null and, under the tracker approximation, the second derivative becomes also null. For the inverse power potential $V=M^{4+\\alpha}/Q^{\\alpha}$, the observed first and second derivatives are used to determine the potential parameter $M$ and $\\alpha$. For the potential of $V=M^4\\exp{(\\beta M/Q)}$, the second derivative is estimated by the observed param...

Hara, Tetsuya; Muromachi, Yusuke; Itoh, Yutaka

2014-01-01

18

Transition of the dark energy equation of state in an interacting holographic dark energy model

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

Bin Wang; Yungui Gong; Elcio Abdalla

2005-01-01

19

Quintessence and tachyon dark energy models with a constant equation of state parameter

In this work we determine the correspondence between quintessence and tachyon dark energy models with a constant dark energy equation of state parameter, we. Although the evolution of both the Hubble parameter and the scalar field potential with redshift is the same, we show that the evolution of quintessence\\/tachyon scalar fields with redshift is, in general, very different. We explicitly

P. P. Avelino; L. Losano; J. J. Rodrigues

2011-01-01

20

Is w{ne}-1 evidence for a dynamical dark energy equation of state?

Current constraints on the dark energy equation of state parameter, w, are expected to be improved by more than 1 order of magnitude in the next decade. If |w-1| > or approx. 0.01 around the present time, but the dark energy dynamics is sufficiently slow, it is possible that future constraints will rule out a cosmological constant while being consistent with a time-independent equation of state parameter. In this paper, we show that although models with such behavior can be constructed, they do require significant fine-tuning. Therefore, if the observed acceleration of the Universe is induced by a dark energy component, then finding w{ne}-1 would, on its own, constitute very strong evidence for a dynamical dark energy equation of state.

Avelino, P. P. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Trindade, A. M. M. [Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto (Portugal); Viana, P. T. P. [Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto (Portugal); Departamento de Matematica Aplicada da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal)

2009-09-15

21

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 result...

Oikonomou, V K; Park, Miok

2014-01-01

22

Dynamical Mutation of Dark Energy

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.

L. R. Abramo; R. C. Batista; L. Liberato; R. Rosenfeld

2007-10-12

23

On the possibility of Dark Energy from corrections to the Wheeler-De Witt equation

We present a method for approximating the effective consequence of generic quantum gravity corrections to the Wheeler-DeWitt equation. We show that in many cases these corrections can produce departures from classical physics at large scales and that this behaviour can be interpreted as additional matter components. This opens up the possibility that dark energy (and possible dark matter) could be large scale manifestations of quantum gravity corrections to classical general relativity. As a specific example we examine the first order corrections to the Wheeler-De Witt equation arising from loop quantum cosmology in the absence of lattice refinement and show how the ultimate breakdown in large scale physics occurs.

William Nelson; Mairi Sakellariadou

2007-09-11

24

Dark energy from cosmological fluids obeying a Shan-Chen non-ideal equation of state

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.

Bini, Donato; Gregoris, Daniele; Succi, Sauro

2014-01-01

25

Observational constraints on scalar field models of dark energy with barotropic equation of state

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

Olga Sergijenko; Ruth Durrer; Bohdan Novosyadlyj

2011-01-01

26

Cosmology with variable parameters and effective equation of state for Dark Energy

A cosmological constant, ?, is the most natural candidate to explain the origin of the dark energy (DE) component in the Universe. However, due to experimental evidence that the equation of state (EOS) of the DE could be evolving with time\\/redshift (including the possibility that it might behave phantom-like near our time) has led theorists to emphasize that there might

Joan Sola

27

Cosmology with variable parameters and effective equation of state for dark energy

A cosmological constant, Lambda, is the most natural candidate to explain the origin of the dark energy (DE) component in the universe. However, due to some experimental evidence that the equation of state (EOS) of the DE could be evolving with time\\/redshift (including the possibility that it might behave phantom-like near our time) has led theorists to emphasize that there

Joan Solà; Hrvoje Stefancic

2006-01-01

28

While there is mounting evidence in all fronts of experimental cosmology for a non-vanishing dark energy component in the Universe, we are still far away from understanding its ultimate nature. A fundamental cosmological constant, Lambda, is the most natural candidate, but many dynamical mechanisms to generate an effective Lambda have been devised which postulate the existence of a peculiar scalar field (so-called quintessence, and generalizations thereof). These models are essentially ad hoc, but they lead to the attractive possibility of a time-evolving dark energy with a non-trivial equation of state (EOS). Most, if not all, future experimental studies on precision cosmology (e.g. the SNAP and PLANCK projects) address very carefully the determination of an EOS parametrized ``a la quintessence''. Here we show that by fitting cosmological data to an EOS of that kind can also be interpreted as a hint of a fundamental, but time-evolving, cosmological term: Lambda=Lambda(t). We exemplify this possibility by studying the effective EOS associated to a renormalization group (RG) model for Lambda. We find that the effective EOS can correspond to both normal quintessence and phantom dark energy, depending on the value of a single parameter of the RG model. We conclude that behind a non-trivial EOS of a purported quintessence or phantom scalar field there can actually be a running cosmological term of a fundamental quantum field theory.

Joan Sola; Hrvoje Stefancic

2005-05-06

29

Can the dark energy equation-of-state parameter w be less than -1?

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

Sean M. Carroll; Mark Hoffman; Mark Trodden

2003-01-01

30

Effect of dark energy sound speed and equation of state on CDM power spectrum

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.

Rizwan Ul Haq Ansari; Sanil Unnikrishnan

2014-07-15

31

NASA Astrophysics Data System (ADS)

Preface; Part I. Theory: 1. Dark energy, gravitation and the Copernican principle J.-P. Uzan; 2. Dark energy and modified gravity R. Maartens and R. Durrer; 3. Some views on dark energy D. Polarski; 4. Emergent gravity and dark energy T. Padmanabhan; Part II. Observations: 5. Foundations of supernova cosmology R. P. Kirshner; 6. Dark energy and supernovae P. Ruiz-Lapuente; 7. The future of supernova cosmology M. Wood-Vasey; 8. The space advantage A. Kim; 9. Baryon acoustic oscillations B. Bassett and R. Hlozek; 10. Weak gravitational lensing A. Heavens; Index.

Ruiz-Lapuente, Pilar

2014-02-01

32

dark matter dark energy inflation

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 Â November 28 Gravitation initial conditions beyond single-field slow roll #12;dark matter dark energy inflation NSF Site

Hu, Wayne

33

Quintessence and tachyon dark energy models with a constant equation of state parameter

NASA Astrophysics Data System (ADS)

In this work we determine the correspondence between quintessence and tachyon dark energy models with a constant dark energy equation of state parameter, w. Although the evolution of both the Hubble parameter and the scalar field potential with redshift is the same, we show that the evolution of quintessence/tachyon scalar fields with redshift is, in general, very different. We explicitly demonstrate that if w?-1 the potentials need to be very fine-tuned for the relative perturbation on the equation of state parameter, ?w/(1+w)?1, to be very small around the present time. We also discuss possible implications of our results for the reconstruction of the evolution of w with redshift using varying couplings.

Avelino, P. P.; Losano, L.; Rodrigues, J. J.

2011-05-01

34

Observational constraints on scalar field models of dark energy with barotropic equation of state

NASA Astrophysics Data System (ADS)

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.

Sergijenko, Olga; Durrer, Ruth; Novosyadlyj, Bohdan

2011-08-01

35

Observational constraints on scalar field models of dark energy with barotropic equation of state

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.

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

36

Alternative high-z cosmic tracers and the dark energy equation of state

NASA Astrophysics Data System (ADS)

We propose to use alternative cosmic tracers to measure the dark energy equation of state and the matter content of the Universe [w(z) & ?m]. Our proposed method consists of two components: (a) tracing the Hubble relation using HII-like starburst galaxies, as an alternative to supernovae type Ia, which can be detected up to very large redshifts, z ~ 4, and (b) measuring the clustering pattern of X-ray selected AGN at a median redshift of ~ 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 a small XMM survey) and the currently largest SNIa sample (Kowalski et al 2008), provides: and w = -1.0 ± 0.1.

Plionis, M.; Terlevich, R.; Basilakos, S.; Bresolin, F.; Terlevich, E.; Melnick, J.; Georgantopoulos, I.

2009-10-01

37

Probing Dark Energy Using Its Density Instead of Its Equation of State

The variation of dark energy density with redshift, $\\rho_X(z)$, provides a critical clue to the nature of dark energy. Since $\\rho_X(z)$ depends on the dark energy equation of state $w_X(z)$ through an integral, $\\rho_X(z)$ can be constrained more tightly than $w_X(z)$ given the same observational data. We demonstrate this explicitly using current type Ia supernova (SN Ia) data [the Tonry/Barris sample], together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We assume a flat universe, and use Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that, while $w_X(z)$ extracted from current data is consistent with a cosmological constant at 68% C.L., $\\rho_X(z)$ (which has far smaller uncertainties) is not. Our results clearly show the advantage of using $\\rho_X(z)$, instead of $w_X(z)$, to probe dark energy.

Yun Wang; Katherine Freese

2004-02-09

38

Properties and uncertainties of scalar field models of dark energy with barotropic equation of state

The dynamics of expansion and large scale structure formation in the multicomponent Universe with dark energy modeled by the minimally coupled scalar field with generalized linear barotropic equation of state are analyzed. It is shown that the past dynamics of expansion and future of the Universe - eternal accelerated expansion or turnaround and collapse - are completely defined by the current energy density of a scalar field and relation between its current and early equation of state parameters. The clustering properties of such models of dark energy and their imprints in the power spectrum of matter density perturbations depend on the same relation and, additionally, on the 'effective sound speed' of a scalar field, defined by its Lagrangian. It is concluded that such scalar fields with different values of these parameters are distinguishable in principle. This gives the possibility to constrain them by confronting the theoretical predictions with the corresponding observational data. For that we have used the 7-year Wilkinson Microwave Anisotropy Probe data on cosmic microwave background anisotropies, the Union2 data set on Supernovae Ia and the seventh data release of the Sloan Digital Sky Survey data on luminous red galaxies space distribution. Using the Markov Chain Monte Carlo technique the marginalized posterior and mean likelihood distributions are computed for the scalar fields with two different Lagrangians: Klein-Gordon and Dirac-Born-Infeld ones. The properties of such scalar field models of dark energy with best fitting parameters and uncertainties of their determination are also analyzed in the paper.

Novosyadlyj, Bohdan; Sergijenko, Olga; Apunevych, Stepan; Pelykh, Volodymyr [Astronomical Observatory of Ivan Franko National University of Lviv, Kyryla i Methodia str., 8, Lviv, 79005 (Ukraine); Ya. S. Pidstryhach Institute for Applied Problems of Mechanics and Mathematics, Naukova Street, 3-b, Lviv, 79060 (Ukraine)

2010-11-15

39

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?

Norbert Straumann

2003-11-26

40

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.

Sapone, Domenico; Kunz, Martin [Departement de Physique Theorique, Universite de Geneve, 24 quai Ernest Ansermet, CH-1211 Geneve 4 (Switzerland); Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH (United Kingdom)

2009-10-15

41

While there is mounting evidence in all fronts of experimental cosmology for a non-vanishing dark energy component in the Universe, we are still far away from understanding its ultimate nature. A fundamental cosmological constant, ?, is the most natural candidate, but many dynamical mechanisms to generate an effective ? have been devised which postulate the existence of a peculiar scalar

Joan Solà; Hrvoje Štefan?i?

2005-01-01

42

Dark-energy thermodynamic models

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.

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

43

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.

Keith A. Olive

2010-01-27

44

Variable equation of state for Bianchi type-VI0 dark energy models

NASA Astrophysics Data System (ADS)

We present dark energy models in an anisotropic Bianchi type-VI0 (B-VI0) space-time with a variable equation of state (EoS). The EoS for dark energy ? is found to be time dependent and its existing range for derived models is in good agreement with the recent observations of SNe Ia data (Knop et al. in Astrophys. J. 598:102 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. in Astrophys. J. 606:702, 2004b) and latest a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al. in Astrophys. J. Suppl. 180:225, 2009; Komatsu et al. in Astrophys. J. Suppl. 180:330, 2009). The cosmological constant ? is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations. The physical and geometric aspects of the models are also discussed in detail.

Amirhashchi, Hassan; Pradhan, Anirudh; Saha, Bijan

2011-05-01

45

Chasing the phantom: A closer look at type Ia supernovae and the dark energy equation of state

NASA Astrophysics Data System (ADS)

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.

Shafer, Daniel L.; Huterer, Dragan

2014-03-01

46

Effective equation of state for running vacuum: "mirage" quintessence and phantom dark energy

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 WMAP observations, in combination with the remaining cosmological observables. Such situation did not completely disappear from the data collected from the Planck satellite mission. In it the EoS analysis may still be interpreted as suggesting wenergy density could explain the persistent phantom anomaly, despite there is no trace of real phantom behavior 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 behavior can also appear as quintessence in transit to phantom, or vice versa.

Spyros Basilakos; Joan Sola

2013-07-17

47

Cosmology with variable parameters and effective equation of state for Dark Energy

A cosmological constant, Lambda, is the most natural candidate to explain the origin of the dark energy (DE) component in the Universe. However, due to experimental evidence that the equation of state (EOS) of the DE could be evolving with time/redshift (including the possibility that it might behave phantom-like near our time) has led theorists to emphasize that there might be a dynamical field (or some suitable combination of them) that could explain the behavior of the DE. While this is of course one possibility, here we show that there is no imperative need to invoke such dynamical fields and that a variable cosmological constant (including perhaps a variable Newton's constant too) may account in a natural way for all these features.

Joan Sola; Hrvoje Stefancic

2006-01-04

48

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.

The Dark Energy Survey Collaboration

2005-10-12

49

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.

Roland de Putter; Eric V. Linder

2008-08-01

50

Unified Description of Dark Energy and Dark Matter

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.

Walter Petry

2008-10-23

51

Interacting Agegraphic Dark Energy

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.

Hao Wei; Rong-Gen Cai

2007-07-27

52

Dark energy and dark matter from cosmological observations

The present status of our knowledge about the dark matter and dark energy is reviewed. Bounds on the content of cold and hot dark matter from cosmological observations are discussed in some detail. I also review current bounds on the physical properties of dark energy, mainly its equation of state and effective speed of sound.

Steen Hannestad

2005-09-13

53

Dark energy without dark energy

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.

Pedro F. Gonzalez-Diaz

2006-08-29

54

Spherical collapse with dark energy

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.

Irit Maor

2006-02-20

55

We propose to use HII galaxies (HIIG) to trace the redshift-distance relation, by means of their $L(\\mathrm{H}\\beta) - \\sigma$ correlation, in an attempt to constrain the dark energy equation of state parameter solution space, as an alternative to the cosmological use of type Ia supernovae. For a sample of 128 local compact HIIG with high equivalent widths of their Balmer emission lines we obtained ionised gas velocity dispersion from high S/N, high-dispersion spectroscopy (Subaru-HDS and ESO VLT-UVES) and integrated H$\\beta$ fluxes from low dispersion wide aperture spectrophotometry. We find that the $L(\\mathrm{H}\\beta) - \\sigma$ relation is strong and stable against restrictions in the sample. The size of the starforming region is an important second parameter, while adding the emission line equivalent width or the continuum colour and metallicity, produces the solution with the smallest rms scatter. We have used the $L(\\mathrm{H}\\beta) - \\sigma$ relation from a local sample of HIIG and a local calibration ...

Chávez, Ricardo

2014-01-01

56

In this paper 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 energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.

Edmund J. Copeland; M. Sami; Shinji Tsujikawa

2006-03-08

57

On dark energy isocurvature perturbation

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.

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

58

Tachyonic Teleparallel Dark Energy

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.

A. Banijamali; B. Fazlpour

2012-06-13

59

Tachyonic teleparallel dark energy

NASA Astrophysics Data System (ADS)

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 Phys. Lett. B 704, 384, 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.

Banijamali, A.; Fazlpour, B.

2012-11-01

60

Reconstructing and deconstructing dark energy

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.

Linder, Eric V.

2004-06-07

61

Beyond two dark energy parameters.

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

Sarkar, Devdeep; Sullivan, Scott; Joudaki, Shahab; Amblard, Alexandre; Holz, Daniel E; Cooray, Asantha

2008-06-20

62

Cosmological Consequences of a Dark Energy Component with Super-Negative Equation of State

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

R. R. Caldwell

63

Correspondence Between Ricci and Other Dark Energies

NASA Astrophysics Data System (ADS)

Purpose of the present paper is to view the correspondence between Ricci and other dark energies. We have considered the Ricci dark energy in presence of dark matter in non-interacting situation. Subsequently, we have derived the pressure and energy density for Ricci dark energy. The equation of state parameter has been generated from these pressure and energy density. Next, we have considered the correspondence between Ricci and other dark energy models, namely tachyonic field, DBI-essence and new agegraphic dark energy without any interaction and investigated possible cosmological consequences.

Chattopadhyay, Surajit; Debnath, Ujjal

2011-02-01

64

Correspondence Between Ricci and Other Dark Energies

Purpose of the present paper is to view the correspondence between Ricci and other dark energies. We have considered the Ricci\\u000a dark energy in presence of dark matter in non-interacting situation. Subsequently, we have derived the pressure and energy\\u000a density for Ricci dark energy. The equation of state parameter has been generated from these pressure and energy density.\\u000a Next, we

Surajit Chattopadhyay; Ujjal Debnath

2011-01-01

65

Using Type Ia SNe and GRBs to place new constraints on the dark energy equation of state

NASA Astrophysics Data System (ADS)

Both the isotropic sky distribution and the large distance distribution of Gamma Ray Bursts (GRBs) make cosmological studies using GRBs tempting. Unfortunately, GRBs are transient events, with the duration of their observed prompt emission in gamma-rays lasting from a few milliseconds to a few hundred seconds. The fact that GRBs are not persistent sources, coupled with a wide range of observed temporal and spectral properties makes cosmological studies using GRBs difficult. However, by utilizing the overlap in redshift between Type Ia Supernovae and GRBs, many observed GRB correlations can be independently calibrated, and the redshift and luminosity distance of many GRBs without observed redshifts can be estimated. An adequate sample of GRBs covering a range in redshift from ~0.1-10 can then be used to study the history of cosmological evolution, particularly the evolution of dark energy and its corresponding equation of state. Using a sample of several hundred GRBs with estimated redshift and distance based on calibration from Type Ia Supernovae, we present improvements in GRB cosmology studies, particularly placing new constraints on the dark energy equation of state.

Goldstein, Adam

66

Introducing a new infrared cut-o for the holographic dark-energy, we study the correspondence between the quintessence, tachyon, K-essence and dilaton en- ergy density with this holographic dark energy density in the at FRW universe.

L. N. Granda; A. Oliverosy

67

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.

Burra G. Sidharth

2011-12-08

68

Measuring the speed of dark: Detecting dark energy perturbations

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.

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

69

Weak lensing and dark energy: The impact of dark energy on nonlinear dark matter clustering

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.

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

70

Inhomogeneous Dark Matter in Nontrivial Interaction with Dark Energy

We study interacting dark energy (DE) and cold dark matter (DM) in the context of an inhomogeneous and anisotropic spacetime.\\u000a DM and DE are modeled as an interactive mixture of inhomogeneous dust (DM) and a generic homogeneous dark energy (DE) fluid.\\u000a By choosing an “equation of state” linking the energy density (?) and pressure (p) of the DE fluid, as

Roberto A. Sussman; Israel Quiros; Osmel Martín González

71

Entropy bounds and dark energy

NASA Astrophysics Data System (ADS)

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, ? in these scenarios might account for the diffuse dark matter component of the cosmological energy density. Permanent address: Institute of Theoretical Science and Department of Physics, University of Oregon, Eugene, OR 97403.

Hsu, Stephen D. H.

2004-07-01

72

Entropy bounds and dark energy

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, ?

Stephen D. H. Hsu

2004-01-01

73

Dark Energy From Fifth Dimension

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.

H. Alavirad; N. Riazi

2008-01-21

74

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.

B. H. J. McKellar; T. Goldman; G. J. Stephenson, Jr.; P. M. Alsing

2009-08-06

75

We discuss the phenomenology of the dark energy in first order perturbation theory, demonstrating that the dark energy cannot be fully constrained unless the dark matter is found, and that there are two functions that characterise the observational properties of the dark sector for cosmological probes. We argue that measuring these two functions should be an important goal for observational cosmology in the next decades.

Martin Kunz; Luca Amendola; Domenico Sapone

2008-06-08

76

How many dark energy parameters?

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.

Linder, Eric V.; Huterer, Dragan

2005-05-16

77

Dark Energy in Global Brane Universe

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.

Yongli Ping; Lixin Xu; Chengwu Zhang; Hongya Liu

2007-07-19

78

Matter Field, Dark Matter and Dark Energy

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.

Masayasu Tsuge

2008-02-01

79

Dark Energy and Mass Generation

We consider a set of solutions for a massless quartic scalar field, recently devised, that satisfy a massive dispersion relation. We show that such solutions have the property to give the correct behavior for the equation of state of the dark energy. It seen that conformal invariance is restored and the mass gap goes to zero on a time scale determined by the Hubble constant and the strength of the self-interaction of the scalar field. When conformal invariance is restored, the equation of state for the dark energy can apply.

Frasca, Marco

2014-01-01

80

Direct reconstruction of dark energy.

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

Clarkson, Chris; Zunckel, Caroline

2010-05-28

81

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.

Neven Bilic

2008-12-30

82

It is now well accepted that both Dark Matter and Dark Energy are required in any successful cosmological model. Although there is ample evidence that both Dark components are necessary, the conventional theories make no prediction for the contributions from each of them. Moreover, there is usually no intrinsic relationship between the two components, and no understanding of the nature of the mysteries of the Dark Sector. Here we suggest that if the Dark Side is so seductive then we should not be restricted to just 2 components. We further suggest that the most natural model has 5 distinct forms of Dark Energy in addition to the usual Dark Matter, each contributing precisely equally to the cosmic energy density budget.

Douglas Scott; Ali Frolop

2007-03-30

83

Introduction to Dark Energy and Dark Matter

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.

Paul H. Frampton

2005-06-28

84

Can Dark Matter Decay in Dark Energy?

We analyze the interaction between Dark Energy and Dark Matter from a thermodynamical perspective. By assuming they have different temperatures, we study the possibility of occurring a decay from Dark Matter into Dark Energy, characterized by a negative parameter $Q$. We find that, if at least one of the fluids has non vanishing chemical potential, for instance $\\mu_x0$, the decay is possible, where $\\mu_x$ and $\\mu_{dm}$ are the chemical potentials of Dark Energy and Dark Matter, respectively. Using recent cosmological data, we find that, for a fairly simple interaction, the Dark Matter decay is favored with a probability of $\\sim 93%$ over the Dark Energy decay. This result comes from a likelihood analysis where only background evolution has been considered.

S. H. Pereira; J. F. Jesus

2008-11-01

85

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

Varun Sahni

2005-01-01

86

Neutrino Models of Dark Energy

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 feautures 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 inportant only when neutrinos become non-relativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented.

R. D. Peccei

2004-11-10

87

Scattering of dark matter and dark energy

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.

Simpson, Fergus [SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)

2010-10-15

88

The Phase Transition of Dark Energy

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.

