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1

Interacting dark energy with inhomogeneous equation of state

NASA Astrophysics Data System (ADS)

We have investigated the model of dark energy interacting with dark matter by choosing inhomogeneous equations of state for the dark energy and a nonlinear interaction term for the underlying interaction. The equations of state have dependencies either on the energy densities, the redshift, the Hubble parameter or the bulk viscosity. We have considered these possibilities and have derived the effective equations of state for the dark energy in each case.

Jamil, Mubasher; Rashid, Muneer Ahmad

2008-08-01

2

Scaling cosmology with variable dark-energy equation of state

NASA Astrophysics Data System (ADS)

Interactions between dark matter and dark energy which result in a power-law behavior (with respect to the cosmic scale factor) of the ratio between the energy densities of the dark components (thus generalizing the ?CDM model) have been considered as an attempt to alleviate the cosmic coincidence problem phenomenologically. We generalize this approach by allowing for a variable equation of state for the dark energy within the CPL-parametrization. Based on analytic solutions for the Hubble rate and using the Constitution and Union2 SNIa sets, we present a statistical analysis and classify different interacting and non-interacting models according to the Akaike (AIC) and the Bayesian (BIC) information criteria. We do not find noticeable evidence for an alleviation of the coincidence problem with the mentioned type of interaction.

Castro, David R.; Velten, Hermano; Zimdahl, Winfried

2012-06-01

3

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

4

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

5

The Unified Equation of State for Dark Matter and Dark Energy

NASA Astrophysics Data System (ADS)

We assume that dark matter and dark energy satisfy the unified equation of state: p = B(z)?, with p = pdE, ? = ?dm+?dE, where the pressure of dark matter pdm = 0 has been taken into account. A special function B=-(A)/((1+z)? ) is presented, which can well describe the evolution of the universe. In this model, the universe will end up with a Big Rip. By further simple analysis, we know other choices of the function B can also describe the universe but lead to a different doomsday.

Wang, Wei; Gui, Yuanxing; Zhang, Suhong; Guo, Guanghai; Shao, Ying

6

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

7

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

8

Note on the equation of state of geometric dark energy in f(R) gravity

NASA Astrophysics Data System (ADS)

We focus on the analysis of three inequivalent equations of state of geometric dark energy in f(R) cosmology that have been considered in the past and discuss their differences, advantages and drawbacks.

Jaime, Luisa G.; Patiño, Leonardo; Salgado, Marcelo

2014-04-01

9

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

10

The dark energy cosmic clock: a new way to parametrise the equation of state

NASA Astrophysics Data System (ADS)

We propose a new parametrisation of the dark energy equation of state, which uses the dark energy density, ?e as a cosmic clock. We expand the equation of state in a series of orthogonal polynomials, with ?e as the expansion parameter and determine the expansion coefficients by fitting to SNIa and H(z) data. Assuming that ?e is a monotonic function of time, we show that our parametrisation performs better than the popular Chevallier-Polarski-Linder (CPL) and Gerke and Efstathiou (GE) parametrisations, and we demonstrate that it is robust to the choice of prior. Expanding in orthogonal polynomials allows us to relate models of dark energy directly to our parametrisation, which we illustrate by placing constraints on the expansion coefficients extracted from two popular quintessence models. Finally, we comment on how this parametrisation could be modified to accommodate high redshift data, where any non-monotonicity of ?e would need to be accounted for.

Tarrant, Ewan R. M.; Copeland, Edmund J.; Padilla, Antonio; Skordis, Constantinos

2013-12-01

11

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.

Abramo, L. R.; Batista, R. C. [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Liberato, L.; Rosenfeld, R. [Instituto de Fisica Teorica, Universidade Estadual Paulista, R. Pamplona 145, 01405-900, Sao Paulo (Brazil)

2008-03-15

12

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

13

Constraints on Dark Energy state equation with varying pivoting redshift

NASA Astrophysics Data System (ADS)

We assume the DE state equations w(a)=w0+wa(ap-a), and study the dependence of the constraints on w0 and wa coefficients on the pivoting redshift 1+zp=1/ap. Coefficients are fitted to data including WMAP7, SNIa (Union 2.1), BAO's (including WiggleZ and SDSS results) and H0 constraints. The fitting algorithm is CosmoMC. We find specific differences between the cases when ?-mass is allowed or disregarded. More in detail: (i) The zp value yielding uncorrelated constraints on w0 and wa is different in the two cases, holding ˜0.25 and ˜0.35, respectively. (ii) If we consider the intervals allowed to w0, we find that they shift when zp increases, in opposite directions for vanishing or allowed ?-mass. This leads to no overlap between 1? intervals already at zp>˜0.4. (iii) The known effect that a more negative state parameter is required to allow for ? mass displays its effects on wa, rather than on w0. (iv) The w0-wa constraints found by using any pivot zp can be translated into constraints holding at a specific zp value (0 or the zp where errors are uncorrelated). When we do so, error ellipses exhibit a satisfactory overlap.