Wei Wang; Yuanxing Gui; Ying Shao

2006-12-05

89

Possible equilibria of interacting dark energy models

Interacting dark energy and the holographic principle offer a possible way of addressing the cosmic coincidence problem as well as accounting for the size of the dark energy component. The equilibrium points of the Friedmann equations which govern the evolution behavior of dark energy, matter, and curvature components can determine the qualitative behavior of the cosmological models. These possible equilibria and their behavior are examined in a general framework, and some illustrative examples are presented.

Berger, Micheal S.; Shojaei, Hamed [Physics Department, Indiana University, Bloomington, Indiana 47405 (United States)

2008-06-15

90

A model of holographic dark energy

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

Miao Li

2004-01-01

91

Dark Energy Coupled with Dark Matter in Viscous Fluid Cosmology

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.

Brevik, I; Timoshkin, A V

2014-01-01

92

Dark Energy Coupled with Dark Matter in Viscous Fluid Cosmology

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.

I. Brevik; V. V. Obukhov; A. V. Timoshkin

2014-10-10

93

Decaying dark matter mimicking time-varying dark energy

NASA Astrophysics Data System (ADS)

A ?CDM model with dark matter that decays into inert relativistic energy on a time scale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, w˜?, as a function of redshift, and show that the evolution of w˜? depends strongly on the assumed value of the dark-matter density, erroneously taken to scale as a-3. Depending on the latter, one can obtain models that mimic quintessence (w˜?>-1), phantom models (w˜?<-1), or models in which the equation of state parameter crosses the phantom divide, evolving from w˜?>-1 at high redshift to w˜?<-1 at low redshift. All of these models generically converge toward w˜??-1 at the present. The degeneracy between the ?CDM model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.

Dutta, Sourish; Scherrer, Robert J.

2010-08-01

94

Dark Energy: Taking SidesDark Energy: Taking SidesDark Energy: Taking Sides Rocky Kolb Barocky The University of Chicago #12;#12; Cold Dark Matter: (CDM) 25% Dark Energy (): 70% Stars: 0.5% H & He: gas 4 For Dark EnergyEvidence For Dark EnergyEvidence For Dark Energy 3) Baryon acoustic oscillations 4) Weak

Yamamoto, Hirosuke

95

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

Edmund J. Copeland; M. Sami; Shinji Tsujikawa

2006-01-01

96

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.

Professor Sean Carroll

2006-11-13

97

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.

Professor Sean Carroll

2010-01-08

98

Dark radiation as a signature of dark energy

We propose a simple dark energy model with the following properties: the model predicts a late-time dark radiation component that is not ruled out by current observational data, but which produces a distinctive time-dependent equation of state w(z) for z<3. The dark energy field can be coupled strongly enough to standard model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

Dutta, Sourish; Scherrer, Robert J. [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States); Hsu, Stephen D. H.; Reeb, David [Institute of Theoretical Science, University of Oregon, Eugene, Oregon 97403-5203 (United States)

2009-05-15

99

Supernova Constraints on Braneworld Dark Energy

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

Ujjaini Alam; Varun Sahni

2002-01-01

100

The Quintom Model of Dark Energy

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.

Bo Feng

2006-02-07

101

Brane-Bulk energy exchange and agegraphic dark energy

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 $w^{\\mathrm{eff}}_D\\geq-1$.

Ahmad Sheykhi

2009-08-09

102

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.

Burra G. Sidharth

2011-11-15

103

Constraining dark energy fluctuations with supernova correlations

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.

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

104

Dark Energy and Dark Matter in Stars Physic

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.

Plamen Fiziev

2014-11-02

105

Dark Energy and Dark Matter in Stars Physic

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.

Fiziev, Plamen

2014-01-01

106

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.

Eric V. Linder

2006-03-21

107

Interacting holographic dark energy with logarithmic correction

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.

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

108

Statefinder Diagnostic for Dilaton Dark Energy

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.

Z. G. Huang; X. M. Song; H. Q. Lu; W. Fang

2008-02-16

109

The dark energy problem may have a simple solution in the model of cosmology based on the space-time interaction hypothesis. The hypothesis throws light on the nature of time (see Time-Transcendence-Truth, to be published).

S. C. Tiwari

2005-02-08

110

Dark energy dynamics in the recent universe is influenced by its evolution through the long, matter-dominated expansion history. A particular dynamical property, the flow variable, remains constant in several classes of scalar field models as long as matter dominates; the dark energy is only free to change this behavior at recent times. This gives natural initial conditions for Monte Carlo studies of dark energy dynamics. We propose a parametrization for the later evolution that covers a wide range of possible behaviors, is tractable in making predictions, and can be constrained by observations. We compare the approach to directly parametrizing the potential, which does not take into account the history of the dark energy dynamics.

Cortes, Marina [Berkeley Lab, Berkeley, California 94720 (United States); Linder, Eric V. [Berkeley Lab, Berkeley, California 94720 (United States); University of California, Berkeley, California 94720 (United States); Institute for the Early Universe, Ewha Womans University, Seoul (Korea, Republic of)

2010-03-15

111

Amplitude of dark energy perturbations

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

Christopher Gordon; David Wands

2005-01-01

112

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

T. Padmanabhan

2008-01-01

113

Field theory model for dark matter and dark energy in interaction

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, baryonic acoustic oscillations, lookback time, and the 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.

Micheletti, Sandro; Abdalla, Elcio [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Wang Bin [Department of Physics, Fudan University, 200433 Shanghai (China)

2009-06-15

114

Interacting Dark Matter and Dark Energy

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

Glennys R. Farrar; P. J. E. Peebles

2003-01-01

115

Thermodynamics of dark energy interacting with dark matter and radiation

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.

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

116

Coupled dark energy field variation

NASA Astrophysics Data System (ADS)

The variation of the dark energy field is found under the assumption that the dark energy is parametric and interacts with the cold dark matter. Considering that the variation of the field could not exceed the Planck mass, we obtain bounds on the coupling and adiabatic coefficients. Three parametrizations of the adiabatic coefficients are considered and two coupling terms where the energy flows from dark energy to dark matter, or the other way around.

García-Zúñiga, Roberto Carlos; Izquierdo, Germán

2014-10-01

117

Unravelling the Dark Matter - Dark Energy Paradigm

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.

Reginald T Cahill

2009-01-26

118

Dark energy from quantum wave function collapse of dark matter

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.

A. S. Majumdar; D. Home; S. Sinha

2005-06-22

119

NASA Astrophysics Data System (ADS)

A system of effective Einstein equations for spatially averaged scalar variables of inhomogeneous cosmological models can be solved by providing a 'cosmic equation of state'. Recent efforts to explain dark energy focus on 'backreaction effects' of inhomogeneities on the effective evolution of cosmological parameters in our Hubble volume, avoiding a cosmological constant in the equation of state. In this letter, it is argued that if kinematical backreaction effects are indeed of the order of the averaged density (or larger as needed for an accelerating domain of the universe), then the state of our regional Hubble volume would have to be in the vicinity of a far-from-equilibrium state that balances kinematical backreaction and average density. This property, if interpreted globally, is shared by a stationary cosmos with effective equation of state {\\bf p}_{\\bf \\rm eff} = -1/3 {\\boldsymbol \\varrho}_{\\bf \\rm eff} . It is concluded that a confirmed explanation of dark energy by kinematical backreaction may imply a paradigmatic change of cosmology.

Buchert, Thomas

2005-10-01

120

Schwarzschild black hole in dark energy background

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.

Ngangbam Ishwarchandra; Ng. Ibohal; K. Yugindro Singh

2014-09-27

121

Schwarzschild black hole in dark energy background

NASA Astrophysics Data System (ADS)

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.

Ishwarchandra, Ngangbam; Ibohal, N.; Yugindro Singh, K.

2014-10-01

122

The concept of an all pervading Aether is age old, and contrary to popular belief, it survived the twentieth century too though with different nuances. Using this concept of a background Quantum Vacuum, the author in 1997 proposed a cosmological model with some resemblance to the Dirac cosmology, which correctly predicted a dark energy driven accelerating universer with a small cosmological constant, as was subsequently confirmed by observation in 1998. Moreover the so called Large Number coincidences including the mysterious Weinberg formula are deduced in this theory, rather than being ad hoc. We examine the concept of Aether in this context and indicate how this dark energy may be harnessed.

Burra G. Sidharth

2004-11-24

123

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

124

Cosmic steps in modeling dark energy

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.

Wang Tower [Center for High-Energy Physics, Peking University, Beijing 100871 (China)

2009-11-15

125

CAPUT DARK ENERGY TOPICS, 2013 1. The Cosmological Constant

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

Weijgaert, Rien van de

126

A new type of dark energy model

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.

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

127

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

Varun Sahni; Alexei Starobinsky

2006-01-01

128

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.

Burra G. Sidharth

2008-08-05

129

Confronting dark energy anisotropic stress

NASA Astrophysics Data System (ADS)

We use currently available cosmic observations to probe and constrain an imperfect dark-energy fluid that is characterized by a constant equation of state w and a constant speed of viscosity cvis2. The model parameter space was scanned using the Markov chain Monte Carlo method. The results show that the speed of viscosity cvis2 was not well constrained when currently available data sets are used (including the cosmic microwave background radiation from Planck-2013 and WMAP9, type Ia supernovae, baryon acoustic oscillations, redshift space distortion f?8(z), and gravitational weak lensing from Canada-France Hawaii Telescope Lensing Survey (CFHTLenS). However, the cosmic data sets slightly favor phantom dark energy with a negative speed of viscosity cvis2.

Chang, Baorong; Xu, Lixin

2014-07-01

130

Roles of dark energy perturbations in dynamical dark energy models: can we ignore them?

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

Park, Chan-Gyung; Hwang, Jai-chan; Lee, Jae-heon; Noh, Hyerim

2009-10-01

131

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.

Rui Neves; Cenalo Vaz

2006-01-06

132

Investigating Dark Energy with Black Hole Binaries

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.

Laura Mersini-Houghton; Adam Kelleher

2009-06-08

133

We explore the fate of the Universe given the possibility that the density associated with `dark energy' may decay slowly with time. Decaying dark energy is modelled by a homogeneous scalar field which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar field rolls down its potential, consequently the

Ujjaini Alam; Varun Sahni; Alexei A. Starobinsky

2003-01-01

134

Dark energy: myths and reality

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].

Lukash, V N

2008-01-01

135

Neutrinos and Dark Energy Dissertation

Neutrinos and Dark Energy Dissertation zur Erlangung des Doktorgrades des Departments Physik der of Dark Energy. As has been widely discussed, the cosmic neutrino background naturally qualifies for a connection with the Dark Energy sector and as a result could play a key role for the origin of cosmic

136

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.

Dragan Huterer

2002-01-01

137

Dark energy: myths and reality

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].

V. N. Lukash; V. A. Rubakov

2008-07-10

138

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.

Wechsler, Risa

2007-10-30

139

We investigate the possibility of a dark energy component that is scaling with epochs. A phenomenological model is introduced whose energy density depends on the redshift in such a way that a smooth transition among the three dominant phases of the universe evolution (radiation era, matter domination, asymptotic de Sitter state) is achieved. We use the WMAP cosmic microwave background data and the luminosity distances of Type Ia Supernovae to test whether this data can provide a signal for scaling.

Salvatore Capozziello; Alessandro Melchiorri; Alice Schirone

2004-08-05

140

Interacting Ricci dark energy with logarithmic correction

NASA Astrophysics Data System (ADS)

Motivated by the holographic principle, it has been suggested that the dark energy density may be inversely proportional to the area A of the event horizon of the universe. However, such a model would have a causality problem. In this work, we consider the entropy-corrected version of the holographic dark energy model in the non-flat FRW universe and we propose to replace the future event horizon area with the inverse of the Ricci scalar curvature. We obtain the equation of state (EoS) parameter ? ?, the deceleration parameter q and ?D' in the presence of interaction between Dark Energy (DE) and Dark Matter (DM). Moreover, we reconstruct the potential and the dynamics of the tachyon, K-essence, dilaton and quintessence scalar field models according to the evolutionary behavior of the interacting entropy-corrected holographic dark energy model.

Pasqua, Antonio; Khodam-Mohammadi, A.; Jamil, Mubasher; Myrzakulov, R.

2012-07-01

141

Can dark energy evolve to the phantom?

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

Alexander Vikman

2005-01-01

142

Black hole and holographic dark energy

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 $\\omega_{\\rm \\Lambda}=-1$.

Yun Soo Myung

2007-02-06

143

The integrated Sachs-Wolfe effect in cosmologies with coupled dark matter and dark energy

NASA Astrophysics Data System (ADS)

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 ? ~ a-3 of the matter density ? with scalefactor 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.

Schäfer, B. M.

2008-08-01

144

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

R. R. Caldwell

2002-01-01

145

Schwarzschild black hole in dark energy background

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...

Ishwarchandra, Ngangbam; Singh, K Yugindro

2014-01-01

146

Signature of the interaction between dark energy and dark matter in observations

We investigate the effect of an interaction between dark energy and dark matter upon the dynamics of galaxy clusters. This effect is computed through the Layser-Irvine equation, which describes how an astrophysical system reaches virial equilibrium and was modified to include the dark interactions. Using observational data from almost 100 purportedly relaxed galaxy clusters we put constraints on the strength of the couplings in the dark sector. We compare our results with those from other observations and find that a positive (in the sense of energy flow from dark energy to dark matter) nonvanishing interaction is consistent with the data within several standard deviations.

Abdalla, Elcio; Abramo, L. Raul; Souza, Jose C. C. de [Instituto de Fisica, Universidade de Sao Paulo, C. P. 66318, 05315-970, Sao Paulo, SP (Brazil)

2010-07-15

147

Structure Formation in Inhomogeneous Dark Energy Models

We investigate how inhomogeneous quintessence models may have a specific signature even in the linear regime of large scale structure formation. The dynamics of the collapse of a dark matter halo is governed by the value or the dynamical evolution of the dark energy equation of state, the energy density's initial conditions and its homogeneity nature in the highly non-linear regime. These have a direct impact on the redshift of collapse, altering in consequence the linearly extrapolated density threshold above which structures will end up collapsing. We compute this quantity for minimally coupled and coupled quintessence models, examining two extreme scenarios: first, when the quintessence field does not exhibit fluctuations on cluster scales and below - homogeneous dark energy; and second, when the field inside the overdensity collapses along with the dark matter - inhomogeneous dark energy. One shows that inhomogeneous dark energy models present distinct features which may be used to confront them with observational data, for instance galaxy number counting. Fitting formulae for the linearly extrapolated density threshold above which structures will end up collapsing are provided for models of dark energy with constant equation of state.

N. J. Nunes; D. F. Mota

2004-09-20

148

Dark Matter and Dark Energy Explained

NASA Astrophysics Data System (ADS)

The standard model of the universe has many mysteries and defects requiring the use of large fudge factors such as Dark Matter and Dark Energy. We will show that Dark Matter is needed when we try to extend Newton's law of gravity (based upon observations in our solar system) to galactic distances. Dark Matter was introduced to explain the observed flat velocity rotation curves of the outer parts of spiral galaxies, as observed by Vera. Rubin. Much earlier, the (under appreciated) Fritz Zwicky introduced the need for large amounts of missing invisible matter to explain the surprising observed motion of groups of remote galaxies. In our hypothesis, the modification of Newton's laws by the addition of a linear term to the gravitational constant that increases with distance will eliminate the need for dark matter. Our hypothesis is different from the MOND theory of Milgrom, which depends upon acceleration. The Red shift observations by Hubble as a function of distance, and interpreted as ``apparent Doppler effect'' led to the unproven belief that the universe is expanding, and thus to the Big Bang. In turn the apparent acceleration of the expansion required the introduction of Dark Energy. Actually there are three additional components of the red shift that are solely due to gravity and distance and can be larger than the Doppler contribution.

Aisenberg, Sol

2006-03-01

149

Dark energy, inflation, and extra dimensions

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 the compact dimensions and vary with time in precise synchrony with the matter and energy density in the noncompact 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.

Steinhardt, Paul J. [Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544 (United States); Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544 (United States); Wesley, Daniel [Centre for Theoretical Cosmology, DAMTP, Cambridge University, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2009-05-15

150

Dark Energy, Inflation and Extra Dimensions

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.

Paul J. Steinhardt; Daniel Wesley

2008-11-11

151

The Dark Side: from Dark Energy & Dark Matter to Washington and Science Policy

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

Collar, Juan I.

152

Dark Energy: The Observational Challenge

Nearly all proposed tests for the nature of dark energy measure some combination of four fundamental observables: the Hubble parameter H(z), the distance-redshift relation d(z), the age-redshift relation t(z), or the linear growth factor D_1(z). I discuss the sensitivity of these observables to the value and redshift history of the equation of state parameter w, emphasizing where these different observables are and are not complementary. Demonstrating time-variability of w is difficult in most cases because dark energy is dynamically insignificant at high redshift. Time-variability in which dark energy tracks the matter density at high redshift and changes to a cosmological constant at low redshift is {\\it relatively} easy to detect. However, even a sharp transition of this sort at z_c=1 produces only percent-level differences in d(z) or D_1(z) over the redshift range 0.4 Estimates of D_1(z) or H(z) at higher redshift, potentially achievable with the Ly-alpha forest, galaxy redshift surveys, and the CMB power spectrum, can add substantial leverage on such models, given precise distance constraints at z space-based studies of Type Ia supernovae, which measure d(z) directly, and of weak gravitational lensing, which is sensitive to d(z), D_1(z), and H(z).

David H. Weinberg

2005-10-06

153

Dark Energy: fiction or reality?

NASA Astrophysics Data System (ADS)

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.

Triay, Roland

2010-06-01

154

We introduce a novel class of field theories where energy always flows along timelike geodesics, mimicking in that respect dust, yet which possess non-zero pressure. This theory comprises two scalar fields, one of which is a Lagrange multiplier enforcing a constraint between the other's field value and derivative. We show that this system possesses no wave-like modes but retains a single dynamical degree of freedom. Thus, the sound speed is always identically zero on all backgrounds. In particular, cosmological perturbations reproduce the standard behaviour for hydrodynamics in the limit of vanishing sound speed. Using all these properties we propose a model unifying Dark Matter and Dark Energy in a single degree of freedom. In a certain limit this model exactly reproduces the evolution history of ?CDM, while deviations away from the standard expansion history produce a potentially measurable difference in the evolution of structure.

Lim, Eugene A. [ISCAP and Department of Physics, Columbia University, 538 West 120th Street, New York, NY 10027 (United States); Sawicki, Ignacy; Vikman, Alexander, E-mail: eugene.a.lim@gmail.com, E-mail: ignacy.sawicki@nyu.edu, E-mail: alexander.vikman@nyu.edu [CCPP, Department of Physics, New York University, 4 Washington Pl., New York, NY 10003 (United States)

2010-05-01

155

NASA Astrophysics Data System (ADS)

The DES Collaboration has completed construction of the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera which is now mounted at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. DECam is comprised of 74 250 micron thick fully depleted CCDs: 62 2k x 4k CCDs for imaging and 12 2k x 2k CCDs for guiding and focus. A filter set of u,g,r,i,z, and Y, a hexapod for focus and lateral alignment as well as thermal management of the cage temperature. DECam will be used to perform the Dark Energy Survey with 30% of the telescope time over a 5 year period. During the remainder of the time, and after the survey, DECam will be available as a community instrument. An overview of the DECam design, construction and initial on-sky performance information will be presented.

Flaugher, Brenna; DES Collaboration

2013-01-01

156

In this talk we present a pedagogical review of scalar field dynamics. The main emphasis is put on the underlying basic features\\u000a rather than on concrete scalar field models. Cosmological dynamics of standard scalar fields, phantoms and tachyon fields\\u000a is developed in detail. Scaling solutions are discussed emphasizing their importance in modelling dark energy. The developed\\u000a concepts are implemented in

M. Sami

2007-01-01

157

1 construct a general description for neutrino dark energy models, that do not require exotic particles or strange couplings. With the help of the above, this class of models is reduced to a single function with several constraints. It is shown that these models lead to some concrete predictions that can be verified (or disproved) within the next decade, using results from PLANK, EUCLID and JDEM.

Gurwich, Ilya [Physics Department, Ben-Gurion University, Beer-Sheva 84105 (Israel)

2010-06-23

158

NASA Astrophysics Data System (ADS)

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.

Piattella, O. F.; Martins, D. L. A.; Casarini, L.

2014-10-01

159

Covariantizing the interaction between dark energy and dark matter

NASA Astrophysics Data System (ADS)

Coupling dark energy and dark matter through an effective fluid description is a very common procedure in cosmology; however, it always remains in comoving coordinates in the special FLRW space. We construct a consistent, general, and covariant formulation, where the interaction is a natural implication of the imperfectness of the fluids. This imperfectness makes difficult the final step towards a robust formulation of interacting fluids, namely the construction of a Lagrangian whose variation would give rise to the interacting equations. Nevertheless, we present a formal solution to this problem for a single fluid, through the introduction of an effective metric.

Faraoni, Valerio; Dent, James B.; Saridakis, Emmanuel N.

2014-09-01

160

Conformal Higgs model: predicted dark energy density

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.

R. K. Nesbet

2010-04-28

161

Probing gravitation, dark energy, and acceleration

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.

Linder, Eric V.

2004-02-20

162

Parameterizing the effect of dark energy perturbations on the growth of structures

We present an analytical fit to the growth function of the dark matter perturbations when dark energy perturbations are present. The growth index ? depends upon the dark energy equation of state w, the speed of sound of the dark energy fluctuations, the dark matter abundance and the observed comoving scale. The growth index changes by O(5%) for small speed

Guillermo Ballesteros; Antonio Riotto

2008-01-01

163

Interacting Dark Energy: Decay into Fermions

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.

A. de la Macorra

2007-02-08

164

Dark Energy and Neutrino Model in SUSY

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.

Ryo Takahashi; Morimitsu Tanimoto

2006-10-26

165

Can Holographic dark energy increase the mass of the wormhole?

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.

Surajit Chattopadhyay; Davood Momeni; Aziza Altaibayeva; Ratbay Myrzakulov

2014-06-27

166

Constraints on generalized dark energy from recent observations

Effects of a generalized dark energy fluid are investigated on cosmic density fluctuations such as a cosmic microwave background. As a general dark energy fluid, we take into consideration the possibility of the anisotropic stress for dark energy, which has not been discussed much in the literature. We comprehensively study its effects on the evolution of density fluctuations along with that of the nonadiabatic pressure fluctuation of dark energy, then give constraints on such a generalized dark energy from current observations. We show that, though we cannot find any stringent limits on the anisotropic stress or the nonadiabatic pressure fluctuation themselves, the constraints on the equation of state of dark energy can be affected in some cases by the nature of dark energy fluctuation characterized by these properties. This may have important implications in the strategy to study the nature of dark energy.

Ichiki, Kiyotomo [Research Center for the Early Universe, University of Tokyo, Tokyo 113-0033 (Japan); Takahashi, Tomo [Department of Physics, Saga University, Saga 840-8502 (Japan)

2007-06-15

167

Can Holographic dark energy increase the mass of the wormhole?

Motivated by the quantum essence of wormholes, 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 accrets 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.

Surajit Chattopadhyay; Davood Momeni; Aziza Altaibayeva; Ratbay Myrzakulov

2014-06-27

168

Can Holographic dark energy increase the mass of the wormhole?

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.

Chattopadhyay, Surajit; Altaibayeva, Aziza; Myrzakulov, Ratbay

2014-01-01

169

Weak Lensing: Dark Matter, Dark Energy

The light rays from distant galaxies are deflected by massive structures along the line of sight, causing the galaxy images to be distorted.Measurements of these distortions, known as weak lensing, provide a way of measuring the distribution of dark matter as well as the spatial geometry of the universe. I will describe the ideas underlying this approach to cosmology. With planned large imaging surveys, weak lensing is a powerful probe of dark energy. I will discuss the observational challenges ahead and recent progress in developing multiple, complementary approaches to lensing measurements.

Bhuvnesh Jain

2006-02-27

170

Self gravitational fluid mechanical methods termed hydro-gravitational-dynamics (HGD) predict plasma fragmentation 0.03 Myr after the turbulent big bang to form protosuperclustervoids, turbulent protosuperclusters, and protogalaxies at the 0.3 Myr transition from plasma to gas. Linear protogalaxyclusters fragment at 0.003 Mpc viscous-inertial scales along turbulent vortex lines or in spirals, as observed. The plasma protogalaxies fragment on transition into white-hot planet-mass gas clouds (PFPs) in million-solar-mass clumps (PGCs) that become globular-star-clusters (GCs) from tidal forces or dark matter (PGCs) by freezing and diffusion into 0.3 Mpc halos with 97% of the galaxy mass. The weakly collisional non-baryonic dark matter diffuses to > Mpc scales and fragments to form galaxy cluster halos. Stars and larger planets form by binary mergers of the trillion PFPs per PGC, mostly on 0.03 Mpc galaxy accretion disks. Stars deaths depend on rates of planet accretion and internal star mixing. Moderate accretion rates produce white dwarfs that evaporate surrounding gas planets by spin-radiation to form planetary nebulae before Supernova Ia events, dimming some events to give systematic distance errors, the dark energy hypothesis, and the Sandage 2006 overestimates of the universe age.

Carl H. Gibson; Rudolph E. Schild

2008-03-30

171

Holographic Dark Energy Model and Scalar-Tensor Theories

We study the holographic dark energy model in a generalized scalar tensor theory. In a universe filled with cold dark matter and dark energy, the effect of potential of the scalar field is investigated in the equation of state parameter. We show that for a various types of potentials, the equation of state parameter is negative and transition from deceleration to acceleration expansion of the universe is possible.

Yousef Bisabr

2008-08-10

172

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.

173

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.

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

174

Dark energy and particle mixing

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.

A. Capolupo; S. Capozziello; G. Vitiello

2008-08-30

175

From Dark Energy and Dark Matter to Dark Metric

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.

S. Capozziello; M. De Laurentis; M. Francaviglia; S. Mercadante

2008-05-23

176

On the similarity of Information Energy to Dark Energy

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.

M. P. Gough; T. D. Carozzi; A. M. Buckley

2006-03-03

177

On cosmic acceleration without dark energy

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

Edward W. Kolb; Sabino Matarrese; Antonio Riotto

2006-01-01

178

Dynamical Dark Energy or Variable Cosmological Parameters?

One of the main aims in the next generation of precision cosmology experiments will be an accurate determination of the equation of state (EOS) for the dark energy (DE). If the latter is dynamical, the resulting barotropic index omega should exhibit a nontrivial evolution with the redshift. This is usually interpreted as a sign that the mechanism responsible for the

Joan Solà; Hrvoje Stefancic

2006-01-01

179

Quintessence Ghost Dark Energy Model

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.

Ahamd Sheykhi; Ali Bagheri

2011-04-26

180

Testing the interaction between dark energy and dark matter via the latest observations

Cosmological analyses based on currently available observations are unable to rule out a sizeable coupling between dark energy and dark matter. However, the signature of the coupling is not easy to grasp, since the coupling is degenerate with other cosmological parameters, such as the dark energy equation of state and the dark matter abundance. We discuss possible ways to break such degeneracy. Based on the perturbation formalism, we carry out the global fitting by using the latest observational data and get a tight constraint on the interaction between dark sectors. We find that the appropriate interaction can alleviate the coincidence problem.

He Jianhua; Wang Bin; Abdalla, Elcio [Institute of Nuclear and Particle Physics, Astronomy, and Cosmology (INPAC) and the Department of Physics, Shanghai Jiao Tong University, 200240 Shanghai (China); Instituto de Fisica, Universidade de Sao Paulo, Caminho Particular 66318, 05315-970, Sao Paulo (Brazil)

2011-03-15

181

Generalized ghost dark energy in Brans-Dicke theory

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.

A. Sheykhi; E. Ebrahimi; Y. Yosefi

2012-09-14

182

Interacting dark matter and holographic dark energy in an anisotropic universe

NASA Astrophysics Data System (ADS)

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.

Adhav, K. S.; Tayade, G. B.; Bansod, A. S.

2014-09-01

183

arXiv:astro-ph/0703364v227Aug2007 Electromagnetic dark energy

arXiv:astro-ph/0703364v227Aug2007 Electromagnetic dark energy Christian Beck School of MathematicalGill University, Montreal, Quebec, Canada (Dated: August 28, 2007) We introduce a new model for dark energy equations, or more generally with the existence of dark energy. The dark energy density consistent

Wright, Francis

184

Amplitude of dark energy perturbations

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.

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

185

Stable dark energy stars: An alternative to black holes?

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/ \\rhodark energy is a possible candidate.

Francisco S. N. Lobo

2006-12-05

186

Dark Energy - Dark Matter Unification: Generalized Chaplygin Gas Model

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.

Orfeu Bertolami

2005-04-12

187

Holographic Dark Energy Like in $f(R)$ Gravity

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.

Kh. Saaidi; A. Aghamohammadi

2010-10-12

188

String allowed discrete symmetry is the mother of acceptable effective global symmetries at low energy. With this philosophy, we discuss dark energy, QCD axion, and inflation, and speculate some implications of the recent BICEP2 data.

Kim, Jihn E

2014-01-01

189

Optimizing New Dark Energy Experiments

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.

Tyson, J. Anthony [University of California, Davis

2013-08-26

190

Ricci Dark Energy in Brans-Dicke theory

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.

Chao-Jun Feng

2008-06-04

191

Fingerprinting dark energy. II. Weak lensing and galaxy clustering tests

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.

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

192

Dark Energy and Non-linear Perturbations

Dark energy might have an influence on the formation of non--linear structures during the cosmic history. For example, in models in which dark energy couples to dark matter, it will be non--homogeneous and will influence the collapse of a dark matter overdensity. We use the spherical collapse model to estimate how much influence dark energy might have.

C. van de Bruck; D. F. Mota

2005-01-14

193

Dark Matter and Dark Energy huncheng@math.mit.edu

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

Cheng, Hung

194

Dark energy models through nonextensive Tsallis' statistics

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.

Rafael da C. Nunes; Edésio M. Barboza Jr.; Everton M. C. Abreu; Jorge Ananias Neto

2014-03-22

195

Spherical Collapse Model And Dark Energy(II)

This is a second paper of a series of two. In this paper, we directly correct the problem pointed out in the first paper of this series, dark energy does not cluster on the scale of galaxy clusters, but the current describing the flowing of dark energies outside the clusters is ignored in almost all the previous papers. We set up and solve a first order differential equation which describes the evolution of the clusters in a back ground universe containing dark energies. From the solution we extract the key parameters of the model and find them depending on the equation of state coefficients of dark energies rather non-trivially. We then apply the results in Press-Scheter theory and calculate the number density of galaxy clusters and its evolutions, we find the observable quantities are strongly affected by the equation of state coefficients of dark energies.

Ding-fang Zeng; Yi-hong Gao

2005-05-09

196

Transient and late time attractor tachyon dark energy: Can we distinguish it from quintessence?

The string inspired tachyon field can serve as a candidate of dark energy. Its equation of state parameter w varies from 0 to -1. In the case of tachyon field potential V()0 slower (faster) than 1\\/² at infinity, dark energy (dark matter) is a late time attractor. We investigate the tachyon dark energy models under the assumption that w is

Amna Ali; M. Sami; A. A. Sen

2009-01-01

197

Dark goo: bulk viscosity as an alternative to dark energy

We present a simple (microscopic) model in which bulk viscosity plays a role in explaining the present acceleration of the universe. The effect of bulk viscosity on the Friedmann equations is to turn the pressure into an 'effective' pressure containing the bulk viscosity. For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic a dark energy equation of state. Our microscopic model includes self-interacting spin-zero particles (for which the bulk viscosity is known) that are added to the usual energy content of the universe. We study both background equations and linear perturbations in this model. We show that a dark energy behavior is obtained for reasonable values of the two parameters of the model (i.e. the mass and coupling of the spin-zero particles) and that linear perturbations are well-behaved. There is no apparent fine tuning involved. We also discuss the conditions under which hydrodynamics holds, in particular that the spin-zero particles must be in local equilibrium today for viscous effects to be important.

Gagnon, Jean-Sebastien [Technische Universität Darmstadt, Schlossgartenstrasse 2, 64289, Darmstadt (Germany); Lesgourgues, Julien, E-mail: jean-sebastien.gagnon@physik.tu-darmstadt.de, E-mail: julien.lesgourgues@cern.ch [École Polytechnique Fédérale de Lausanne, CH-1015, Lausanne (Switzerland)

2011-09-01

198

Quantum Informational Dark Energy: Dark energy from forgetting

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.

Jae-Weon Lee; Jungjai Lee; Hyeong-Chan Kim

2007-09-01

199

Holographic Dark Energy with Time Varying n^2 Parameter in Non-Flat Universe

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.

Bushra Majeed; Mubasher Jamil; Azad A. Siddiqui

2014-11-01

200

Holographic Dark Energy with Time Varying n^2 Parameter in Non-Flat Universe

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.

Majeed, Bushra; Siddiqui, Azad A

2014-01-01

201

Properties of singularities in the (phantom) dark energy universe

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

Shinichi Nojiri; Sergei D. Odintsov; Shinji Tsujikawa

2005-01-01

202

Future supernovae observations as a probe of dark energy

We study the potential impact of improved future supernovae data on our understanding of the dark energy problem. We carefully examine the relative utility of different fitting functions that can be used to parametrize the dark energy models, and provide concrete reasons why a particular choice (based on a parametrization of the equation of state) is better in almost all

Jochen Weller; Andreas Albrecht

2002-01-01

203

Axially symmetric cosmological model with anisotropic dark energy

NASA Astrophysics Data System (ADS)

The axially symmetric cosmological model is investigated in the presence of anisotropic dark energy. Exact solutions of the Einstein field equations are obtained by a special law of variation for the mean Hubble parameter, which yields a constant value of the deceleration parameter. Geometrical and kinematic properties of the model and the anisotropy of the dark energy have been carried out.

Sahoo, P. K.; Mishra, B.

2014-09-01

204

Quantum Haplodynamics, Dark Matter and Dark Energy

In quantum haplodynamics (QHD) the weak bosons, quarks and leptons are bound states of fundamental constituents, denoted as haplons. The confinement scale of the associated gauge group SU(2)_h is of the order of $\\Lambda_h\\simeq 0.3$ TeV. One scalar state has zero haplon number and is the resonance observed at the LHC. In addition, there exist new bound states of haplons with no counterpart in the SM, having a mass of the order of 0.5 TeV up to a few TeV. In particular, a neutral scalar state with haplon number 4 is stable and can provide the dark matter in the universe. The QHD, QCD and QED couplings can unify at the Planck scale. If this scale changes slowly with cosmic time, all of the fundamental couplings, the masses of the nucleons and of the DM particles, including the cosmological term (or vacuum energy density), will evolve with time. This could explain the dark energy of the universe.

Harald Fritzsch; Joan Sola

2014-02-17

205

Interacting Tachyon Dark Energy Model in Gauss-Bonnet Cosmology

NASA Astrophysics Data System (ADS)

We investigate the tachyon scalar filed model of dark energy in the framework of Gauss-Bonnet cosmology. We consider a spatially non-flat universe containing interacting tachyon dark energy with pressureless dark matter. We obtain the equation of state and deceleration parameters. We also reconstruct the potential and the dynamics for the tachyon scalar field model, which describe accelerated expansion of the universe.

Fayaz, V.; Felegary, F.; Hossienkhani, H.

2013-01-01

206

Numerical study of halo concentrations in dark-energy cosmologies

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

Klaus Dolag; Matthias Bartelmann; Francesca Perrotta; Carlo Baccigalupi; Lauro Moscardini; Massimo Meneghetti; Giuseppe Tormen

2004-01-01

207

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.

Aaron Trout

2012-08-08

208

Dark Energy Rules the Universe

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

Linder, Eric

2008-01-01

209

Decoupling dark energy from matter

We examine the embedding of dark energy in high energy models based upon supergravity and extend the usual phenomenological setting comprising an observable sector and a hidden supersymmetry breaking sector by including a third sector leading to the acceleration of the expansion of the universe. We find that gravitational constraints on the non-existence of a fifth force naturally imply that the dark energy sector must possess an approximate shift symmetry. When exact, the shift symmetry provides an example of a dark energy sector with a runaway potential and a nearly massless dark energy field whose coupling to matter is very weak, contrary to the usual lore that dark energy fields must couple strongly to matter and lead to gravitational inconsistencies. Moreover, the shape of the potential is stable under one-loop radiative corrections. When the shift symmetry is slightly broken by higher order terms in the Kähler potential, the coupling to matter remains small. However, the cosmological dynamics are largely affected by the shift symmetry breaking operators leading to the appearance of a minimum of the scalar potential such that dark energy behaves like an effective cosmological constant from very early on in the history of the universe.

Brax, Philippe [Institut de Physique Théorique, CEA, IPhT, CNRS, URA 2306, F-91191Gif/Yvette Cedex (France); Bruck, Carsten van de [Department of Applied Mathematics, University of Sheffield, Hounsfield Road, Sheffield S3 7RH (United Kingdom); Davis, Anne-Christine [Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, Cambridge CB3 0WA (United Kingdom); Martin, Jérôme, E-mail: philippe.brax@cea.fr, E-mail: c.vandebruck@sheffield.ac.uk, E-mail: a.c.davis@damtp.cam.ac.uk, E-mail: jmartin@iap.fr [Institut d'Astrophysique de Paris, UMR 7095-CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris (France)

2009-09-01

210

Extraterrestrial life contradicts dark energy

NASA Astrophysics Data System (ADS)

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.

Gibson, Carl H.

2012-10-01

211

Anisotropic charged dark energy star

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

Kanika Das; Nawsad Ali

2014-02-02

212

Dark energy from gravitoelectromagnetic inflation?

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.

Federico Agustin Membiela; Mauricio Bellini

2008-05-23

213

Dynamical behavior of interacting dark energy in loop quantum cosmology

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.

Kui Xiao; Jian-Yang Zhu

2010-06-28

214

Dark Energy and Dark Matter as Inertial Effects

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.

Serkan Zorba

2012-10-10

215

Is there Supernova Evidence for Dark Energy Metamorphosis ?

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.

Alam, U; Saini, T D; Starobinsky, A A; Alam, Ujjaini; Sahni, Varun; Saini, Tarun Deep

2003-01-01

216

Is there Supernova Evidence for Dark Energy Metamorphosis ?

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.

Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

2003-11-16

217

Interacting Ricci Dark Energy Models with an Effective ?-term in Lyra Manifold

NASA Astrophysics Data System (ADS)

In this paper, we consider a universe filled with barotropic dark matter and Ricci dark energy in Lyras geometry with varying ?. We assume two different kinds of interactions between dark matter and dark energy. Then, by using numerical analysis, we investigate some cosmological parameters of the models such as equation of state, Hubble and deceleration parameters.

Khurshudyan, M.; Sadeghi, J.; Pasqua, A.; Chattopadhyay, S.; Myrzakulov, R.; Farahani, H.

2014-09-01

218

Cosmological constraints on a dark matter -- dark energy interaction

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.

Mark B. Hoffman

2003-07-18

219

Voids and overdensities of coupled Dark Energy

We investigate the clustering properties of dynamical Dark Energy even in association of a possible coupling between Dark Energy and Dark Matter. We find that within matter inhomogeneities, Dark Energy migth form voids as well as overdensity depending on how its background energy density evolves. Consequently and contrarily to what expected, Dark Energy fluctuations are found to be slightly suppressed if a coupling with Dark Matter is permitted. When considering density contrasts and scales typical of superclusters, voids and supervoids, perturbations amplitudes range from |{delta}{sub {phi}}| {approx} O(10{sup -6}) to |{delta}{sub {phi}}| {approx} O(10{sup -4}) indicating an almost homogeneous Dark Energy component.

Mainini, Roberto, E-mail: roberto.mainini@astro.uio.no [Institute of Theoretical Astrophysics, University of Oslo, Box 1029, 0315 Oslo (Norway)] [Institute of Theoretical Astrophysics, University of Oslo, Box 1029, 0315 Oslo (Norway)

2009-04-15

220

Strong gravitational lensing and dark energy complementarity

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.

Linder, Eric V.

2004-01-21

221

Dark energy and formation of classical scalar fields

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.

Houri Ziaeepour

2006-04-03

222

NASA Astrophysics Data System (ADS)

Much evidence has accumulated that within the context of general relativistic Friedmann-Robertson-Walker (FRW) cosmology there must exist a new, and gravitationally repulsive, substance in the Universe. The effect of this new type of energy density on the expansion of the Universe is to cause its acceleration, and the name that is given to it is ‘Dark Energy’. To say whether or not Dark Energy really exists, however, requires a definite model for the Universe. That is, to be sure of the existence of Dark Energy, and the cosmological acceleration it causes, we must first be sure of the cosmological model we are using to interpret our observations. This is the subject of the present contribution, which will concentrate on the observational status of the Copernican Principle, which is at the heart of the FRW model. In particular, we will outline recent progress that has been made toward answering the question ‘can the observations usually requiring the existence of Dark Energy be accounted for without introducing any new and exotic types of energy density, if we are prepared to give up some of the assumptions of the standard cosmological model?’, or, alternatively, ‘is Dark Energy an illusion?’.

Clifton, Timothy

2011-04-01

223

Interaction Between DBI-Essence and Other Dark Energies

NASA Astrophysics Data System (ADS)

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 ?. 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).

Chattopadhyay, Surajit; Debnath, Ujjal

2010-07-01

224

Tachyon dark energy models: Dynamics and constraints

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

Gianluca Calcagni; Andrew R. Liddle

2006-01-01

225

Dark Energy vs. Dark Matter: Towards a Unifying Scalar Field?

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.

A. Arbey

2008-12-18

226

Dark Energy and Dark Matter in Galaxy Halos

We consider the possibility that the dark matter is coupled through its mass to a scalar field associated with the dark energy of the Universe. In order for such a field to play a role at the present cosmological distances, it must be effectively massless at galactic length scales. We discuss the effects of the field on the distribution of dark matter in galaxy halos. We show that the profile of the distribution outside the galaxy core remains largely unaffected and the approximately flat rotation curves persist. The dispersion of the dark matter velocity is enhanced by a potentially large factor relative to the case of zero coupling between dark energy and dark matter. The counting rates in terrestrial dark matter detectors are similarly enhanced. Existing bounds on the properties of dark matter candidates can be extended to the coupled case, by taking into account the enhancement factor.

N. Tetradis

2005-07-25

227

DARK ENERGY VS. DARK MATTER: TOWARDS A UNIFYING SCALAR FIELD?

Abstract. 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. 1

E. Pecontal; T. Buchert; Y. Copin; P. Di Stefano (eds; Arbey A

228

Thermodynamics of Viscous Dark Energy in AN Rsii Braneworld

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 = ThdSh + WdV, at the apparent horizon. In addition, the generalized second law of thermodynamics can be fulfilled in a region enclosed by the apparent horizon

M. R. Setare; A. Sheykhi

2010-01-01

229

Anisotropic Dark Energy and the Generalized Second Law of Thermodynamics

We consider a Bianchi type $I$ model in which anisotropic dark energy is interacting with dark matter and anisotropic radiation. With this scenario, we investigate the validity of the generalized second law of thermodynamics. It is concluded that the validity of this law depends on different parameters like shear, skewness and equation of state.

M. Sharif; Farida Khanum

2011-11-12

230

Interacting Ghost Dark Energy Models in the Higher Dimensional Cosmology

We investigate interacting ghost dark energy models in higher dimensional cosmology. We attempt to model dark matter within a barotropic fluid with $P_{b}=\\omega(t)_{b}\\rho$. In this work we consider four different models based on choosing equation of state parameter and interaction term. We confirm that our models agree with observational data.

J. Sadeghi; M. Khurshudyan; A. Movsisyan; H. Farahani

2014-01-26

231

Dressed Dark Solitons of the Defocusing Nonlinear Schrödinger Equation

NASA Astrophysics Data System (ADS)

The nonlinear Schrödinger equation is proved to be consistent-tanh-expansion (CTE) solvable. Some types of dark soliton solutions dressed by cnoidal periodic waves are obtained by means of the CTE method.

Lou, Sen-Yue; Cheng, Xue-Ping; Tang, Xiao-Yan

2014-07-01

232

Black holes in the presence of dark energy

The new, rapidly developing field of theoretical research --- studies of dark energy interacting with black holes (and, in particular, accreting onto black holes) --- is reviewed. The term `dark energy' is meant to cover a wide range of field theory models, as well as perfect fluids with various equations of state, including cosmological dark energy. Various accretion models are analyzed in terms of the simplest test field approximation or by allowing back reaction on the black-hole metric. The behavior of various types of dark energy in the vicinity of Schwarzschild and electrically charged black holes is examined. Nontrivial effects due to the presence of dark energy in the black hole vicinity are discussed. In particular, a physical explanation is given of why the black hole mass decreases when phantom energy is being accreted, a process in which the basic energy conditions of the famous theorem of nondecreasing horizon area in classical black holes are violated. The theoretical possibility of a signal esc...

Babichev, E O; Eroshenko, Yu N

2014-01-01

233

Dark matter, dark energy and gravitational proprieties of antimatter

We suggest that the eventual gravitational repulsion between matter and antimatter may be a key for understanding of the nature of dark matter and dark energy. If there is gravitational repulsion, virtual particle-antiparticle pairs in the vacuum, may be considered as gravitational dipoles. We use a simple toy model to reveal a first indication that the gravitational polarization of such a vacuum, caused by baryonic matter in a Galaxy, may produce the same effect as supposed existence of dark matter. In addition, we argue that cancellation of gravitational charges in virtual particle-antiparticle pairs, may be a basis for a solution of the cosmological constant problem and identification of dark energy with vacuum energy. Hence, it may be that dark matter and dark energy are not new, unknown forms of matter-energy but an effect of complex interaction between quantum vacuum and known baryonic matter.