Scovacricchi, Dario; Bonometto, Silvio A.; Mezzetti, Marino; La Vacca, Giuseppe

2014-01-01

14

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

15

NASA Astrophysics Data System (ADS)

The discovery that the Universe is speeding up and not slowing down was greeted with open arms by theorists. First, because the dark energy powering the acceleration provided the "missing stuff" needed to make the Universe flat, in accord with a key prediction of inflation. Second, because theorists now have a new puzzle to solve, the nature of the mysterious dark energy. I have no doubt that the dark energy problem will be just as fundamental and just as interesting as the dark matter problem. Determining its nature will require the work of both astronomers and particle physics and will shed light on both fundamental physics and the fate of the Universe.

Turner, Michael S.

2001-04-01

16

High-redshift investigation on the dark energy equation of state

NASA Astrophysics Data System (ADS)

The understanding of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. Whether or not the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, many models have been proposed to solve this problem, both in the context of General Relativity and alternative theories of gravity. Actually, a further possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state (EOS), w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here, we investigate different models of dark energy, described through several parametrizations of the EOS. Our high-redshift analysis is based on the Union2 Type Ia supernovae data set (Suzuki et al.), the Hubble diagram constructed from some gamma-ray bursts luminosity-distance indicators, and Gaussian priors on the distance from the baryon acoustic oscillations, and the Hubble constant h (these priors have been included in order to help to break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the EOS parameters, we use the Markov Chain Monte Carlo Method. It turns out that, if exact flatness is assumed, the dark energy EOS is evolving for all the parametrizations that we considered. We finally compare our results with the ones obtained by previous cosmographic analyses performed on the same astronomical data sets, showing that the latter ones are sufficient to test and compare the new parametrizations.

Piedipalumbo, E.; Della Moglie, E.; De Laurentis, M.; Scudellaro, P.

2014-07-01

17

National Technical Information Service (NTIS)

Dark Energy is the dominant constituent of the universe and we have little understanding of it. We describe a new project aimed at measuring the dark energy equation of state parameter to a statistical precision of approx. 5 %, with four separate techniqu...

B. Flaugher

2004-01-01

18

Reconstruction of the deceleration parameter and the equation of state of dark energy

The new 182 gold supernova Ia data, the baryon acoustic oscillation measurement and the shift parameter determined from the Sloan Digital Sky Survey, and the three-year Wilkinson Microwave Anisotropy Probe data are combined to reconstruct the dark energy equation of state parameter w(z) and the deceleration parameter q(z). We find that the strongest evidence of acceleration happens around the redshift z{approx}0.2 and the stringent constraints on w(z) lie in the redshift range z{approx}0.2-0.5. At the sweet spot, -1.2

Gong Yungui; Wang Anzhong [College of Electronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China) and CASPER, Physics Department, Baylor University, Waco, Texas 76798 (United States); CASPER, Physics Department, Baylor University, Waco, Texas 76798 (United States)

2007-02-15

19

NASA Astrophysics Data System (ADS)

Our ideas about the mass and energy make-up of our universe and the way in which it will evolve in the future have undergone a marked change in the last few years. A wide diversity of experiments have now shown that the universe is dominated by a mysterious `dark energy', and that the normal matter which makes up the stars, planets and ourselves accounts for only some 4% of its total mass and energy content.

Morison, Ian

2003-05-01

20

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

21

Effects of voids on the reconstruction of the equation of state of dark energy

We quantify the effects of the voids known to exist in the Universe upon the reconstruction of the dark energy equation of state w. We show that the effect can start to be comparable with some of the other errors taken into account when analyzing supernova data, depending strongly upon the low redshift cutoff used in the sample. For the supernova data alone, the error induced in the reconstruction of w is much larger than the percent level. When the Baryonic Acoustic Oscillations and the Cosmic Microwave Background data are included in the fit, the effect of the voids upon the determination of w is much lessened but is not much smaller than some of the other errors taken into consideration when performing such fits. We also look at the effect of voids upon the estimation of the equation of state when we allow w to vary over time and show that even when supernova, Cosmic Microwave Background, and Baryonic Acoustic Oscillations data are used to constrain the equation of state, the best fit points in parameter space can change at the 10% level due to the presence of voids, and error-bars increase significantly.