Dragan Slavkov Hajdukovic

2008-10-19

234

Spherical Collapse Model And Dark Energy(I)

In existing literatures about the top-hat spherical collapse model of galaxy clusters formation in cosmology containing dark energies, dark energies are usually assumed not to cluster on this scale. But all these literatures ignored the current describing the flowing of dark energies outside the clusters which should exist under this assumption, so the conclusions of these literatures are worth further explorations. In this paper we study this model in QCDM or Phantom-CDM cosmologies(flat) by assuming that dark energies will cluster synchronously with ordinary matters on the scale of galaxy clusters so the dark energy current flowing outside the clusters does not exist at all and find that in this case, the key parameters of the model exhibit rather non-trivial and remarkable dependence on the equation of state coefficients of dark energies. We then apply the results in Press-Scheter theory and calculate the number density of galaxy clusters and its evolutions. We find that this two quantities are both affected exponentially by the equation of state coefficients of dark energies. We leave the study of this model with the assumption that dark energies do not cluster on the scale of galaxy clusters at all as the topic of another paper where similar conclusions will be obtained also.

Ding-fang Zeng; Yi-hong Gao

2005-05-09

235

Dark energy interacting with neutrinos and dark matter: a phenomenological theory

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.

G. M. Kremer

2007-04-03

236

On the determination of dark energy

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.

Clarkson, Chris [Cosmology and Gravity Group, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch 7701 (South Africa)

2010-06-23

237

Cosmology with Coupled Gravity and Dark Energy

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.

Ti-Pei Li

2014-09-01

238

Cosmology with Coupled Gravity and Dark Energy

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-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.

Ti-Pei Li

2014-09-01

239

Cosmology with Coupled Gravity and Dark Energy

The dark energy is a fundamental constitution of our universe, its role in the cosmological field equation should just like the gravity. Here we construct a dark energy and gravity coupling (DEMC) model of cosmology in a way that gravity and dark energy are introduced into the cosmological field equation in parallel to each other from the beginning. The DEMC universe possesses a composite symmetry constituted from the global Galileo invariance and local Lorentz invariance. The observed evolution trend of expansion rate at redshift z>1 is in tension with the standard LCDM model, but can be well predicted from measurements for only near epoch by the DEMC model. The so far most precise measured expansion rate at high z is quite a bit slower than the expectation from LCDM, but remarkably consistent with that from DEMC. It is hopeful that the DEMC scenario can also help to solve existed challenges to cosmology: large scale anomalies in CMB maps and large structures with dimension up to about 10^3 Mpc of a quasar group. The DEMC universe is a definite mechanical system, from measurements we can quantitatively evaluate its total rest energy, present absolute radius and expanding speed.

Ti-Pei Li

2014-09-01

240

The Hubble constant and dark energy from cosmological distance measures

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.

Ichikawa, Kazuhide [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan); Takahashi, Tomo, E-mail: kazuhide@icrr.u-tokyo.ac.jp, E-mail: tomot@cc.saga-u.ac.jp [Department of Physics, Saga University, Saga 840-8502 (Japan)

2008-04-15

241

QCD modified ghost scalar field dark energy models

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.

K. Karami; S. Asadzadeh; M. Mousivand; Z. Safari

2012-09-05

242

QCD Modified Ghost Scalar Field Dark Energy Models

NASA Astrophysics Data System (ADS)

Within the framework of FRW cosmology, we study the QCD modified ghost scalar field models of dark energy (DE) in the presence of both interaction and viscosity. For a spatially nonflat FRW universe containing modified ghost dark energy (MGDE) and dark matter (DM), 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.

Karami, K.; Asadzadeh, S.; Mousivand, M.; Safari, Z.

2013-04-01

243

Dark energy rest frame and the CMB dipole

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.

Antonio L. Maroto

2006-09-08

244

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.

Cahn, Robert N.; de Putter, Roland; Linder, Eric V.

2008-07-08

245

Inhomogeneous models of interacting dark matter and dark energy

We derive and analyze a class of spherically symmetric cosmological models whose source is an interactive mixture of inhomogeneous cold dark matter (DM) and a generic homogeneous dark energy (DE) fluid. If the DE fluid corresponds to a quintessense scalar field, the interaction term can be associated with a well motivated non--minimal coupling to the DM component. By constructing a suitable volume average of the DM component we obtain a Friedman evolution equation relating this average density with an average Hubble scalar, with the DE component playing the role of a repulsive and time-dependent $\\Lambda$ term. Once we select an ``equation of state'' linking the energy density ($\\mu$) and pressure ($p$) of the DE fluid, as well as a free function governing the radial dependence, the models become fully determinate and can be applied to known specific DE sources, such as quintessense scalar fields or tachyonic fluids. Considering the simple equation of state $p= (\\gamma-1) \\mu$ with $0\\leq\\gamma <2/3$, we show that the free parameters and boundary conditions can be selected for an adequate description of a local DM overdensity evolving in a suitable cosmic background that accurately fits current observational data. While a DE dominated scenario emerges in the asymptotic future, with total $\\Omega$ and $q$ tending respectively to 1 and -1/2 for all cosmic observers, the effects of inhomogeneity and anisotropy yield different local behavior and evolution rates for these parameters in the local overdense region. We suggest that the models presented can be directly applied to explore the effects of various DE formalisms on local DM cosmological inhomogeneities.

Roberto A Sussman; Israel Quiros; Osmel Martin Gonzalez

2005-03-29

246

w and w' of Scalar Field Models of Dark Energy

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.

Takeshi Chiba

2005-10-20

247

It is proposed that after the macroscopic fluctuation of energy density that is responsible for inflation dies away, a class of microscopic fluctuations, always present, survives to give the present day dark energy. This latter is simply a reinterpretation of the causet mechanism of Ahmed, Dodelson, Green and Sorkin, wherein the emergence of space is dropped but only energy considerations are maintained. At postinflation times, energy is exchanged between the "cisplanckian" cosmos and an unknown foam-like transplanckian reservoir. Whereas during inflation, the energy flows only from the latter to the former after inflation it fluctuates in sign thereby accounting for the tiny effective cosmological constant that seems to account for dark energy.

Robert Brout

2005-08-04

248

Tachyon Reconstruction of Interacting Ghost Dark Energy

Recently it has been argued that a possible source for the dark energy may arise due to the contribution to the vacuum energy of the QCD ghost in a time-dependent background. In this paper we establish a connection between interacting ghost dark energy and tachyon field. It is demonstrated that the evolution of the ghost dark energy dominated universe can

A. Sheykhi; M. Sadegh Movahed; E. Ebrahimi

2011-01-01

249

The cosmological constant and dark energy

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

P. J. Peebles; Bharat Ratra

2003-01-01

250

Agegraphic Chaplygin gas model of dark energy

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.

Ahmad Sheykhi

2010-02-07

251

Astrophysikalisches Institut Potsdam Probes of Dark Energy

Astrophysikalisches Institut Potsdam Probes of Dark Energy using Cosmological Simulations Nonlinear component, called dark energy. This unknown energy causes the expansion of the universe to accelerate theoretical model of dark energy has been developed. Instead a number of models have been proposed that range

252

Dark energy and 3-manifold topology

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.

Torsten Asselmeyer-Maluga; Helge Rose

2007-10-08

253

Cosmology with Coupled Gravity and Dark Energy

The dark energy is a fundamental constitution of our universe, its role in the cosmological field equation should just like the gravity. Here we construct a dark energy and gravity coupling (DEMC) model of cosmology in a way that gravity and dark energy are introduced into the cosmological field equation in parallel to each other from the beginning. The DEMC universe possesses a composite symmetry constituted from the global Galileo invariance and local Lorentz invariance. The observed evolution trend of expansion rate at redshift z>1 is in tension with the standard LCDM model, but can be well predicted from measurements for only near epoch by the DEMC model. The so far most precise measured expansion rate at high z is quite a bit slower than the expectation from LCDM, but remarkably consistent with that from DEMC. It is hopeful that the DEMC scenario can also help to solve existed challenges to cosmology: large scale anomalies in CMB maps and large structures with dimension up to about 10^3 Mpc of a quasar g...

Li, Ti-Pei

2014-01-01

254

Cosmological constraints on dark energy

NASA Astrophysics Data System (ADS)

It has been only 15 years since the discovery of dark energy (although some may argue there were strong indications even earlier). In the short time since measurements of type Ia supernovae indicated an accelerating universe, many other techniques have now confirmed the acceleration is real. The variety of ways in which dark energy has been confirmed is one of the reasons we are so confident in the statement that most of the energy in the universe is in a form we can not see except through its gravitational influence. This review aims to summarise briefly the many varied ways we now have measured dark energy. The fact that these different techniques all indicate that the simplest model remains the best—that dark energy contributes a constant background acceleration—is remarkable, since each of these different types of measurements represented opportunities for this simplest model to fail. Although we currently lack a compelling theoretical explanation for this acceleration, any explanation will have to explain the wide variety of complementary observations that we review here. This is an informal presentation, following the lines of the talk I presented at the General Relativity and Gravitation (GR20) conference in Warsaw in July 2013.

Davis, Tamara M.

2014-06-01

255

Interacting Ghost Dark Energy in Brans-Dicke Theory

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.

Esmaeil Ebrahimi; Ahmad Sheykhi

2011-05-28

256

Constraints on the sound speed of dark energy

We have studied constraints on the equation of state, $w$, and speed of sound, c_s, of the dark energy from a joint analysis of data from the cosmic microwave background, large scale structure and type-Ia supernovae. We find that current observations have no significant sensitivity to c_s. However, there is a slight difference between models in which there are no dark energy perturbations and models in which dark energy behaves as a fluid. Assuming that there are no dark energy perturbations shifts the allowed region for $w$ to slightly higher values. At present models with and without dark energy perturbations provide roughly equally good fits to observations, but the difference is potentially important for future parameter estimations. Finally, we have also performed error forecasts for future measurements of c_s.

Steen Hannestad

2005-04-01

257

Analysis of Generalized Ghost Version of Pilgrim Dark Energy

The proposal of pilgrim dark energy is based on the speculation that phantom-like dark energy possesses enough resistive force to preclude the black hole formation in the later universe. We explore this phenomenon by assuming the generalized ghost version of pilgrim dark energy. We find that most of the values of the interacting ($\\xi^2$) as well as pilgrim dark energy ($u$) parameters push the equation of state parameter towards phantom region. The squared speed of sound shows that this model remains stable in most of the cases of $\\xi^2$ and $u$. We also develop $\\omega_\\Lambda-\\omega'_\\Lambda$ plane and observe that this model corresponds to thawing as well as freezing regions. Finally, it is shown that the non-interacting and interacting generalized ghost versions of pilgrim dark energy correspond to $\\Lambda$CDM limit on the statefinder plane.

M. Sharif; Abdul Jawad

2013-12-20

258

Analysis of Generalized Ghost Version of Pilgrim Dark Energy

The proposal of pilgrim dark energy is based on the speculation that phantom-like dark energy possesses enough resistive force to preclude the black hole formation in the later universe. We explore this phenomenon by assuming the generalized ghost version of pilgrim dark energy. We find that most of the values of the interacting ($\\xi^2$) as well as pilgrim dark energy ($u$) parameters push the equation of state parameter towards phantom region. The squared speed of sound shows that this model remains stable in most of the cases of $\\xi^2$ and $u$. We also develop $\\omega_\\Lambda-\\omega'_\\Lambda$ plane and observe that this model corresponds to thawing as well as freezing regions. Finally, it is shown that the non-interacting and interacting generalized ghost versions of pilgrim dark energy correspond to $\\Lambda$CDM limit on the statefinder plane.

Sharif, M

2014-01-01

259

Dark Energy: Is It of Torsion Origin?

{\\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.

M. I. Wanas

2010-06-10

260

Curvature force and dark energy

A curvature self-interaction of the cosmic gas is shown to mimic a cosmological constant or other forms of dark energy, such as a rolling tachyon condensate or a Chaplygin gas. Any given Hubble rate and deceleration parameter can be traced back to the action of an effective curvature force on the gas particles. This force self-consistently reacts back on the

Alexander B. Balakin; Diego Pavón; Dominik J Schwarz; Winfried Zimdahl

2003-01-01

261

NASA Astrophysics Data System (ADS)

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. With reasonable prior information on other cosmological parameters, we find that a survey covering 1000 sq. deg. down to a limiting magnitude of R = 27 can impose constraints comparable to those expected from upcoming type Ia supernova and number-count surveys. This result, however, is contingent on the control of both observational and theoretical systematics. Concentrating on the latter, we find that the nonlinear power spectrum of matter perturbations and the redshift distribution of source galaxies both need to be determined accurately in order for weak lensing to achieve its full potential. Finally, we discuss the sensitivity of the three-point statistics to dark energy.

Huterer, Dragan

262

Scale Dependence of Dark Energy Antigravity

NASA Astrophysics Data System (ADS)

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.

Perivolaropoulos, L.

2002-09-01

263

Scale Dependence of Dark Energy Antigravity

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.

L. Perivolaropoulos

2001-06-25

264

Scale Dependence of Dark Energy Antigravity

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.

Perivolaropoulos, L

2001-01-01

265

Simple implementation of general dark energy models

NASA Astrophysics Data System (ADS)

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.

Bloomfield, Jolyon K.; Pearson, Jonathan A.

2014-03-01

266

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.

Ni, G

2005-01-01

267

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.

Guang-jiong Ni

2005-06-02

268

Interacting agegraphic tachyon model of dark energy

NASA Astrophysics Data System (ADS)

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.

Sheykhi, Ahmad

2010-01-01

269

Interacting agegraphic tachyon model of dark energy

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.

A. Sheykhi

2009-07-15

270

Dark energy and dark matter as curvature effects

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.

S. Capozziello; V. F. Cardone; A. Troisi

2006-02-15

271

Metamaterial Model of Tachyonic Dark Energy

NASA Astrophysics Data System (ADS)

Dark energy with negative pressure and positive energy density is believed to be responsible for the accelerated expansion of the universe. Quite a few theoretical models of dark energy are based on tachyonic fields interacting with itself and normal (bradyonic) matter. Here we propose an experimental model of tachyonic dark energy based on hyperbolic metamaterials. Wave equation describing propagation of extraordinary light inside hyperbolic metamaterials exhibits 2+1 dimensional Lorentz symmetry. The role of time in the corresponding effective 3D Minkowski spacetime is played by the spatial coordinate aligned with the optical axis of the metamaterial. Nonlinear optical Kerr effect bends this spacetime resulting in effective gravitational force between extraordinary photons. We demonstrate that this model has a self-interacting tachyonic sector having negative effective pressure and positive effective energy density. Moreover, a composite multilayer SiC-Si hyperbolic metamaterial exhibits closely separated tachyonic and bradyonic sectors in the long wavelength infrared range. This system may be used as a laboratory model of inflation and late time acceleration of the universe.

Smolyaninov, Igor

2014-02-01

272

Metamaterial model of tachyonic dark energy

Dark energy with negative pressure and positive energy density is believed to be responsible for the accelerated expansion of the universe. Quite a few theoretical models of dark energy are based on tachyonic fields interacting with itself and normal (bradyonic) matter. Here we propose an experimental model of tachyonic dark energy based on hyperbolic metamaterials. Wave equation describing propagation of extraordinary light inside hyperbolic metamaterials exhibits 2+1 dimensional Lorentz symmetry. The role of time in the corresponding effective 3D Minkowski spacetime is played by the spatial coordinate aligned with the optical axis of the metamaterial. Nonlinear optical Kerr effect bends this spacetime resulting in effective gravitational force between extraordinary photons. We demonstrate that this model has a self-interacting tachyonic sector having negative effective pressure and positive effective energy density. Moreover, a composite multilayer SiC-Si hyperbolic metamaterial exhibits closely separated tachyonic and bradyonic sectors in the long wavelength infrared range. This system may be used as a laboratory model of inflation and late time acceleration of the universe.

Igor I. Smolyaninov

2013-10-30

273

We explore the fate of the universe given the possibility that the density associated with `dark energy' may decay slowly with time. Decaying dark energy is modeled by a homogeneous scalar field which couples minimally to gravity and whose potential has {\\em at least one} local quadratic maximum. Dark energy decays as the scalar field rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the first, the dark energy potential is modeled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the field evolves, ensuring that a spatially flat universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to find allowed regions in the $\\lbrace m, \\phi_0\\rbrace$ plane ($m$ is the tachyon mass modulus and $\\phi_0$ the initial scalar field value; $m\\sim H_0$ and $\\phi_0\\sim M_P$ by order of magnitude). For the first model, the time for the potential to drop to half its maximum value is larger than $\\sim 8$ Gyrs. In the case of the cosine potential, the time left until the universe collapses is always greater than $\\sim 18$ Gyrs (both estimates are presented for $\\om = 0.3$, $m/H_0 \\sim 1$, $H_0 \\simeq 70$ km/sec/Mpc, and at the 95.4% confidence level).

Ujjaini Alam; Varun Sahni; A. A. Starobinsky

2003-02-15

274

Interacting agegraphic tachyon model of dark energy

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

Ahmad Sheykhi

2010-01-01

275

Cosmic structure growth and dark energy

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

Eric V. Linder; Adrian Jenkins

2003-01-01

276

Observational approaches to understanding dark energy

Illuminating the nature of dark energy is one of the most important challenges in cosmology today. In this review I discuss several promising observational approaches to understanding dark energy, in the context of the recommendations by the U.S. Dark Energy Task Force and the ESA-ESO Working Group on Fundamental Cosmology.

Yun Wang

2007-12-01

277

On the Ricci dark energy model

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.

Kyoung Yee Kim; Hyung Won Lee; Yun Soo Myung

2008-12-22

278

Figures of merit for present and future dark energy probes

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.

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

279

Studies of dark energy with X-ray observatories.

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

Vikhlinin, Alexey

2010-04-20

280

Studies of dark energy with x-ray observatories

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

Vikhlinin, Alexey

2010-01-01

281

Cosmological evolution of a quintom model of dark energy

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,

Zong-Kuan Guo; Yun-Song Piao; Xinmin Zhang; Yuan-Zhong Zhang

2005-01-01

282

Interaction Between DBI-Essence and Other Dark Energies

The present work considers interaction between DBI-essence and other candidates of dark energies like modified Chaplygin gas,\\u000a hessence, tachyonic field, and new agegraphic dark energy. The potentials of the fields have been reconstructed under interaction\\u000a and their evolutions have been viewed against cosmic time t and scalar field ?. Equation of state parameters have also been obtained. The nature of

Surajit Chattopadhyay; Ujjal Debnath

2010-01-01

283

Holographic Dark Energy with Generalized Chaplygin Gas in Higher Dimensions

NASA Astrophysics Data System (ADS)

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.

Ghose, S.; Saha, A.; Paul, B. C.

2014-11-01

284

From confinement to dark energy

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.

B. Holdom

2010-12-02

285

Dark energy from discrete spacetime.

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

Trout, Aaron D

2013-01-01

286

Dark Energy from Discrete Spacetime

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

Trout, Aaron D.

2013-01-01

287

Dark energy and the quietness of the local Hubble flow

NASA Astrophysics Data System (ADS)

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.

Axenides, M.; Perivolaropoulos, L.

2002-06-01

288

Reconstructing Quintom from Ricci Dark Energy

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.

Chao-Jun Feng

2008-10-15

289

Cosmic Constraint on Ricci Dark Energy Model

In this paper, a holographic dark energy model, dubbed Ricci dark energy, is confronted with cosmological observational data from type Ia supernovae (SN Ia), baryon acoustic oscillations (BAO) and cosmic microwave background (CMB). By using maximum likelihood method, it is found out that Ricci dark energy model is a viable candidate of dark energy model with the best fit parameters: $\\Omega_{m0}=0.34\\pm 0.04$, $\\alpha=0.38\\pm 0.03$ with $1\\sigma$ error. Here, $\\alpha$ is a dimensionless parameter related with Ricci dark energy $\\rho_{R}$ and Ricci scalar $R$, i.e., $\\rho_{R}\\propto \\alpha R$.

Lixin Xu; Wenbo Li; Jianbo Lu; Baorong Chang

2008-10-26

290

Inhomogeneous models of interacting dark matter and dark energy

We derive and analyze a class of spherically symmetric cosmological models whose source is an interactive mixture of inhomogeneous cold dark matter (DM) and a generic homogeneous dark energy (DE) fluid. If the DE fluid corresponds to a quintessence scalar field, the interaction term can be associated with a well motivated non-minimal coupling to the DM component. By constructing a

Roberto A. Sussman; Israel Quiros; Osmel Martín González

2005-01-01

291

Interacting Holographic Tachyon Model of Dark Energy

We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann-Robertson-Walker universe. We show that this model can describe the observed accelerated expansion

Alberto Rozas-Fernández; David Brizuela; Norman Cruz

2010-01-01

292

REPORT OF THE DARK ENERGY TASK FORCE

Bernstein, University of Pennsylvania Robert Cahn, Lawrence Berkeley National Laboratory Wendy L. Freedman. Suntzeff, Texas A&M University Dark energy appears to be the dominant component of the physical Universe of the Dark Energy Task Force..............................................11 V. Recommendations of the Dark

Hu, Wayne

293

Tachyon dark energy models: dynamics and constraints

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 behaviour 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.

Gianluca Calcagni; Andrew R. Liddle

2006-06-01

294

Dark energy versus modified gravity.

There is now strong observational evidence that the expansion of the Universe is accelerating. The standard explanation invokes an unknown "dark energy" component. But such scenarios are faced with serious theoretical problems, which has led to increased interest in models where instead general relativity is modified in a way that leads to the observed accelerated expansion. The question then arises whether the two scenarios can be distinguished. Here we show that this may not be so easy, demonstrating explicitly that a generalized dark energy model can match the growth rate of the Dvali-Gabadadze-Porrati model and reproduce the 3+1 dimensional metric perturbations. Cosmological observations are then unable to distinguish the two cases. PMID:17501110

Kunz, Martin; Sapone, Domenico

2007-03-23

295

Radiative inflation and dark energy

We propose a model based on radiative symmetry breaking that combines inflation with dark energy and is consistent with the Wilkinson Microwave Anisotropy Probe 7-year regions. The radiative inflationary potential leads to the prediction of a spectral index 0.955 < or approx. n{sub S} < or approx. 0.967 and a tensor to scalar ratio 0.142 < or approx. r < or approx. 0.186, both consistent with current data but testable by the Planck experiment. The radiative symmetry breaking close to the Planck scale gives rise to a pseudo Nambu-Goldstone boson with a gravitationally suppressed mass which can naturally play the role of a quintessence field responsible for dark energy. Finally, we present a possible extra dimensional scenario in which our model could be realized.

Di Bari, Pasquale; King, Stephen F. [School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ (United Kingdom); Luhn, Christoph [Institute for Particle Physics Phenomenology, Ogden Centre for Fundamental Physics, Department of Physics, University of Durham, Science Laboratories, South Road, Durham DH1 3LE (United Kingdom); Merle, Alexander [Max-Planck-Institut fuer Kernphysik, Postfach 10 39 80, 69029 Heidelberg (Germany); Department of Theoretical Physics, School of Engineering Sciences, Royal Institute of Technology (KTH) - AlbaNova University Centre, Roslagstullsbacken 21, 106 91 Stockholm (Sweden); Schmidt-May, Angnis [Department of Physics and Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm (Sweden)

2011-10-15

296

Designing dark energy afterglow experiments

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.

Amol Upadhye; Jason H. Steffen; Aaron S. Chou

2012-04-24

297

Fingerprinting Dark Energy III: distinctive marks of viscosity

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.

Domenico Sapone; Elisabetta Majerotto

2012-03-09

298

Genesis of Dark Energy: Dark Energy as a Consequence of Cosmological Nuclear Energy

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.

R. C. Gupta

2004-12-07

299

Energy Conservation (Bernoulli's Equation)

NSDL National Science Digital Library

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.

Shih, Chiang

2008-09-24

300

Clustering properties of dynamical dark energy models

We provide a generic but physically clear discussion of the clustering properties of dark energy models. We explicitly show that in quintessence-type models the dark energy fluctuations, on scales smaller than the Hubble radius, are of the order of the perturbations to the Newtonian gravitational potential, hence necessarily small on cosmological scales. Moreover, comparable fluctuations are associated with different gauge choices. We also demonstrate that the often used homogeneous approximation is unrealistic, and that the so-called dark energy mutation is a trivial artifact of an effective, single fluid description. Finally, we discuss the particular case where the dark energy fluid is nonminimally coupled to dark matter.

Avelino, P. P.; Beca, L. M. G. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Martins, C. J. A. P. [Centro de Astrofisica, Universidade do Porto, Rua das Estrelas s/n, 4150-762 Porto (Portugal); DAMTP, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2008-05-15

301

We propose the Gauss-Bonnet dark energy model inspired by string\\/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with a Gauss-Bonnet invariant. It is demonstrated that the effective phantom (or quintessence) phase of the late universe may occur in the presence of such a term when the scalar is phantom or for nonzero potential (for canonical scalar). However, with

Shin'ichi Nojiri; Sergei D. Odintsov; Misao Sasaki

2005-01-01

302

Dark Energy, Gravitation and Electromagnetism

In the context of the fact that the existence of dark energy causing the accelerated expansion of the universe has been confirmed by the WMAP and the Sloan Digital Sky Survey, we re-examine gravitation itself, starting with the formulation of Sakharov and show that it is possible to obtain gravitation in terms of the electromagnetic charge of elementary particles, once the ZPF and its effects at the Compton scale are taken into account.

B. G. Sidharth

2004-01-08

303

Transient and late time attractor tachyon dark energy: Can we distinguish it from quintessence?

The string inspired tachyon field can serve as a candidate of dark energy. Its equation of state parameter w varies from 0 to -1. In the case of tachyon field potential V(varphi)-->0 slower (faster) than 1\\/varphi2 at infinity, dark energy (dark matter) is a late time attractor. We investigate the tachyon dark energy models under the assumption that w is

Amna Ali; M. Sami; A. A. Sen

2009-01-01

304

Holographic dark energy with varying gravitational constant in Ho?ava-Lifshitz cosmology

We investigate the holographic dark energy scenario with a varying gravitational constant in a flat background in the context of Ho?ava-Lifshitz gravity. We extract the exact differential equation determining the evolution of the dark energy density parameter, which includes G variation term. Also we discuss a cosmological implication of our work by evaluating the dark energy equation of state for low redshifts containing varying G corrections.

Setare, M.R. [Department of Physics, University of Kurdistan, Pasdaran Ave., Sanandaj (Iran, Islamic Republic of); Jamil, Mubasher, E-mail: rezakord@ipm.ir, E-mail: mjamil@camp.nust.edu.pk [Center for Advanced Mathematics and Physics, National University of Sciences and Technology, Rawalpindi, 46000 (Pakistan)

2010-02-01

305

Holographic scalar field models of dark energy

NASA Astrophysics Data System (ADS)

Many theoretical attempts toward reconstructing the potential and dynamics of the scalar fields have been done in the literature by establishing a connection between holographic/agegraphic energy density and a scalar-field model of dark energy. However, in most of these cases the analytical form of the potentials in terms of the scalar field have not been reconstructed due to the complexity of the equations involved. In this paper, by taking Hubble radius as system’s IR cutoff, we are able to reconstruct the analytical form of the potentials as a function of scalar field, namely, V=V(?) as well as the dynamics of the scalar fields as a function of time, namely, ?=?(t), by establishing the correspondence between holographic energy density and quintessence, tachyon, K-essence, and dilaton energy density in a flat Friedmann-Robertson-Walker universe.

Sheykhi, Ahmad

2011-11-01

306

Bulk-Brane Interaction and Holographic Dark Energy

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.

M R Setare

2006-09-14

307

Spacetime Foam and Dark Energy

Due to quantum fluctuations, spacetime is foamy on small scales. The degree of foaminess is found to be consistent with the holographic principle. One way to detect spacetime foam is to look for halos in the images of distant quasars. Applying the holographic foam model to cosmology we "predict" that the cosmic energy density takes on the critical value; and basing only on existing archived data on active galactic nuclei from the Hubble Space Telescope, we also "predict" the existence of dark energy which, we argue, is composed of an enormous number of inert "particles" of extremely long wavelength. We speculate that these "particles" obey infinite statistics.

Y. Jack Ng

2008-08-08

308

Constraining dark energy with clusters: Complementarity with other probes

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.

Cunha, Carlos; Huterer, Dragan [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan 48109-1040 (United States); Frieman, Joshua A. [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, Illinois 60510 (United States); Kavli Institute for Cosmological Physics, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 (United States)

2009-09-15

309

Dark Energy Constraints from Galaxy Cluster Peculiar Velocities

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.

Suman Bhattacharya; Arthur Kosowsky

2007-12-01

310

New agegraphic dark energy in Ho?ava-Lifshitz cosmology

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.

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

311

A Kinematical Approach to Dark Energy Studies

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.

Rapetti, David; Allen, Steven W.; Amin, Mustafa A.; Blandford, Roger D.; /KIPAC, Menlo Park

2006-06-06

312

Dark energy might directly interact with cold dark matter. However, in such a scenario, an early-time large-scale instability occurs occasionally, which may be due to the incorrect treatment for the pressure perturbation of dark energy as a nonadiabatic fluid. To avoid this nonphysical instability, we establish a new framework to correctly calculate the cosmological perturbations in the interacting dark energy models. Inspired by the well-known parameterized post-Friedmann approach, the condition of the dark energy pressure perturbation is replaced with the relationship between the momentum density of dark energy and that of other components on large scales. By reconciling the perturbation evolutions on the large and small scales, one can complete the perturbation equations system. The large-scale instability can be successfully avoided and the well-behaved density and metric perturbations are obtained within this framework. Our test results show that this new framework works very well and is applicable to al...

Li, Yun-He; Zhang, Xin

2014-01-01

313

Holographic Scalar Fields Models of Dark Energy

Many theoretical attempts toward reconstructing the potential and dynamics of the scalar fields have been done in the literature by establishing a connection between holographic/agegraphic energy density and a scalar field model of dark energy. However, in most of these cases the analytical form of the potentials in terms of the scalar field have not been reconstructed due to the complexity of the equations involved. In this paper, by taking Hubble radius as system's IR cutoff, we are able to reconstruct the analytical form of the potentials as a function of scalar field, namely $V=V(\\phi)$ as well as the dynamics of the scalar fields as a function of time, namely $\\phi=\\phi(t)$ by establishing the correspondence between holographic energy density and quintessence, tachyon, K-essence and dilaton energy density in a flat FRW universe. The reconstructed potentials are quite reasonable and have scaling solutions. Our study further supports the viability of the holographic dark energy model with Hubble radius as IR cutoff.

Ahmad Sheykhi

2011-06-26

314

Cosmological zoo -- accelerating models with dark energy

ecent observations of type Ia supernovae indicate that the Universe is in an accelerating phase of expansion. The fundamental quest in theoretical cosmology is to identify the origin of this phenomenon. In principle there are two possibilities: 1) the presence of matter which violates the strong energy condition (a substantial form of dark energy), 2) modified Friedmann equations (Cardassian models -- a non-substantial form of dark matter). We classify all these models in terms of 2-dimensional dynamical systems of the Newtonian type. We search for generic properties of the models. It is achieved with the help of Peixoto's theorem for dynamical system on the Poincar{\\'e} sphere. We find that the notion of structural stability can be useful to distinguish the generic cases of evolutional paths with acceleration. We find that, while the $\\Lambda$CDM models and phantom models are typical accelerating models, the cosmological models with bouncing phase are non-generic in the space of all planar dynamical systems. We derive the universal shape of potential function which gives rise to presently accelerating models. Our results show explicitly the advantages of using a potential function (instead of the equation of state) to probe the origin of the present acceleration. We argue that simplicity and genericity are the best guide in understanding our Universe and its acceleration.

Marek Szydlowski

2006-10-09

315

A unifying description of dark energy

We review and extend a novel approach that we introduced recently, to describe general dark energy or scalar-tensor models. Our approach relies on an ADM formulation based on the hypersurfaces where the underlying scalar field is uniform. The advantage of this approach is that it can describe in the same language and in a minimal way a vast number of existing models, such as quintessence models, $F(R)$ theories, scalar tensor theories, their Horndeski extensions and beyond. It also naturally includes Horava-Lifshitz theories. As summarized in this review, our approach provides a unified treatment of the linear cosmological perturbations about a FLRW universe, obtained by a systematic expansion of our general action up to quadratic order. This shows that the behaviour of these linear perturbations is generically characterized by five time-dependent functions. We derive the full equations of motion in the Newtonian gauge, and obtain in particular the equation of state for dark energy perturbations, in the Hornd...

Gleyzes, Jérôme; Vernizzi, Filippo

2014-01-01

316

Stability of Ghost Dark Energy in CBD Model of Gravity

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 <0 $ the model is instable. However, we study the interacting and non-interacting version of chameleon Brans-Dicke 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.

Khaled Saaidi

2012-02-18

317

Stability of Ghost Dark Energy in CBD Model of Gravity

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 <0 $ the model is instable. However, we study the interacting and non-interacting version of chameleon Brans-Dicke 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.

Saaidi, Khaled

2012-01-01

318

No evidence for Dark Energy Metamorphosis ?

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.

Jonsson, J; Amanullah, R; Bergström, L

2004-01-01

319

No evidence for Dark Energy Metamorphosis ?

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.

J. Jonsson; A. Goobar; R. Amanullah; L. Bergstrom

2004-04-23

320

Dark energy in some integrable and nonintegrable FRW cosmological models

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.

Kuralay Esmakhanova; Nurgissa Myrzakulov; Gulgasyl Nugmanova; Yerlan Myrzakulov; Leonid Chechin; Ratbay Myrzakulov

2011-04-19

321

Parametrized post-Friedmann framework for interacting dark energy

NASA Astrophysics Data System (ADS)

Dark energy might directly interact with cold dark matter. However, in such a scenario, an early-time large-scale instability occurs occasionally, which may be due to the incorrect treatment for the pressure perturbation of dark energy as a nonadiabatic fluid. To avoid this nonphysical instability, we establish a new framework to correctly calculate the cosmological perturbations in the interacting dark energy models. Inspired by the well-known parametrized post-Friedmann approach, the condition of the dark energy pressure perturbation is replaced with the relationship between the momentum density of dark energy and that of other components on large scales. By reconciling the perturbation evolutions on the large and small scales, one can complete the perturbation equations system. The large-scale instability can be successfully avoided and the well-behaved density and metric perturbations are obtained within this framework. Our test results show that this new framework works very well and is applicable to all the interacting dark energy models.

Li, Yun-He; Zhang, Jing-Fei; Zhang, Xin

2014-09-01

322

Parametrized Post-Friedmann Framework for Interacting Dark Energy

Dark energy might directly interact with cold dark matter. However, in such a scenario, an early-time large-scale instability occurs occasionally, which may be due to the incorrect treatment for the pressure perturbation of dark energy as a nonadiabatic fluid. To avoid this nonphysical instability, we establish a new framework to correctly calculate the cosmological perturbations in the interacting dark energy models. Inspired by the well-known parametrized post-Friedmann approach, the condition of the dark energy pressure perturbation is replaced with the relationship between the momentum density of dark energy and that of other components on large scales. By reconciling the perturbation evolutions on the large and small scales, one can complete the perturbation equations system. The large-scale instability can be successfully avoided and the well-behaved density and metric perturbations are obtained within this framework. Our test results show that this new framework works very well and is applicable to all the interacting dark energy models.

Yun-He Li; Jing-Fei Zhang; Xin Zhang

2014-04-21

323

A two measure model of dark energy and dark matter

NASA Astrophysics Data System (ADS)

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, phi with a non-conventional kinetic term, as in the string theory tachyon; an arbitrary potential, V(phi); two measures — a metric measure ((-g)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.

Guendelman, Eduardo; Singleton, Douglas; Yongram, N.

2012-11-01

324

Leptogenesis, Dark Energy, Dark Matter and the neutrinos

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.

Utpal Sarkar

2007-12-17

325

A two measure model of dark energy and dark matter

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.

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

326

On the Chemical Potential of Dark Energy

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.

S. H. Pereira

2008-06-23

327

Curvature force and dark energy

A curvature self-interaction of the cosmic gas is shown to mimic a cosmological constant or other forms of dark energy, such as a rolling tachyon condensate or a Chaplygin gas. Any given Hubble rate and deceleration parameter can be traced back to the action of an effective curvature force on the gas particles. This force self-consistently reacts back on the cosmological dynamics. The links between an imperfect fluid description, a kinetic description with effective antifriction forces, and curvature forces, which represent a non-minimal coupling of gravity to matter, are established.

Alexander B. Balakin; Diego Pavon; Dominik J. Schwarz; Winfried Zimdahl

2003-02-07

328

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

Zibin, James P; Moss, Adam; Scott, Douglas

2008-12-19

329

Dark energy interacting with dark matter and unparticle

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.

Songbai Chen; Jiliang Jing

2009-03-01

330

Dark Energy and Neutrino CPT Violation

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.

Pei-Hong Gu; Xiao-Jun Bi; Xinmin Zhang

2005-11-03

331

Black holes in the presence of dark energy

The new, rapidly developing field of theoretical research --- studies of dark energy interacting with black holes (and, in particular, accreting onto black holes) --- is reviewed. The term `dark energy' is meant to cover a wide range of field theory models, as well as perfect fluids with various equations of state, including cosmological dark energy. Various accretion models are analyzed in terms of the simplest test field approximation or by allowing back reaction on the black-hole metric. The behavior of various types of dark energy in the vicinity of Schwarzschild and electrically charged black holes is examined. Nontrivial effects due to the presence of dark energy in the black hole vicinity are discussed. In particular, a physical explanation is given of why the black hole mass decreases when phantom energy is being accreted, a process in which the basic energy conditions of the famous theorem of nondecreasing horizon area in classical black holes are violated. The theoretical possibility of a signal escaping from beneath the black hole event horizon is discussed for a number of dark energy models. Finally, the violation of the laws of thermodynamics by black holes in the presence of noncanonical fields is considered.

E. O. Babichev; V. I. Dokuchaev; Yu. N. Eroshenko

2014-06-03

332

Unification of Gravitation, Gauge Field and Dark Energy

This paper is composed of two correlated topics: 1. unification of gravitation with gauge fields; 2. the coupling between the daor field and other fields and the origin of dark energy. After introducing the concept of ``daor field" and discussing the daor geometry, we indicate that the complex daor field has two kinds of symmetry transformations. Hence the gravitation and SU(1,3) gauge field are unified under the framework of the complex connection. We propose a first-order nonlinear coupling equation of the daor field, which includes the coupling between the daor field and SU(1,3) gauge field and the coupling between the daor field and the curvature, and from which Einstein's gravitational equation can be deduced. The cosmological observations imply that dark energy cannot be zero, and which will dominate the doom of our Universe. The real part of the daor field self-coupling equation can be regarded as Einstein's equation endowed with the cosmological constant. It shows that dark energy originates from the self-coupling of the space-time curvature, and the energy-momentum tensor is proportional to the square of coupling constant \\lambda. The dark energy density given by our scenario is in agreement with astronomical observations. Furthermore, the Newtonian gravitational constant G and the coupling constant \\epsilon of gauge field satisfy G= \\lambda^{2}\\epsilon^{2}.

Xin-Bing Huang

2004-10-27

333

Raytracing simulations of coupled dark energy models

Dark matter and dark energy are usually assumed to be independent, coupling only gravitationally. An extension to this simple picture is to model dark energy as a scalar field which is directly coupled to the cold dark matter fluid. Such a non-trivial coupling in the dark sector leads to a fifth force and a time-dependent dark matter particle mass. In this work we examine the impact that dark energy-dark matter couplings have on weak lensing statistics by constructing realistic simulated weak-lensing maps using raytracing techniques through a suite of N-body cosmological simulations. We construct maps for an array of different lensing quantities, covering a range of scales from a few arcminutes to several degrees. The concordance $\\Lambda$CDM model is compared to different coupled dark energy models, described either by an exponential scalar field potential (standard coupled dark energy scenario) or by a SUGRA potential (bouncing model). We analyse several statistical quantities, in particular the power spect...

Pace, Francesco; Moscardini, Lauro; Bacon, David; Crittenden, Robert

2014-01-01

334

The Dark Energy Camera (DECam)

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.

K. Honscheid; D. L. DePoy; for the DES Collaboration

2008-10-20

335

Nonparametric dark energy reconstruction from supernova data.

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

Holsclaw, Tracy; Alam, Ujjaini; Sansó, Bruno; Lee, Herbert; Heitmann, Katrin; Habib, Salman; Higdon, David

2010-12-10

336

Dark energy and supermassive black holes

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 accelerate 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.

Pedro F. Gonzalez-Diaz

2004-08-24

337

Dark energy and supermassive black holes

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.

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

338

Can cosmological observations uniquely determine the nature of dark energy?

The observational effect of all minimally coupled scalar field models of dark energy can be determined by the behavior of the following two parameters: (1) equation of state parameter w, which relates dark energy pressure to its energy density, and (2) effective speed of sound c{sub e}{sup 2}, which relates dark energy pressure fluctuation to its density fluctuation. In this paper we show that these two parameters do not uniquely determine the form of a scalar field dark energy Lagrangian even after taking into account the perturbation in the scalar field. We present this result by showing that two different forms of scalar field Lagrangian can lead to the same values for these paired parameters. It is well known that from the background evolution the Lagrangian of the scalar field dark energy cannot be uniquely determined. The two models of dark energy presented in this paper are indistinguishable from the evolution of background as well as from the evolution of perturbations from a Friedmann-Robertson-Walker metric.

Unnikrishnan, Sanil [Department of Physics and Astrophysics, University of Delhi, Delhi 110007 (India)

2008-09-15

339

Topology and dark energy: testing gravity in voids.

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

Spolyar, Douglas; Sahlén, Martin; Silk, Joe

2013-12-13

340

Holographic Dark Energy Model: State Finder Parameters

In this work, we have studied interacting holographic dark energy model in the background of FRW model of the universe. The interaction is chosen either in linear combination or in product form of the matter densities for dark matter and dark energy. The IR cut off for holographic dark energy is chosen as Ricci's length scale or radius of the future event horizon. The analysis is done using the state finder parameter and coincidence problem has been graphically presented. Finally, universal thermodynamics has been studied using state finder parameters.

Nairwita Mazumder; Ritabrata Biswas; Subenoy Chakraborty

2011-10-19

341

Dark MaGICC: the effect of Dark Energy on galaxy formation. Cosmology does matter

We present the Dark MaGICC project, which aims to investigate the effect of Dark Energy (DE) modeling on galaxy formation via hydrodynamical cosmological simulations. Dark MaGICC includes four dynamical Dark Energy scenarios with time varying equations of state, one with a self-interacting Ratra-Peebles model. In each scenario we simulate three galaxies with high resolution using smoothed particle hydrodynamics (SPH). The baryonic physics model is the same used in the Making Galaxies in a Cosmological Context (MaGICC) project, and we varied only the background cosmology. We find that the Dark Energy parameterization has a surprisingly important impact on galaxy evolution and on structural properties of galaxies at z=0, in striking contrast with predictions from pure Nbody simulations. The different background evolutions can (depending on the behavior of the DE equation of state) either enhance or quench star formation with respect to a LCDM model, at a level similar to the variation of the stellar feedback pa...

Penzo, Camilla; Casarini, Luciano; Stinson, Greg S; Wadsley, James

2014-01-01

342

A Quantum Cosmology: No Dark Matter, Dark Energy nor Accelerating Universe

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.

Reginald T Cahill

2007-09-18

343

Thermodynamics of interacting holographic dark energy

The thermodynamics of a scheme of dark matter-dark energy interaction is studied considering a holographic model for the dark energy in a flat Friedmann-Lemaitre-Robertson-Walker background. We obtain a total entropy rate for a general horizon and study the Generalized Second Law of Thermodynamics. In addition, we study two horizons related to the Ricci and Ricci-like model and its effect on an interacting system.

Arevalo, Fabiola; Pena, Francisco

2014-01-01

344

Dark Energy From Vacuum Fluctuations

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.

S. G. Djorgovski; V. G. Gurzadyan

2006-10-06

345

Dark matter and dark energy via nonperturbative (flavor) vacua

NASA Astrophysics Data System (ADS)

A nonperturbative field theoretical approach to flavor physics (Blasone-Vitiello formalism) has been shown to imply a highly nontrivial vacuum state. Although still far from representing a satisfactory framework for a coherent and complete characterization of flavor states, in recent years the formalism has received attention for its possible implications at cosmological scales. In a previous work, we implemented the approach on a simple supersymmetric model (free Wess-Zumino), with flavor mixing, which was regarded as a model for free neutrinos and sneutrinos. The resulting effective vacuum (called flavor vacuum) was found to be characterized by a strong supersymmetry breaking. In this paper we explore the phenomenology of the model and we argue that the flavor vacuum is a consistent source for both dark energy (thanks to the bosonic sector of the model) and dark matter (via the fermionic one). Quite remarkably, besides the parameters connected with neutrino physics, in this model no other parameters have been introduced, possibly leading to a predictive theory of dark energy/matter. Despite its oversimplification, such a toy model already seems capable to shed some light on the observed energy hierarchy between neutrino physics, dark energy and dark matter. Furthermore, we move a step forth in the construction of a more realistic theory, by presenting a novel approach for calculating relevant quantities and hence extending some results to interactive theories, in a completely nonperturbative way.

Tarantino, Walter

2012-02-01

346

Nonlocal String Tachyon as a Model for Cosmological Dark Energy

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<-1, i.e.

I. Ya. Aref'eva

2004-01-01

347

Nonlocal String Tachyon as a Model for Cosmological Dark Energy

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 <

Irina Ya. Aref’eva

2006-01-01

348

Cosmological Tests for Minimally Coupled Perturbed Dark Energy

A dark energy is treated as a minimally coupled scalar fields. Both the cases of classical or tachyonic scalar fields have been considered. The evolution of scalar linear perturbations is studied, for a multi-component cosmological model, with the fluid approach used for description of components. For these scalar fields, the parameter ``equation of state'' is assumed to vary with time,

S. Apunevych; B. Novosyadlyj; O. Sergijenko

2010-01-01

349

Perturbed dark energy: Classical scalar field versus tachyon

The evolution of scalar linear perturbations is studied in gauge-invariant approach for 2-component models with nonrelativistic matter and minimally coupled scalar fields, the potentials of which were constructed for either constant dark energy equation of state parameter w or its adiabatic sound speed ca2 equal to zero. The numerical solutions show that such fields are almost smoothed out on subhorizon

Olga Sergijenko; Bohdan Novosyadlyj

2009-01-01

350

Hessence: a new view of quintom dark energy

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

Hao Wei; Rong-Gen Cai; Ding-Fang Zeng

2005-01-01

351

Nonlocal String Tachyon as a Model for Cosmological Dark Energy

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 <

I. Ya

352

Cosmological Surprises from Braneworld models of Dark Energy

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.

Varun Sahni

2005-01-01

353

Dark energy: Vacuum fluctuations, the effective phantom phase, and holography

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

Emilio Elizalde; Shin'ichi Nojiri; S. D. Odintsov; Peng Wang

2005-01-01

354

GALAXY CLUSTERS AS A PROBE OF EARLY DARK ENERGY

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.

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

355

Instability of holographic dark energy models

We investigate the difference between holographic dark energy, Chaplygin gas, and tachyon model with constant potential. For this purpose, we examine their squared speeds of sound which are evaluated to zeroth order in perturbation theory and hence depends only on time. We find that the squared speed for holographic dark energy is always negative when choosing the future event horizon

Yun Soo Myung; Hyung Won Lee

2007-01-01

356

Statefinder Parameters for Tachyon Dark Energy Model

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,

Ying Shao; Yuanxing Gui

2008-01-01

357

The Amplitude of Dark Energy Perturbations

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 anti-correlation 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.

Christopher Gordon; David Wands

2005-04-06

358

Probing dark energy: Methods and strategies

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

Dragan Huterer; Michael S. Turner

2001-01-01

359

The case for dynamical dark energy revisited

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

Ujjaini Alam; Varun Sahni; A. A. Starobinsky

2004-01-01

360

Singularity-free dark energy star

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.

Farook Rahaman; Anil Kumar Yadav; Saibal Ray; Raju Maulick; Ranjan Sharma

2011-02-07

361

The possible nature of dark energy and dark matter Nathalie Olivi-Tran1,2

The possible nature of dark energy and dark matter Nathalie Olivi-Tran1,2 1 Laboratoire Charles: Nathalie.Olivi-Tran@univ-montp2.fr (Dated: July 23, 2012) Abstract The nature of dark matter is linked the nature of time. Keywords: dark matter; dark energy; nature of time 1 hal-00719998,version1-23Jul2012

Paris-Sud XI, UniversitÃ© de

362

Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios T. Jenke,1,*

Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios T. Jenke,1,* G) dark matter axionlike spin-mass coupling is excluded for the coupling strength gsgp > 3.7 Ã? 10-16 (5 this work, we link these new measurements to dark matter and dark energy searches. Observational cosmology

Rotter, Stefan

363

Inferring "Dark Matter" and "Dark Energy" from Videos Dan Xie , Sinisa Todorovic

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 to approach "dark matter" and thus sat- isfy their needs, where their motions are subject to a com- posite

Zhu, Song Chun

364

First-order formalism for dark energy and dust

This work deals with first-order formalism for dark energy and dust in standard cosmology, for models described by real scalar field in the presence of dust in spatially flat space. The field dynamics may be standard or tachyonic, and we show how the equations of motion can be solved by first-order differential equations. We investigate a model to illustrate how the dustlike matter may affect the cosmic evolution using this framework.

D. Bazeia; L. Losano; J. J. Rodrigues; R. Rosenfeld

2006-11-24

365

Is there supernova evidence for dark energy metamorphosis?

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

Ujjaini Alam; Varun Sahni; Tarun Deep Saini; A. A. Starobinsky

2004-01-01

366

CMB lensing constraints on neutrinos and dark energy

Signatures of lensing of the cosmic microwave background radiation by gravitational potentials along the line of sight carry with them information on the matter distribution, neutrino masses, and dark energy properties. We examine the constraints that Planck, PolarBear, and CMBpol future data, including from the B-mode polarization or the lensing potential, will be able to place on these quantities. We simultaneously fit for neutrino mass and dark energy equation of state including time variation and early dark energy density, and compare the use of polarization power spectra with an optimal quadratic estimator of the lensing. Results are given as a function of systematics level from residual foreground contamination. A realistic CMBpol experiment can effectively constrain the sum of neutrino masses to within 0.05 eV and the fraction of early dark energy to 0.002. We also present a surprisingly simple prescription for calculating dark energy equation of state constraints in combination with supernova distances from JDEM.

Putter, Roland de; Zahn, Oliver; Linder, Eric V. [Berkeley Lab and Berkeley Center for Cosmological Physics, University of California, Berkeley, California 94720 (United States)

2009-03-15

367

Examining the evidence for dynamical dark energy.

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

Zhao, Gong-Bo; Crittenden, Robert G; Pogosian, Levon; Zhang, Xinmin

2012-10-26

368

The Dark Energy Survey instrument design

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.

Flaugher, B.; /Fermilab

2006-05-01

369

Tachyon reconstruction of ghost dark energy

NASA Astrophysics Data System (ADS)

Recently it has been argued that a possible source for the dark energy may arise due to the contribution to the vacuum energy of the QCD ghost in a time-dependent background. In this paper we establish a connection between interacting ghost dark energy and tachyon field. It is demonstrated that the evolution of the ghost dark energy dominated universe can be described completely by a single tachyon scalar field. The potential and the dynamics of the tachyon field are reconstructed according to the evolutionary behavior of ghost energy density.

Sheykhi, A.; Sadegh Movahed, M.; Ebrahimi, E.

2012-05-01

370

Vacuum quantum fluctuation energy in expanding universe and dark energy

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.

Shun-Jin Wang

2013-01-02

371

Holographic tachyon model of dark energy

In this Letter 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.

M. R. Setare

2007-01-01

372

Holographic tachyon model of dark energy

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.

M R Setare

2007-05-24

373

On cosmic acceleration without dark energy

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.

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

374

Entropic-force dark energy reconsidered

We reconsider the entropic-force model in which both kind of Hubble terms ${\\dot H}$ and $H^{2}$ appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions, namely the scale factor, deceleration parameter, matter density and growth of linear matter perturbations. However, we find that the entropic-force model is not viable at the background and perturbation levels due to the fact that the entropic formulation does not add a constant term in the Friedmann equations. On the other hand, if on mere phenomenological grounds we replace the ${\\dot H}$ dependence of the effective DE density with a linear term $H$ without including a constant additive term, we find that the transition from deceleration to acceleration becomes possible but the recent structure formation data \

Basilakos, Spyros

2014-01-01

375

Testing the Dark-Energy Cosmology by the Solar-System Experiments

NASA Astrophysics Data System (ADS)

According to the recent astronomical data, the most part of energy in the Universe is in the 'dark' form, which is effectively described by ?-term in Einstein equations. All arguments in favor of the dark energy were obtained so far from the observational data related to very large (intergalactic) scales. Is it possible to find a manifestation of the dark energy at much less scales (e.g. inside the Solar system)?... Note from Publisher: This article contains the abstract and references only.

Dumin, Yurii V.

2008-09-01

376

Statefinder diagnosis for the interacting model of holographic dark energy

In this Letter, we investigate the holographic dark energy model with interaction between dark energy and dark matter, from the statefinder viewpoint. We plot the trajectories of the interacting holographic dark energy model for different interaction cases as well as for different values of the parameter c in the statefinder-plane. The statefinder diagrams characterize the properties of the holographic dark

Jingfei Zhang; Xin Zhang; Hongya Liu

2008-01-01

377

Quintom dark energy models with nearly flat potentials

We examine quintom dark energy models, produced by the combined consideration of a canonical and a phantom field, with nearly flat potentials and dark energy equation-of-state parameter w{sub DE} close to -1. We find that all such models converge to a single expression for w{sub DE}(z), depending only on the initial field values and their derivatives. We show that this quintom paradigm allows for a description of the transition through -1 in the near cosmological past. In addition, we provide the necessary conditions for the determination of the direction of the -1 crossing.

Setare, M. R.; Saridakis, E. N. [Department of Science, Payame Noor University, Bijar (Iran, Islamic Republic of); Department of Physics, University of Athens, GR-15771 Athens (Greece)

2009-02-15

378

Can we test Dark Energy with Running Fundamental Constants ?

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.

Michael Doran

2004-11-22

379

A quantum model of dark energy

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.

Chang-Yu Zhu; Heng Fan

2009-11-20

380

Dark energy and dark matter from Bose-Einstein condensate

We propose that gravitons with a very small mass (but consistent with experiments) may form a Bose-Einstein condensate, with its macroscopic wave function spanning the universe and producing a quantum potential. The latter's contribution to the cosmological constant accounts for the observed dark energy content and the accelerated expansion of our universe. Further, the density of the Bose-Einstein condensed gravitons in the lowest zero momentum state accounts for its cold dark matter content. In the far future this condensate is all that remains of our universe.

Saurya Das; Rajat K. Bhaduri

2014-11-04

381

Dark energy and dark matter from Bose-Einstein condensate

We propose that gravitons with a very small mass (but consistent with experiments) may form a Bose-Einstein condensate, with its macroscopic wave function spanning the universe and producing a quantum potential. The latter's contribution to the cosmological constant accounts for the observed dark energy content and the accelerated expansion of our universe. Further, the density of the Bose-Einstein condensed gravitons in the lowest zero momentum state accounts for its cold dark matter content. In the far future this condensate is all that remains of our universe.

Das, Saurya

2014-01-01

382

Dark Matter and Dark Energy in the Universe

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

Michael S. Turner

2000-01-01

383

Comparing holographic dark energy models with statefinder

We apply the statefinder diagnostic to the holographic dark energy models, including the original holographic dark energy (HDE) model, the new holographic dark energy model, the new agegraphic dark energy (NADE) model, and the Ricci dark energy model. In the low-redshift region the holographic dark energy models are degenerate with each other and with the $\\Lambda$CDM model in the $H(z)$ and $q(z)$ evolutions. In particular, the HDE model is highly degenerate with the $\\Lambda$CDM model, and in the HDE model the cases with different parameter values are also in strong degeneracy. Since the observational data are mainly within the low-redshift region, it is very important to break this low-redshift degeneracy in the $H(z)$ and $q(z)$ diagnostics by using some quantities with higher order derivatives of the scale factor. It is shown that the statefinder diagnostic $r(z)$ is very useful in breaking the low-redshift degeneracies. By employing the statefinder diagnostic the holographic dark energy models can be differentiated efficiently in the low-redshift region. The degeneracy between the holographic dark energy models and the $\\Lambda$CDM model can also be broken by this method. Especially for the HDE model, all the previous strong degeneracies appearing in the $H(z)$ and $q(z)$ diagnostics are broken effectively. But for the NADE model, the degeneracy between the cases with different parameter values cannot be broken, even though the statefinder diagnostic is used. A direct comparison of the holographic dark energy models in the $r$--$s$ plane is also made, in which the separations between the models (including the $\\Lambda$CDM model) can be directly measured in the light of the current values $\\{r_0,s_0\\}$ of the models.

Jing-Lei Cui; Jing-Fei Zhang

2014-02-08

384

Tachyon as a Dark Energy Source

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''.

S. K. Srivastava

2004-09-20

385

Tachyon as a Dark Energy Source

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''.

S. K. Srivastava

2004-01-01

386

Tachyon as a Dark Energy Source

It is demonstrated that dark energy, driven by tachyons having non-minimal\\u000acoupling with curvature and self-interacting inverse cubic potential, decays to\\u000acold dark matter in the late universe. It is found that this penomenon yields a\\u000asolution to `` cosmic coincidence problem''.

S. K. Srivastava

2004-01-01

387

Report of the Dark Energy Task Force

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

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

388

Tommaso Treu, UC Santa Barbara What is the nature of dark energy and dark matter?

Tommaso Treu, UC Santa Barbara What is the nature of dark energy and dark matter? I will describe of the cold dark matter model, i.e. that galaxies should be surrounded by large numbers of dark satellite exotic alternatives like warm dark matter. I will show first results from this experiment based on Keck

Glashausser, Charles

389

Instability of holographic dark energy models

We investigate the difference between holographic dark energy, Chaplygin gas, and tachyon model with constant potential. For this purpose, we examine their squared speeds of sound which are evaluated to zeroth order in perturbation theory and hence depends only on time. We find that the squared speed for holographic dark energy is always negative when choosing the future event horizon as the IR cutoff, while those for Chaplygin gas and tachyon are non-negative. This means that the perfect fluid for holographic dark energy is classically unstable. Hence the holographic interpretation for Chaplygin gas and tachyon is problematic.

Yun Soo Myung

2007-06-26

390

Spinor dark energy and cosmological coincidence problem

NASA Astrophysics Data System (ADS)

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.

Wei, Hao

2011-01-01

391

Spinor Dark Energy and Cosmological Coincidence Problem

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.

Hao Wei

2010-02-23

392

Statefinder Diagnosis for Ricci Dark Energy

Statefinder diagnostic is a useful method which can differ one dark energy model from each others. In this letter, we apply this method to a holographic dark energy model from Ricci scalar curvature, called the Ricci dark energy model(RDE). We plot the evolutionary trajectories of this model in the statefinder parameter-planes, and it is found that the parameter of this model plays a significant role from the statefinder viewpoint. In a very special case, the statefinder diagnostic fails to discriminate LCDM and RDE models, thus we apply a new diagnostic called the Om diagnostic proposed recently to this model in this case in Appendix A and it works well.

Chao-Jun Feng

2008-09-15

393

Is Hubble's Expansion due to Dark Energy

{\\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'.

R. C. Gupta; Anirudh Pradhan

2010-10-19

394

What We Know About Dark Energy from Supernovae

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.

Filippenko, Alex (University of California, Berkeley) [University of California, Berkeley

2008-05-21

395

Voids as a precision probe of dark energy

The shapes of cosmic voids, as measured in spectroscopic galaxy redshift surveys, constitute a promising new probe of dark energy (DE). We forecast constraints on the DE equation of state and its variation from current and future surveys and find that the promise of void shape measurements compares favorably to that of standard methods such as supernovae and cluster counts even for currently available data. Owing to the complementary nature of the constraints, void shape measurements improve the Dark Energy Task Force figure of merit by 2 orders of magnitude for a future large scale experiment such as EUCLID when combined with other probes of dark energy available on a similar time scale. Modeling several observational and theoretical systematics has only moderate effects on these forecasts. We discuss additional systematics which will require further study using simulations.

Biswas, Rahul; Alizadeh, Esfandiar [Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, Illinois 61801 (United States); Wandelt, Benjamin D. [UPMC Universite Paris-06, Institut d'Astrophysique de Paris, 98 bis, boulevard Arago, 75014 Paris (France); Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, Illinois 61801 (United States); Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, Illinois 61801 (United States)

2010-07-15

396

What We Know About Dark Energy From Supernovae

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.

Alex Filippenko

2008-05-21

397

Dark Energy, A Cosmological Constant, and Type Ia Supernovae

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.

Lawrence M. Krauss; Katherine Jones-Smith; Dragan Huterer

2007-01-24

398

What We Know About Dark Energy From Supernovae

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.

Alex Filippenko

2010-01-08

399

Dark Energy and Tachyon Field in Bianchi Type-V Space-time

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.

J. Sadeghi; H. Farahani

2014-04-15

400

Constraining dark energy evolution with gravitational lensing by large scale structures

We study the sensitivity of weak lensing by large scale structures as a probe of the evolution of dark energy. We explore a two-parameters model of dark energy evolution, inspired by tracking quintessence models. To this end, we compute the likelihood of a few fiducial models with varying and nonvarying equation of states. For the different models, we investigate the

Karim Benabed; Ludovic van Waerbeke

2004-01-01

401

Reconstruction of f-essence and fermionic Chaplygin gas models of dark energy

Recently, it was proposed a new fermionic model of dark energy, the so-called f-essence. In this work, we explicitly reconstruct the different f-essence models. In particular, these models include the fermionic Chaplygin gas and the fermionic generalized Chaplygin gas models of dark energy. We also derive the equation of state parameter of the selected f-essence models.

P. Tsyba; K. Yerzhanov; K. Esmakhanova; I. Kulnazarov; G. Nugmanova; R. Myrzakulov

2011-03-30

402

Photo Credit: Peter GinterSLAC National Accelerator Laboratory Dark Energy

Photo Credit: Peter GinterSLAC National Accelerator Laboratory #12;Dark Energy 70% Dark Matter 26 and Advanced Camera for Surveys #12;Dark Energy 70% Dark Matter 26% Ordinary Matter 4% #12;Dark Energy 70% Dark Matter 26% Ordinary Matter 4% #12;Dark Energy 70% Dark Matter 26% Ordinary Matter 4% #12;Dark Energy 70

Osheroff, Douglas D.

403

Dark Energy Experiments: The Road Ahead

NASA Astrophysics Data System (ADS)

The nature of Dark Energy is currently being investigated by a number of ongoing experiments, including the Dark Energy Survey discussed in the preceding talk. In order to better probe this frontier physics, a variety of new projects will begin operations over the next decade. These experiments will utilize multiple methods both to test whether the explanation for cosmic acceleration is consistent with General Relativity and to constrain the properties of Dark Energy if it is the cause of the Universe's accelerating expansion. In this talk, I will give a broad overview of the new instruments and telescopes now being designed that should revolutionize our understanding of Dark Energy in the next two decades. I will focus most on the LSST and MS-DESI projects, which are currently advancing through the U.S. Department of Energy's Critical Decision process.

Newman, Jeffrey

2013-04-01

404

G-corrected holographic dark energy model

NASA Astrophysics Data System (ADS)

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.

Malekjani, M.; Honari-Jafarpour, M.

2013-08-01

405

Direct Detection Rates of Dark Matter Coupled to Dark Energy

We investigate the effect of a coupling between dark matter and dark energy on the rates for the direct detection of dark matter. The magnitude of the effect depends on the strength $\\kappa$ of this new interaction relative to gravity. The resulting isothermal velocity distribution for dark matter in galaxy halos is still Maxwell-Boltzmann (M-B), but the characteristic velocity and the escape velocity are increased by $\\sqrt{1+\\kappa^2}$. We adopt a phenomenological approach and consider values of $\\kappa$ near unity. For such values we find that: (i) The (time averaged) event rate increases for light WIMPs, while it is somewhat reduced for WIMP masses larger than 100 GeV. (ii) The time dependence of the rate arising from the modulation amplitude is decreased compared to the standard M-B velocity distribution. (iii) The average and maximum WIMP energy increase proportionally to $1+\\kappa^2$, which, for sufficiently massive WIMPs, allows the possibility of designing experiments measuring $\\gamma$ rays following nuclear de-excitation.

N. Tetradis; J. D. Vergados; Amand Faessler

2006-09-07

406

Ghost dark energy in $f(R)$ model of gravity

We study a correspondence between $f(R)$ model of gravity and a phenomenological kind of dark energy (DE), which is known as QCD ghost dark energy. Since this kind of dark energy is not stable in the context of Einsteinian theory of gravity and Brans-Dicke model of gravity, we consider two kinds of correspondence between modified gravity and DE. By studding the dynamical evolution of model and finding relevant quantities such as, equation of state parameter, deceleration parameter, dimensionless density parameter, we show that the model can describe the present Universe and also the EoS parameter can cross the phantom divide line without needs to any kinetic energy with negative sign. Furthermore, by obtaining the adiabatic squared sound speed of the model for different cases of interaction, we show that this model is stable. Finally, we fit this model with supernova observational data in a non interaction case and we find the best values of parameter at $1\\sigma$ confidence interval as; $f_0=0.958^{+0.07}_{-0.25}$, $\\beta=-0,256^{+0.2}_{-0.1}$, and $\\Om_{m_0} = 0.23^{+0.3}_{-0.15}$. These best-fit values show that dark energy equation of state parameter, $\\om_{d_0}$, can cross the phantom divide line at the present time.

Kh. Saaidi; Ali. Aghamohammadi; B. Sabet; O. Farooq

2012-03-17

407

In this work, we consider the cosmological constraints on the holographic Ricci dark energy proposed by Gao et al.[Phys. Rev. D 79, 043511 (2009)], by using the observational data currently available. The main characteristic of holographic Ricci dark energy is governed by a positive numerical parameter {alpha} in the model. When {alpha}<1/2, the holographic Ricci dark energy will exhibit a quintomlike behavior; i.e., its equation of state will evolve across the cosmological-constant boundary w=-1. The parameter {alpha} can be determined only by observations. Thus, in order to characterize the evolving feature of dark energy and to predict the fate of the Universe, it is of extraordinary importance to constrain the parameter {alpha} by using the observational data. In this paper, we derive constraints on the holographic Ricci dark energy model from the latest observational data including the Union sample of 307 type Ia supernovae, the shift parameter of the cosmic microwave background given by the five-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey. The joint analysis gives the best-fit results (with 1{sigma} uncertainty): {alpha}=0.359{sub -0.025}{sup +0.024} and {omega}{sub m0}=0.318{sub -0.024}{sup +0.026}. That is to say, according to the observations, the holographic Ricci dark energy takes on the quintom feature. Finally, in light of the results of the cosmological constraints, we discuss the issue of the scalar-field dark energy reconstruction, based on the scenario of the holographic Ricci vacuum energy.

Zhang Xin [Department of Physics, College of Sciences, Northeastern University, Shenyang 110004 (China); Kavli Institute for Theoretical Physics China, Chinese Academy of Sciences, Beijing 100080 (China)

2009-05-15

408

Holographic Dark Energy in Brans-Dicke Theory

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.

Lixin Xu; Wenbo Li; Jianbo Lu

2008-04-18

409

Collapsing Inhomogeneous Dust Fluid in the Background of Dark Energy

In the present work, gravitational collapse of an inhomogeneous spherical star model, consisting of inhomogeneous dust fluid (dark matter) in the background of dark energy is considered. The collapsing process is examined first separately for both dark matter and dark energy and then under the combined effect of dark matter and dark energy with or without interaction. The dark energy is considered in the form of perfect fluid and both marginally and non-marginally bound cases are considered for the collapsing model. Finally dark energy in the form of anisotropic fluid is investigated and it is found to be similar to ref. [12

Tanwi Bandyopadhyay; Subenoy Chakraborty

2006-05-11

410

Neutrino mixing, flavor states and dark energy

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.

M. Blasone; A. Capolupo; S. Capozziello; G. Vitiello

2007-11-06

411

Particle mixing, flavor condensate and dark energy

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.

Massimo Blasone; Antonio Capolupo; Giuseppe Vitiello

2009-12-08

412

Holographic dark energy in the DGP model

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

Norman Cruz; Samuel Lepe; Francisco Peña

2011-09-09

413

Here we consider the entropy-corrected version of the holographic dark energy model in the non-flat universe. We obtain the equation of state parameter in the presence of interaction between dark energy and dark matter. Moreover, we reconstruct the potential and the dynamics of the quintessence, tachyon, K-essence and dilaton scalar field models according to the evolutionary behavior of the interacting entropy-corrected holographic dark energy model.

Kayoomars Karami; M. S. Khaledian; Mubasher Jamil

2011-01-19

414

Towards Dark Energy from String-Theory

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.

Axel Krause

2007-12-31

415

Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios

NASA Astrophysics Data System (ADS)

We report on precision resonance spectroscopy measurements of quantum states of ultracold neutrons confined above the surface of a horizontal mirror by the gravity potential of Earth. Resonant transitions between several of the lowest quantum states are observed for the first time. These measurements demonstrate that Newton's inverse square law of gravity is understood at micron distances on an energy scale of 10-14 eV. At this level of precision, we are able to provide constraints on any possible gravitylike interaction. In particular, a dark energy chameleon field is excluded for values of the coupling constant ? >5.8×108 at 95% confidence level (C.L.), and an attractive (repulsive) dark matter axionlike spin-mass coupling is excluded for the coupling strength gsgp>3.7×10-16 (5.3×10-16) at a Yukawa length of ? =20 ?m (95% C.L.).

Jenke, T.; Cronenberg, G.; Burgdörfer, J.; Chizhova, L. A.; Geltenbort, P.; Ivanov, A. N.; Lauer, T.; Lins, T.; Rotter, S.; Saul, H.; Schmidt, U.; Abele, H.

2014-04-01

416

Gravity resonance spectroscopy constrains dark energy and dark matter scenarios.

We report on precision resonance spectroscopy measurements of quantum states of ultracold neutrons confined above the surface of a horizontal mirror by the gravity potential of Earth. Resonant transitions between several of the lowest quantum states are observed for the first time. These measurements demonstrate that Newton's inverse square law of gravity is understood at micron distances on an energy scale of 10-14??eV. At this level of precision, we are able to provide constraints on any possible gravitylike interaction. In particular, a dark energy chameleon field is excluded for values of the coupling constant ?>5.8×108 at 95% confidence level (C.L.), and an attractive (repulsive) dark matter axionlike spin-mass coupling is excluded for the coupling strength gsgp>3.7×10-16 (5.3×10-16) at a Yukawa length of ?=20???m (95% C.L.). PMID:24785025

Jenke, T; Cronenberg, G; Burgdörfer, J; Chizhova, L A; Geltenbort, P; Ivanov, A N; Lauer, T; Lins, T; Rotter, S; Saul, H; Schmidt, U; Abele, H

2014-04-18

417

Evolution of perturbations in distinct classes of canonical scalar field models of dark energy

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.

Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)

2010-04-15

418

Holographic dark energy in chameleon tachyon cosmology

NASA Astrophysics Data System (ADS)

We propose in this paper an interacting holographic dark energy (IHDE) model in chameleon-tachyon cosmology by interaction between the components of the dark sectors. In the formalism, the interaction term emerges from the scalar field coupling matter Lagrangian in the model rather than being inserted into the formalism as an external source for the interaction. The correspondence between the tachyon field and the holographic dark energy (HDE) densities allows to reconstruct the tachyon scalar field and its potential in a flat FRW universe. The model can show the accelerated expansion of the universe and satisfies the observational data.

Farajollahi, H.; Ravanpak, A.; Fadakar, G. F.

2011-12-01

419

Essential building blocks of dark energy

NASA Astrophysics Data System (ADS)

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.

Gleyzes, Jerome; Langlois, David; Piazza, Federico; Vernizzi, Filippo

2013-08-01

420

We study the holographic dark energy on the subject of Ho$\\check{r}$ava-Lifshitz gravity with a time dependent gravitational constant (G(t)), in the non-flat space-time. We obtain the differential equation that specify the evolution of the dark energy density parameter based on varying gravitational constant. we find out a relation for the state parameter of the dark energy equation to low redshifts which containing varying $G$ correction.

Kh. Saaidi; A. Aghamohammadi; M. R. Setare

2010-06-09

421

Dark Energy-Dark Matter Interaction from the Abell Cluster A586

We find that deviation from the virial equilibrium of the Abell Cluster A586 yields evidence of the interaction between dark matter and dark energy. We argue that this interaction might imply a violation of the Equivalence Principle. Our analysis show that evidence is found in the context of two different models of dark energy-dark matter interaction.

Orfeu Bertolami; Francisco Gil Pedro; Morgan Le Delliou

2007-12-31

422

DARK MATTER AND DARK ENERGY AS EFFECTS OF QUANTUM GRAVITY Max I. Fomitchev1

DARK MATTER AND DARK ENERGY AS EFFECTS OF QUANTUM GRAVITY Max I. Fomitchev1 Submitted March 12th of high matter density expected in the early Universe I show that primordial inflation and dark energy (i. Subject headings: cosmology: dark matter Â galaxies: kinematics and dynamics Â X-rays: galaxies: clusters

Giles, C. Lee

423

Thermodynamics of viscous dark energy in an RSII braneworld

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.

M. R. Setare; A. Sheykhi

2011-03-05

424

Dark Energy, Dark Matter and Science with Constellation-X

NASA Technical Reports Server (NTRS)

Constellation-X, with more than 100 times the collecting area of any previous spectroscopic mission operating in the 0.25-40 keV bandpass, will enable highthroughput, high spectral resolution studies of sources ranging from the most luminous accreting supermassive black holes in the Universe to the disks around young stars where planets form. This talk will review the updated Constellation-X science case, released in booklet form during summer 2005. The science areas where Constellation-X will have major impact include the exploration of the space-time geometry of black holes spanning nine orders of magnitude in mass and the nature of the dark energy and dark matter which govern the expansion and ultimate fate of the Universe. Constellation-X will also explore processes referred to as "cosmic feedback" whereby mechanical energy, radiation, and chemical elements from star formation and black holes are returned to interstellar and intergalactic medium, profoundly affecting the development of structure in the Universe, and will also probe all the important life cycles of matter, from stellar and planetary birth to stellar death via supernova to stellar endpoints in the form of accreting binaries and supernova remnants. This talk will touch upon all these areas, with particular emphasis on Constellation-X's role in the study of Dark Energy.

Cardiff, Ann Hornschemeier

2005-01-01

425

The accelerating universe and dark energy

NASA Astrophysics Data System (ADS)

The recent discovery by Riess et al.1 and Perlmutter et al.2 that the expansion of the universe is accelerating is one of the most significant discoveries in cosmology in the last few decades. To explain this acceleration a mysterious new component of the universe, dark energy, was hypothesized. Using general relativity (GR), the measured rate of acceleration translates to the present understanding that the baryonic matter, of which the familiar world is made of, is a mere 4% of the total mass-energy of the universe, with nonbaryonic dark matter making up 24% and dark energy making up the majority 72%. Dark matter, by definition, has attractive gravity, and even though we presently do not know what it is, it could be made of the next heavy particles discovered by particle physicists. Dark energy, however, is much more mysterious, in that even though we do not know what it is, it must have some kind of repulsive gravity and negative pressure, very unusual properties that are not part of the present understanding of physics. Investigating the nature of dark energy is therefore one of the most important areas of cosmology. In this review, the cosmology of an expanding universe, based on GR, is discussed. The methods of studying the acceleration of the universe, and the nature of dark energy, are presented. A large amount of experimentation on this topic has taken place in the decade since the discovery of the acceleration. These are discussed and the present state of knowledge of the cosmological parameters is summarized in Table 7 below. A vigorous program to further these studies is under way. These are presented and the expected results are summarized in Table 10 below. The hope is that at the end of this program, it would be possible to tell whether dark energy is due to Einstein's cosmological constant or is some other new constituent of the universe, or alternately the apparent acceleration is due to some modification of GR.

Baltay, Charles

2014-05-01

426

Spiral Galaxies from a Dark Matter Solution of Einstein's Equations

NASA Astrophysics Data System (ADS)

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.

Ringermacher, H. I.; Mead, L. R.

2005-12-01

427

Coupling dark energy with Standard Model states

In this contribution one examines the coupling of dark energy to the gauge fields, to neutrinos, and to the Higgs field. In the first case, one shows how a putative evolution of the fundamental couplings of strong and weak interactions via coupling to dark energy through a generalized Bekenstein-type model may cause deviations on the statistical nuclear decay Rutherford-Soddy law. Existing bounds for the weak interaction exclude any significant deviation. For neutrinos, a perturbative approach is developed which allows for considering viable varying mass neutrino models coupled to any quintessence-type field. The generalized Chaplygin model is considered as an example. For the coupling with the Higgs field one obtains an interesting cosmological solution which includes the unification of dark energy and dark matter.

M. C. Bento; A. E. Bernardini; O. Bertolami

2009-04-13

428

Report of the Dark Energy Task Force

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 of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible.

Albrecht, A; Cahn, R; Freedman, W L; Hewitt, J; Hu, W; Huth, J; Kamionkowski, M; Kolb, Edward W; Knox, L; Mather, J C; Staggs, S; Suntzeff, N B; Albrecht, Andreas; Bernstein, Gary; Cahn, Robert; Freedman, Wendy L.; Hewitt, Jacqueline; Hu, Wayne; Huth, John; Kamionkowski, Marc; Kolb, Edward W.; Knox, Lloyd; Mather, John C.; Staggs, Suzanne; Suntzeff, Nicholas B.

2006-01-01

429

We present constraints on the dark energy equation-of-state parameter, w=P\\/(rhoc2), using 60 SNe Ia from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on (OmegaM, w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w=-1.05+0.13-0.12 (stat 1 sigma)+\\/-0.13 (sys)

W. Michael Wood-Vasey; G. Miknaitis; C. W. Stubbs; S. Jha; A. G. Riess; P. M. Garnavich; R. P. Kirshner; C. Aguilera; A. C. Becker; J. W. Blackman; S. Blondin; P. Challis; A. Clocchiatti; A. Conley; R. Covarrubias; T. M. Davis; A. V. Filippenko; R. J. Foley; A. Garg; M. Hicken; K. Krisciunas; B. Leibundgut; W. Li; T. Matheson; A. Miceli; G. Narayan; G. Pignata; J. L. Prieto; A. Rest; M. E. Salvo; B. P. Schmidt; R. C. Smith; J. Sollerman; J. Spyromilio; J. L. Tonry; N. B. Suntzeff; A. Zenteno

2007-01-01

430

First-order formalism for the quintom model of dark energy

The present paper deals to the quintom model of dark energy. We introduce a first-order formalism, which shows how to relate the potential that specifies the scalar field model to Hubble parameter. Reviewing briefly the quintom scenario of dark energy, we present a general procedure to solve the equations of motion for quintom model driven by a couple scalar fields with first-order differential equations.

M. R. Setare; J. Sadeghi

2008-05-08

431

Dark energy and future singularity of the universe in Kaluza-Klein space time

NASA Astrophysics Data System (ADS)

The dark energy model with the equation of state is studied in Kaluza-Klein space time. The model comprises and provides realization of several types of singularities in different parameter regimes. We discuss 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. Also, we discussed the models in terms of the cosmological redshift and some observational parameters.

Samnata, G. C.

2014-10-01

432

Impact of the matter density uncertainty on the dark energy reconstruction

In this paper we study the impact of the fractional matter density uncertainty in the reconstruction of the equation of state of dark energy. We consider both standard reconstruction methods, based on the dynamical effect that dark energy has on the expansion of the Universe, as well as nonstandard methods, in which the evolution of the dark energy equation of state with redshift is inferred through the variation of fundamental couplings such as the fine-structure constant, {alpha}, or the proton-to-electron mass ratio, {mu}. We show that the negative impact of the matter density uncertainty in the dark energy reconstruction using varying couplings may be very small compared to standard reconstruction methods. We also briefly discuss other fundamental questions which need to be answered before varying couplings can be successfully used to probe the nature of the dark energy.

Avelino, P. P. [Centro de Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal) and Departamento de Fisica da Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal)

2009-04-15

433

A Casimir approach to dark energy

We calculate the gravitational self-energy of vacuum quantum field fluctuations using a Casimir approach. We find that the Casimir gravitational self-energy density can account for the measured dark energy density when the SUSY-breaking energy is approximately 5 TeV, in good agreement with current estimates. Furthermore, the Casimir gravitational self-energy appears to provide a quantum mechanism for the well-know geometric relation between the Planck, SUSY and cosmological constant energy scales.

Allan Rosencwaig

2006-06-26

434

Dark Matter and Dark Energy from Gravitational Symmetry Breaking

NASA Astrophysics Data System (ADS)

We build a mechanism of gravitational symmetry breaking (GSB) of a global U(1) symmetry based on the relaxation of the equivalence principle due to the mass variation of pseudo Nambu-Goldstone dark matter (DM) particles. This GSB process is described by the modified cosmological convergence mechanism of the Abnormally Weighting Energy (AWE) Hypothesis previously introduced by the authors. Several remarkable constraints from the Hubble diagram of faraway supernovae are derived, notably on the explicit and gravitational symmetry breaking energy scales of the model. We then briefly present some consequences on neutrino masses when this mechanism is applied to the particular case of the breaking of lepton number symmetry.

Füzfa, A.; Alimi, J.-M.

2010-06-01

435

Clarifying spherical collapse in coupled dark energy cosmologies

The spherical collapse model is often used to follow the evolution of overdensities into the nonlinear regime. We describe the correct approach to be used in coupled dark energy cosmologies, where a fifth force, different from gravity and mediated by the dark energy scalar field, influences the collapse. We reformulate the spherical collapse description by deriving it directly from the set of nonlinear hydrodynamical Navier-Stokes equations. By comparing with the corresponding relativistic equations, we show how the fifth force should be taken into account within the spherical collapse picture and clarify the problems arising when an inhomogeneous scalar field is considered within a spherical collapse picture. We then apply our method to the case of coupled quintessence, where the fifth force acts among cold dark matter particles, and to growing neutrino quintessence, where the fifth force acts between neutrinos. Furthermore, we review this method within standard cosmologies and apply our analysis to minimally coupled quintessence. We also check past results for early dark energy parametrizations.

Wintergerst, Nico [Arnold-Sommerfeld-Center, Ludwig-Maximilians-Universitaet, Theresienstrasse 37, D-80333 Muenchen (Germany); Institut fuer Theoretische Physik, Universitaet Heidelberg, Philosophenweg 16, D-69120 Heidelberg (Germany); Pettorino, Valeria [SISSA, Via Bonomea 265, 34136 Trieste (Italy); Institut fuer Theoretische Physik, Universitaet Heidelberg, Philosophenweg 16, D-69120 Heidelberg (Germany)

2010-11-15

436

Dark matter and dark energy: approaches and constraints

NASA Astrophysics Data System (ADS)

We will introduce problems of Dark Matter (DM) and Dark Energy (DE), namely we will describe a development of these concepts and their present status. We will demonstrate ap-proaches to these problems. As specific issues we will discuss limits on DM concentration near the black hole at the Galactic Center and ways to solve DE problem introducing alternative theories of gravity such as f (R)-theories. The existence of dark matter (DM) at scales of few pc down to 10-5 pc around the centers of galaxies and in particular in the Galactic Center region has been considered in the literature. Under the assumption that such a DM clump, principally constituted by non-baryonic matter (like WIMPs) does exist at the center of our galaxy, the study of the ?-ray emission from the Galactic Center region allows us to constrain both the mass and the size of this DM sphere. Moreover, if a DM cusp does exist around the Galactic Center it could modify the trajectories of stars moving around it in a sensible way depending on the DM mass distribution. Here, we discuss the constraints that can be obtained with the orbit analysis of stars (as S2 and S16) moving inside the DM concentration with present and next generations of large telescopes. In particular, consideration of the S2 star apoastron shift may allow improving limits on the DM mass and size. We will describe severe constraints from Solar system data on parameters f (R) = Rn theories, where n = 1 corresponds to the standard general relativistic case. 1. A. F. Zakharov, A.A. Nucita, F. De Paolis, G. Ingrosso: Solar system constraints on Rn gravity, Phys. Rev. D 74, 107101, (2006). 2. A. F. Zakharov, A.A. Nucita, F. De Paolis, G. Ingrosso: Apoastron shift constraints on dark matter distribution at the Galactic Center, Phys. Rev. D 76, 062001, (2007). 3. A.F. Zakharov, S. Capozziello, F. De Paolis, G. Ingrosso, A.A. Nucita, The Role of Dark Matter and Dark Energy in Cosmological Models: Theoretical Overview, Space Sci. Rev. 148, 301-313(2009).

Zakharov, Alexander

437

Fermion field as inflaton, dark energy and dark matter

NASA Astrophysics Data System (ADS)

The search for constituents that can explain the periods of accelerating expansion of the Universe is a fundamental topic in cosmology. In this context, we investigate how fermionic fields minimally and non-minimally coupled with the gravitational field may be responsible for accelerated regimes during the evolution of the Universe. The forms of the potential and coupling of the model are determined through the technique of the Noether symmetry for two cases. The first case comprises a Universe filled only with the fermion field. Cosmological solutions are straightforwardly obtained for this case and an exponential inflation mediated by the fermion field is possible with a non-minimal coupling. The second case takes account of the contributions of radiation and baryonic matter in the presence of the fermion field. In this case the fermion field plays the role of dark energy and dark matter, and when a non-minimal coupling is allowed, it mediates a power-law inflation.

Grams, Guilherme; de Souza, Rudinei C.; Kremer, Gilberto M.

2014-09-01

438

Transient and late time attractor tachyon dark energy: Can we distinguish it from quintessence?

The string inspired tachyon field can serve as a candidate of dark energy. Its equation of state parameter w varies from 0 to -1. In the case of tachyon field potential V({phi}){yields}0 slower (faster) than 1/{phi}{sup 2} at infinity, dark energy (dark matter) is a late time attractor. We investigate the tachyon dark energy models under the assumption that w is close to -1. We find that all the models exhibit unique behavior around the present epoch which is exactly the same as that of the thawing quintessence.

Ali, Amna; Sami, M.; Sen, A. A. [Centre of Theoretical Physics, Jamia Millia Islamia, New Delhi-110025 (India)

2009-06-15

439

Entropic-force dark energy reconsidered

We reconsider the entropic-force model in which both kind of Hubble terms ${\\dot H}$ and $H^{2}$ appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions, namely the scale factor, deceleration parameter, matter density and growth of linear matter perturbations. However, we find that the entropic-force model is not viable at the background and perturbation levels due to the fact that the entropic formulation does not add a constant term in the Friedmann equations. On the other hand, if on mere phenomenological grounds we replace the ${\\dot H}$ dependence of the effective DE density with a linear term $H$ without including a constant additive term, we find that the transition from deceleration to acceleration becomes possible but the recent structure formation data strongly disfavors this cosmological scenario. Finally, we briefly compare the entropic-force models with some related DE models (based on dynamical vacuum energy) which overcome these difficulties and are compatible with the present observations.

Spyros Basilakos; Joan Sola

2014-02-26

440

Dark energy perturbations and cosmic coincidence

While there is plentiful evidence in all fronts of experimental cosmology for the existence of a non-vanishing dark energy (DE) density \\rho_D in the Universe, we are still far away from having a fundamental understanding of its ultimate nature and of its current value, not even of the puzzling fact that \\rho_D is so close to the matter energy density \\rho_M at the present time (i.e. the so-called "cosmic coincidence" problem). The resolution of some of these cosmic conundrums suggests that the DE must have some (mild) dynamical behavior at the present time. In this paper, we examine some general properties of the simultaneous set of matter and DE perturbations (\\delta\\rho_M, \\delta\\rho_D) for a multicomponent DE fluid. Next we put these properties to the test within the context of a non-trivial model of dynamical DE (the LXCDM model) which has been previously studied in the literature. By requiring that the coupled system of perturbation equations for \\delta\\rho_M and \\delta\\rho_D has a smooth solution throughout the entire cosmological evolution, that the matter power spectrum is consistent with the data on structure formation and that the "coincidence ratio" r=\\rho_D/\\rho_M stays bounded and not unnaturally high, we are able to determine a well-defined region of the parameter space where the model can solve the cosmic coincidence problem in full compatibility with all known cosmological data.

Javier Grande; Ana Pelinson; Joan Sola

2008-09-20

441

Coupled dark energy with perturbed Hubble expansion rate

The coupling between dark sectors provides a possible approach to mitigate the coincidence problem of cosmological standard model. In this paper, dark energy is treated as a fluid with a constant equation of state, whose coupling with dark matter is proportional the Hubble parameter and energy density of dark energy, that is, $\\bar{Q}=3\\xi_x\\bar{H}\\bar{\\rho}_x$. Particularly, we consider the Hubble expansion rate to be perturbed in the perturbation evolutions of dark sectors. Using jointing data sets which include cosmic microwave background radiation, baryon acoustic oscillation, type Ia supernovae, and redshift-space distortions, we perform a full Monte Carlo Markov Chain likelihood analysis for the coupled model. The results show that the mean value with errors of interaction rate is: $\\xi_x=0.00305_{-0.00305-0.00305-0.00305}^{+0.000645+0.00511+0.00854}$ for $Q^{\\mu}_A\\parallel u^{\\mu}_c$; $\\xi_x=0.00317_{-0.00317-0.00317-0.00317}^{+0.000628+0.00547+0.00929}$ for $Q^{\\mu}_A\\parallel u^{\\mu}_x$, which means...

Yang, Weiqiang

2014-01-01

442

Coupled dark energy with perturbed Hubble expansion rate

NASA Astrophysics Data System (ADS)

The coupling between dark sectors provides a possible approach to mitigate the coincidence problem of the cosmological standard model. In this paper, dark energy is treated as a fluid with a constant equation of state, whose coupling with dark matter is proportional the Hubble parameter and energy density of dark energy, that is, Q ¯ =3 ?xH ¯ ?¯ x . In particular, we consider the Hubble expansion rate to be perturbed in the perturbation evolutions of dark sectors. Using joint data sets which include cosmic microwave background radiation, baryon acoustic oscillation, type Ia supernovae, and redshift-space distortions, we perform a full Markov chain Monte Carlo likelihood analysis for the coupled model. The results show that the mean value with errors of the interaction rate is ?x=0.0030 5-0.00305-0.00305 -0.00305 +0.000645 +0.00511 +0.00854 for QA??uc? and ?x=0.0031 7-0.00317-0.00317 -0.00317 +0.000628 +0.00547 +0.00929 for QA??ux?, which means that the recent cosmic observations favor a small interaction rate which is up to the order of 10-3. Moreover, in contrast to the coupled model with an unperturbed expansion rate, we find the perturbed Hubble expansion rate can bring about a negligible impact on the model parameter space.

Yang, Weiqiang; Xu, Lixin

2014-10-01

443

Falsification of dark energy by fluid mechanics

The 2011 Nobel Prize in Physics was awarded for the discovery of accelerating supernovae dimness, suggesting a remarkable change in the expansion rate of the Universe from a decrease since the big bang to an increase, driven by anti-gravity forces of a mysterious dark energy material comprising 70% of the Universe mass-energy. Fluid mechanical considerations falsify both the accelerating expansion and dark energy concepts. Kinematic viscosity is neglected in current standard models of self-gravitational structure formation, which rely on cold dark matter CDM condensations and clusterings that are also falsified by fluid mechanics. Weakly collisional CDM particles do not condense but diffuse away. Photon viscosity predicts superclustervoid fragmentation early in the plasma epoch and protogalaxies at the end. At the plasma-gas transition, the plasma fragments into Earth-mass gas planets in trillion planet clumps (proto-globular-star-cluster PGCs). The hydrogen planets freeze to form the dark matter of galaxies and merge to form their stars. Dark energy is a systematic dimming error for Supernovae Ia caused by dark matter planets near hot white dwarf stars at the Chandrasekhar carbon limit. Evaporated planet atmospheres may or may not scatter light from the events depending on the line of sight.

Carl H. Gibson

2012-03-23

444

"Dark energy" in the Local Void

NASA Astrophysics Data System (ADS)

The unexpected discovery of the accelerated cosmic expansion in 1998 has filled the Universe with the embarrassing presence of an unidentified "dark energy", or cosmological constant, devoid of any physical meaning. While this standard cosmology seems to work well at the global level, improved knowledge of the kinematics and other properties of our extragalactic neighborhood indicates the need for a better theory. We investigate whether the recently suggested repulsive-gravity scenario can account for some of the features that are unexplained by the standard model. Through simple dynamical considerations, we find that the Local Void could host an amount of antimatter (˜5×1015 M ?) roughly equivalent to the mass of a typical supercluster, thus restoring the matter-antimatter symmetry. The antigravity field produced by this "dark repulsor" can explain the anomalous motion of the Local Sheet away from the Local Void, as well as several other properties of nearby galaxies that seem to require void evacuation and structure formation much faster than expected from the standard model. At the global cosmological level, gravitational repulsion from antimatter hidden in voids can provide more than enough potential energy to drive both the cosmic expansion and its acceleration, with no need for an initial "explosion" and dark energy. Moreover, the discrete distribution of these dark repulsors, in contrast to the uniformly permeating dark energy, can also explain dark flows and other recently observed excessive inhomogeneities and anisotropies of the Universe.

Villata, M.

2012-05-01

445

Isotropic and anisotropic dark energy models

NASA Astrophysics Data System (ADS)

In this review we discuss the evolution of the universe filled with dark energy with or without perfect fluid. In doing so we consider a number of cosmological models, namely Bianchi type I, III, V, VI0, VI and FRW ones. For the anisotropic cosmological models we have used proportionality condition as an additional constrain. The exact solutions to the field equations in quadrature are found in case of a BVI model. It was found that the proportionality condition used here imposed severe restriction on the energy-momentum tensor, namely it leads to isotropic distribution of matter. Anisotropic BVI0, BV, BIII and BIDE models with variable EoS parameter ? have been investigated by using a law of variation for the Hubble parameter. In this case the matter distribution remains anisotropic, though depending on the concrete model there appear different restrictions on the components of energy-momentum tensor. That is why we need an extra assumption such as variational a law for the Hubble parameter. It is observed that, at the early stage, the EoS parameter v is positive i.e. the universe was matter dominated at the early stage but at later time, the universe is evolving with negative values, i.e., the present epoch. DE model presents the dynamics of EoS parameter ? whose range is in good agreement with the acceptable range by the recent observations. A spatially homogeneous and anisotropic locally rotationally symmetric Bianchi-I space time filled with perfect fluid and anisotropic DE possessing dynamical energy density is studied. In the derived model, the EoS parameter of DE (?( de)) is obtained as time varying and it is evolving with negative sign which may be attributed to the current accelerated expansion of Universe. The distance modulus curve of derived model is in good agreement with SNLS type Ia supernovae for high redshift value which in turn implies that the derived model is physically realistic. A system of two fluids within the scope of a spatially flat and isotropic FRW model is studied. The role of the two fluids, either minimally or directly coupled in the evolution of the dark energy parameter, has been investigated. In doing so we have used three different ansatzs regarding the scale factor that gives rise to a variable decelerating parameter. It is observed that, in the non-interacting case, both the open and flat universes can cross the phantom region whereas in the interacting case only the open universe can cross the phantom region. The stability and acceptability of the obtained solution are also investigated.

Saha, Bijan

2014-03-01

446

February 2006, NRAO, VA (or why H0 is the Dark Energy)

Wayne Hu February 2006, NRAO, VA (or why H0 is the Dark Energy) Dark Energy in Light of the CMB #12;If its not dark, it doesn't matter! Â· Cosmic matter-energy budget: Dark Energy Dark Matter Dark provide the high redshift cornerstone to cosmological inferences on the dark matter and dark energy WMAP

Hu, Wayne

447

Perturbed dark energy: Classical scalar field versus tachyon

The evolution of scalar linear perturbations is studied in gauge-invariant approach for 2-component models with nonrelativistic matter and minimally coupled scalar fields, the potentials of which were constructed for either constant dark energy equation of state parameter w or its adiabatic sound speed c{sub a}² equal to zero. The numerical solutions show that such fields are almost smoothed out on

Olga Sergijenko; Bohdan Novosyadlyj

2009-01-01

448

Dark energy constraints from the cosmic age and supernova

Using the low limit of cosmic ages from globular cluster and the white dwarfs: t0>12Gyr, 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 0.2?m0.4 or 0.1?mh20.16. We find cosmic age limit plays a

Bo Feng; Xiulian Wang; Xinmin Zhang

2005-01-01

449

Can Strong Gravitational Lensing Constrain Dark Energy?

We discuss the ratio of the angular diameter distances from the source to the lens, $D_{ds}$, and to the observer at present, $D_{s}$, for various dark energy models. It is well known that the difference of $D_s$s between the models is apparent and this quantity is used for the analysis of Type Ia supernovae. However we investigate the difference between the ratio of the angular diameter distances for a cosmological constant, $(D_{ds}/D_{s})^{\\Lambda}$ and that for other dark energy models, $(D_{ds}/D_{s})^{\\rm{other}}$ in this paper. It has been known that there is lens model degeneracy in using strong gravitational lensing. Thus, we investigate the model independent observable quantity, Einstein radius ($\\theta_E$), which is proportional to both $D_{ds}/D_s$ and velocity dispersion squared, $\\sigma_v^2$. $D_{ds}/D_s$ values depend on the parameters of each dark energy model individually. However, $(D_{ds}/D_s)^{\\Lambda} - (D_{ds}/D_{s})^{\\rm{other}}$ for the various dark energy models, is well within the error of $\\sigma_v$ for most of the parameter spaces of the dark energy models. Thus, a single strong gravitational lensing by use of the Einstein radius may not be a proper method to investigate the property of dark energy. However, better understanding to the mass profile of clusters in the future or other methods related to arc statistics rather than the distances may be used for constraints on dark energy.

Seokcheon Lee; Kin-Wang Ng

2007-07-12

450

NASA Astrophysics Data System (ADS)

The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein's field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.

Sarkar, Sanjay

2014-02-01

451

Holographic dark energy in chameleon tachyon cosmology

We propose in this paper an interacting holographic dark energy (IHDE) model in chameleon–tachyon cosmology by interaction\\u000a between the components of the dark sectors. In the formalism, the interaction term emerges from the scalar field coupling\\u000a matter Lagrangian in the model rather than being inserted into the formalism as an external source for the interaction. The\\u000a correspondence between the tachyon

H. Farajollahi; A. Ravanpak; G. F. Fadakar

2011-01-01

452

Braneworlds, Conformal Fields and Dark Energy

In the Randall-Sundrum scenario we analize the dynamics of a spherically symmetric 3-brane when matter fields propagate in the bulk. For a well defined class of conformal fields of weight -4 we determine a new set of exact 5-dimensional solutions which localize gravity in the vicinity of the brane and are stable under radion field perturbations. Geometries which describe the dynamics of inhomogeneous dust, generalized dark radiation and homogeneous polytropic dark energy are shown to belong to this set.

Rui Neves

2006-01-06

453

Dark Energy and the Accelerating Universe

The discovery ten years ago that the expansion of the Universe is\\u000aaccelerating put in place the last major building block of the present\\u000acosmological model, in which the Universe is composed of 4% baryons, 20% dark\\u000amatter, and 76% dark energy. At the same time, it posed one of the most\\u000aprofound mysteries in all of science, with deep

Joshua A. Frieman; Michael S. Turner; Dragan Huterer

2008-01-01

454

Dark Energy Update / Edward Larson's Evolution

NSDL National Science Digital Library

These two radio broadcasts discusse new evidence supporting the existence of dark energy from the Chandra X-ray Observatory and Darwin and his theory of evolution. First, The new observatory data from measured distances between galactic clusters suggest the universe started accelerating 6 billion years ago and that the amount of dark energy - the mysterious repulsive force that appears to be pushing the universe apart - may in fact be constant in time. The show discusses how this dark energy may be related to the energy of the quantum vacuum; how there may actually be no dark energy but what is needed is a better understanding of gravity; and how dark energy may provide the pivotal clue to developing a theory of quantum gravity. This segment is 15 minutes in length. The second part of the broadcast discusses how Charles Darwin came to create the theory of evolution. It covers the state of science leading up to the Beagle voyage made by Darwin; which scientists influenced his thinking, and what he saw that others before him had not; the agnostic religious views of Darwin; and how the theory of descent with modification itself has been modified. There is also discussion of the biggest misunderstandings about evolution and the debate over classroom teaching of evolution. The broadcast is 34 minutes in length.

455

Some features of new holographic dark energy model in Ho?ava-Lifshitz gravity

NASA Astrophysics Data System (ADS)

In this work, we have considered the interaction of new holographic dark energy (NHDE) with dark matter and tachyonic field in both interacting and non-interacting cases in Ho?ava-Lifshitz gravity. In all cases we have studied the nature of equation of state parameters, statefinder parameters, deceleration parameter, Om diagnostic and speed of sound.

Bagchi Khatua, Piyali; Debnath, Ujjal

2012-05-01

456

Behaviour of interacting Ricci dark energy in logarithmic f(T) gravity

In the present work we have considered a modified gravity dubbed as "logarithmic $f(T)$ gravity" and investigated the behavior of Ricci dark energy interacting with pressureless dark matter. We have chosen the interaction term in the form $Q\\propto H\\delta\\rho_{m}$ and investigated the behavior of the Hubble parameter $H$ as a function of the redshift $z$. For this reconstructed $H$ we have investigated the behavior of the density of the Ricci dark energy $\\rho_{RDE}$ and density contribution due to torsion $\\rho_{T}$. All of the said cosmological parameters are seen to have increasing behavior from higher to lower redshifts for all values of $c^{2}$ pertaining to the Ricci dark energy. Subsequently, we observed the equation of state parameter $w_{RDE}$ in this situation. The equation of state parameter is found to behave like phantom for all choices of $c^{2}$ in the Ricci dark energy.

Rahul Ghosh; Surajit Chattopadhyay

2012-10-07

457

Cosmological dynamics of tachyonic teleparallel dark energy

NASA Astrophysics Data System (ADS)

A detailed dynamical analysis of the tachyonic teleparallel dark energy model, in which a noncanonical scalar field (tachyon field) is nonminimally coupled to gravitation, is performed. It is found that, when the nonminimal coupling is ruled by a dynamically changing coefficient ??f,?/f, with f(?) an arbitrary function of the scalar field ?, the Universe may experience a field-matter-dominated era “?MDE,” in which it has some portions of the energy density of ? in the matter dominated era. This is the most significant difference in relation to the so-called teleparallel dark energy scenario, in which a canonical scalar field (quintessence) is nonminimally coupled to gravitation.

Otalora, G.

2013-09-01

458

Dark Energy Density in Brane World

We present a possible explanation to the tiny positive cosmological constant under the frame of AdS$_5$ spacetime embedded by a dS$_4$ brane. We calculate the dark energy density by summing the zero point energy of massive scalar fields in AdS$_5$ spacetime. Under the assumption that the radius of AdS$_5$ spacetime is of the same magnitude as the radius of observable universe, the dark energy density in dS$_4$ brane is obtained, which is smaller than the observational value. The reasons are also discussed.

Hai-Bao Wen; Xin-Bing Huang

2005-02-08

459

Distinguishing modified gravity from dark energy

The acceleration of the Universe can be explained either through dark energy or through the modification of gravity on large scales. In this paper we investigate modified gravity models and compare their observable predictions with dark energy models. Modifications of general relativity are expected to be scale independent on superhorizon scales and scale dependent on subhorizon scales. For scale-independent modifications, utilizing the conservation of the curvature scalar and a parametrized post-Newtonian formulation of cosmological perturbations, we derive results for large-scale structure growth, weak gravitational lensing, and cosmic microwave background anisotropy. For scale-dependent modifications, inspired by recent f(R) theories we introduce a parametrization for the gravitational coupling G and the post-Newtonian parameter {gamma}. These parametrizations provide a convenient formalism for testing general relativity. However, we find that if dark energy is generalized to include both entropy and shear stress perturbations, and the dynamics of dark energy is unknown a priori, then modified gravity cannot in general be distinguished from dark energy using cosmological linear perturbations.

Bertschinger, Edmund; Zukin, Phillip [Department of Physics, MIT, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States)

2008-07-15

460

Distinguishing Modified Gravity from Dark Energy

The acceleration of the universe can be explained either through dark energy or through the modification of gravity on large scales. In this paper we investigate modified gravity models and compare their observable predictions with dark energy models. Modifications of general relativity are expected to be scale-independent on super-horizon scales and scale-dependent on sub-horizon scales. For scale-independent modifications, utilizing the conservation of the curvature scalar and a parameterized post-Newtonian formulation of cosmological perturbations, we derive results for large scale structure growth, weak gravitational lensing, and cosmic microwave background anisotropy. For scale-dependent modifications, inspired by recent $f(R)$ theories we introduce a parameterization for the gravitational coupling $G$ and the post-Newtonian parameter $\\gamma$. These parameterizations provide a convenient formalism for testing general relativity. However, we find that if dark energy is generalized to include both entropy and shear stress perturbations, and the dynamics of dark energy is unknown a priori, then modified gravity cannot in general be distinguished from dark energy using cosmological linear perturbations.

Edmund Bertschinger; Phillip Zukin

2008-01-16

461

Dark Energy and the Hubble Law

NASA Astrophysics Data System (ADS)

The Big Bang predicted by Friedmann could not be empirically discovered in the 1920th, since global cosmological distances (more than 300-1000 Mpc) were not available for observations at that time. Lemaitre and Hubble studied receding motions of galaxies at local distances of less than 20-30 Mpc and found that the motions followed the (nearly) linear velocity-distance relation, known now as Hubble's law. For decades, the real nature of this phenomenon has remained a mystery, in Sandage's words. After the discovery of dark energy, it was suggested that the dynamics of local expansion flows is dominated by omnipresent dark energy, and it is the dark energy antigravity that is able to introduce the linear velocity-distance relation to the flows. It implies that Hubble's law observed at local distances was in fact the first observational manifestation of dark energy. If this is the case, the commonly accepted criteria of scientific discovery lead to the conclusion: In 1927, Lemaitre discovered dark energy and Hubble confirmed this in 1929.

Chernin, A. D.; Dolgachev, V. P.; Domozhilova, L. M.

462

Dark matter density profiles from the Jeans equation

We make a simple analytical study of radial profiles of dark matter structures, with special attention to the question of the central radial density profile. We let our theoretical assumptions be guided by results from numerical simulations, and show that at any radius where both the radial density profile, rho, and the phase-space-like density profile, rho/sigma^epsilon, are exact power laws, the only allowed density slopes in agreement with the spherical symmetric and isotropic Jeans equation are in the range 1< beta <3, where beta = - dln(rho)/dln(r). We also allow for a radial variation of these power laws, as well as anisotropy, and show how this allows for more shallow central slopes.

Steen H. Hansen

2004-05-19

463

$\\sigma$CDM coupled to radiation. Dark energy and Universe acceleration

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 ...

Abbyazov, Renat R; Müller, Volker

2014-01-01

464

New Perspectives: Wave Mechanical Interpretations of Dark Matter, Baryon and Dark Energy

NASA Astrophysics Data System (ADS)

We model the cosmic components: dark matter, dark energy and baryon distributions in the Cosmic Web by means of highly nonlinear Schrodinger type and reaction diffusion type wave mechanical descriptions. The construction of these wave mechanical models of the structure formation is achieved by introducing the Fisher information measure and its comparison with highly nonlinear term which has dynamical analogy to infamous quantum potential in the wave equations. Strikingly, the comparison of this nonlinear term and the Fisher information measure provides a dynamical distinction between lack of self-organization and self-organization in the dynamical evolution of the cosmic components. Mathematically equivalent to the standard cosmic fluid equations, these approaches make it possible to follow the evolution of the matter distribution even into the highly nonlinear regime by circumventing singularities. Also, numerical realizations of the emerging web-like patterns are presented from the nonlinear dynamics of the baryon component while dark energy component shows Gaussian type dynamics corresponding to soliton-like solutions.

Russell, Esra

465

Dark energy and supermassive black holes

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

Gonzalez-Diaz; Pedro F

2004-01-01

466

Dark energy and supermassive black holes

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

Pedro F. Gonzalez-D ´ õaz

467

Dark Energy: Mystery of the Millennium

Nearly seventy per cent of the energy density in the universe is unclustered and exerts negative pressure. This conclusion - now supported by numerous observations - poses the greatest challenge for theoretical physics today. I discuss this issue with special emphasis on the cosmological constant as the possible choice for the dark energy. Several curious features of a universe with

T. Padmanabhan

2006-01-01

468

Dark energy perturbations and cosmic coincidence

While there is plentiful evidence in all fronts of experimental cosmology for the existence of a nonvanishing dark energy (DE) density rhoD in the Universe, we are still far away from having a fundamental understanding of its ultimate nature and of its current value, not even of the puzzling fact that rhoD is so close to the matter energy density

Javier Grande; Ana Pelinson; Joan Solà

2009-01-01

469

Cosmic Acceleration, Dark Energy, and Fundamental Physics

A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic ( p˜-rho), very smooth form of energy called ``dark energy'' accounting for about 75% of the Universe.

Michael S. Turner; Dragan Huterer

2007-01-01

470

Dark energy and quantum vacuum fluctuations

It is suggested that the vacuum expectation of the quantum vacuum energy-momentum is zero, but quantum fluctuations give rise to a space-time curvature equivalent to that of a cosmological constant or dark energy. Calculations within quantized gravity, following a few plausible hypotheses, provide results compatible with cosmological observations.

Emilio Santos

2009-02-25

471

Probing dark energy with lensing magnification in photometric surveys.

I present an estimator for the angular cross correlation of two tracers of the cosmological large-scale structure that utilizes redshift information to isolate separate physical contributions. The estimator is derived by solving the Limber equation for a reweighting of the foreground tracer that nulls either clustering or lensing contributions to the cross correlation function. Applied to future photometric surveys, the estimator can enhance the measurement of gravitational lensing magnification effects to provide a competitive independent constraint on the dark energy equation of state. PMID:24580685

Schneider, Michael D

2014-02-14

472

Axion-Dilaton Cosmology and Dark Energy

We discuss a class of flat FRW cosmological models based on D=4 axion-dilaton gravity universally coupled to cosmological background fluids. In particular, we investigate the possibility of recurrent acceleration, which was recently shown to be generically realized in a wide class of axion-dilaton models, but in absence of cosmological background fluids. We observe that, once we impose the existence of radiation -and matter- dominated earlier stages of cosmic evolution, the axion-dilaton dynamics is altered significantly with respect to the case of pure axion-dilaton gravity. During the matter dominated epoch the scalar fields remain either frozen, due to the large expansion rate, or enter a cosmological scaling regime. In both cases, oscillations of the effective equation of state around the acceleration boundary value are impossible. Models which enter an oscillatory stage in the low redshift regime, on the other hand, are disfavored by observations. We also comment on the viability of the axion-dilaton system as a candidate for dynamical dark energy. In a certain subclass of models, an intermediate scaling regime is succeeded by eternal acceleration. We also briefly discuss the issue of dependence on initial conditions.

Riccardo Catena; Jan Möller

2007-09-12

473

Dynamical dark energy or variable cosmological parameters?

One of the main aims in the next generation of precision cosmology experiments will be an accurate determination of the equation of state (EOS) for the dark energy (DE). If the latter is dynamical, the resulting barotropic index \\omega should exhibit a non-trivial evolution with the redshift. Usually this is interpreted as a sign that the mechanism responsible for the DE is related to some dynamical scalar field, and in some cases this field may behave non-canonically (phantom field). Present observations seem to favor an evolving DE with a potential phantom phase near our time. In the literature there is a plethora of dynamical models trying to describe this behavior. Here we show that the simplest option, namely a model with a variable cosmological term, \\Lambda=\\Lambda(t), leads in general to a non-trivial effective EOS, with index \\omega_e, which may naturally account for these data features. We prove that in this case there is always a ``crossing'' of the \\omega_e=-1 barrier near our time. We also show how this effect is modulated (or even completely controled) by a variable Newton's constant G=G(t).

Joan Sola; Hrvoje Stefancic

2005-07-05

474

Dynamics of Bianchi I Universe with Magnetized Anisotropic Dark Energy

We study Bianchi type $I$ cosmological model in the presence of magnetized anisotropic dark energy. The energy-momentum tensor consists of anisotropic fluid with anisotropic EoS $p=\\omega{\\rho}$ and a uniform magnetic field of energy density $\\rho_B$. We obtain exact solutions to the field equations using the condition that expansion is proportional to the shear scalar. The physical behavior of the model is discussed with and without magnetic field. We conclude that universe model as well as anisotropic fluid do not approach isotropy through the evolution of the universe.

M. Sharif; M. Zubair

2010-05-25

475

Observables and unobservables in dark energy cosmologies

NASA Astrophysics Data System (ADS)

The aim of this paper is to answer the following two questions: (1) Given cosmological observations of the expansion history and linear perturbations in a range of redshifts and scales as precise as is required, which of the properties of dark energy could actually be reconstructed without imposing any parameterization? (2) Are these observables sufficient to rule out not just a particular dark energy model, but the entire general class of viable models comprising a single scalar field? This paper bears both good and bad news. On one hand, we find that the goal of reconstructing dark energy models is fundamentally limited by the unobservability of the present values of the matter density ?m0, the perturbation normalization ?8 as well as the present matter power spectrum. On the other, we find that, under certain conditions, cosmological observations can nonetheless rule out the entire class of the most general single scalar-field models, i.e., those based on the Horndeski Lagrangian.

Amendola, Luca; Kunz, Martin; Motta, Mariele; Saltas, Ippocratis D.; Sawicki, Ignacy

2013-01-01

476

Singularity problem in teleparallel dark energy models

NASA Astrophysics Data System (ADS)

We study future singularity in teleparallel dark energy models, particularly its behavior and its (non)occurrence in the observationally viable models. For the models with a general self-potential of the scalar field, we point out that both at early times and in the future near the singularity the behavior of dark energy can be described by the analytic solutions of the scalar field we obtained for the model with no self-potential. As to the (non)occurrence in the viable models, we consider a natural binding-type self-potential, the quadratic potential, when fitting observational data, and illustrate the constraining region up to the 3? confidence level as well as the region where a singularity will occur. As a result, the singularity region is outside the 3? constraint. Thus, although the future singularity problem potentially exists in teleparallel dark energy models, the observationally viable models may not suffer this problem.

Geng, Chao-Qiang; Gu, Je-An; Lee, Chung-Chi

2013-07-01

477

A novel teleparallel dark energy model

Although equivalent to general relativity, teleparallel gravity is conceptually speaking a completely different theory. In this theory, the gravitational field is described by torsion, not by curvature. By working in this context, a new model is proposed in which the four-derivative of a canonical scalar field representing dark energy is nonminimally coupled to the "vector torsion". This type of coupling is motivated by the fact that, in teleparallel gravity, the scalar field couples to torsion through its four-derivative. It is found that the current state of accelerated expansion of the Universe corresponds to a late-time attractor that can be (i) a dark-energy-dominated de Sitter solution ($\\omega_{\\phi}=-1$), (ii) a quintessence-type solution with $\\omega_{\\phi}\\geq-1$, or (iii) a phantom-type $\\omega_{\\phi}<-1$ dark energy.

G. Otalora

2014-02-10