Lavallaz, Arnaud de; Fairbairn, Malcolm [Department of Physics, King's College London, Strand, London, WC2R 2LS (United Kingdom)

2011-10-15

22

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

NASA Astrophysics Data System (ADS)

Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not incompatible with a phantom phase near our time. This has been the case in the years of Wilkinson Microwave Anisotropy Probe observations, in combination with the remaining cosmological observables. Such situations did not completely disappear from the data collected from the Planck satellite mission. In it the EoS analysis may still be interpreted as suggesting ?D ? -1, and so a mildly evolving DE cannot be discarded. In our opinion, the usual ansatzs made on the structure of the EoS for dynamical DE models (e.g. quintessence and the like) is too simplified. In this work, we examine in detail some of these issues and suggest that a general class of models with a dynamical vacuum energy density could explain the persistent phantom anomaly, despite this there is no trace of real phantom behaviour in them. The spurious or `mirage' effect is caused by an attempt to describe them as if the DE would be caused by fundamental phantom scalar fields. Remarkably, the effective DE behaviour can also appear as quintessence in transit to phantom, or vice versa.

Basilakos, Spyros; Solà, Joan

2014-02-01

23

Fundamental Constants, New Physics and the Dark Energy Equation of State

NASA Astrophysics Data System (ADS)

One of the most urgent quests in modern astrophysics and cosmology is the determination of the dark energy equation of state, w, as a function of time. Standard ?CDM cosmology fixes w at -1 for all epochs of the universe, whereas, rolling scalar field cosmologies have values of w that evolve with time. Most rolling scalar fields also couple with the electromagnetic field to produce changes in the fundamental constants such as the fine structure constant ? and the proton to electron mass ratio ?. If the strength of the coupling is given as ? then the change of the fundamental constants is proportional to the integral of ?^2(w+1) integrated over the interval of change. Note that the value of w does not have to change over time to alter the values of the fundamental constants, it just has to be different from -1. This talk examines the present state of limits on w and the variation of fundamental constants to evaluate the parameter space available to new cosmologies and physics. The current limits on the variation of the fundamental constants greatly restricts that space.

Thompson, Rodger I.

2014-01-01

24

Thermodynamical properties of dark energy

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature T{approx}a{sup -n} and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state w>-1 and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation, and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.

Gong Yungui; Wang Bin; Wang Anzhong [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China) and CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States); Department of Physics, Fudan University, Shanghai 200433 (China); CASPER, Department of Physics, Baylor University, Waco, Texas 76798 (United States)

2007-06-15

25

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

26

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

27

Dark energy perturbations revisited

NASA Astrophysics Data System (ADS)

In this Letter we study the evolution of cosmological perturbations in the presence of dynamical dark energy, and revisit the issue of dark energy perturbations. For a generally parameterized equation of state (EoS) such as w(z)=w+wz1+z (for a single fluid or a single scalar field), the dark energy perturbation diverges when its EoS crosses the cosmological constant boundary w=-1. In this Letter we present a method of treating the dark energy perturbations during the crossing of the w=-1 surface by imposing matching conditions which require the induced 3-metric on the hypersurface of w=-1 and its extrinsic curvature to be continuous. These matching conditions have been used widely in the literature to study perturbations in various models of early universe physics, such as Inflation, the Pre-Big-Bang and Ekpyrotic scenarios, and bouncing cosmologies. In all of these cases the EoS undergoes a sudden change. Through a detailed analysis of the matching conditions, we show that ? and ? are continuous on the matching hypersurface. This justifies the method used (Zhao et al., 2005, 2007; Xia et al., 2006, 2008) [1-4] in the numerical calculation and data fitting for the determination of cosmological parameters. We discuss the conditions under which our analysis is applicable.

Li, Mingzhe; Cai, Yifu; Li, Hong; Brandenberger, Robert; Zhang, Xinmin

2011-08-01

28

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

29

Alternatives to dark matter and dark energy

NASA Astrophysics Data System (ADS)

We review the underpinnings of the standard Newton Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy momentum tensor which serves as the source of gravity. We identify the a priori assumption of the validity of standard gravity on all distance scales as the root cause of the dark matter and dark energy problems, and discuss how the freedom currently present in gravitational theory can enable us to construct candidate alternatives to the standard theory in which the dark matter and dark energy problems could then be resolved. We identify three generic aspects of these alternate approaches: that it is a universal acceleration scale which determines when a luminous Newtonian expectation is to fail to fit data, that there is a global cosmological effect on local galactic motions which can replace galactic dark matter, and that to solve the cosmological constant problem it is not necessary to quench the cosmological constant itself, but only the amount by which it gravitates.

Mannheim, Philip D.

2006-04-01

30

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

31

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

32

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

33

NASA Astrophysics Data System (ADS)

In cosmology we are facing the dark energy crisis: How can we survive huge vacuum energy, meanwhile living with tiny dark energy? For the solution to this crisis, we raise several clues and hints, in particular, supersymmetry and the double hierarchy, Mp-MSM-MDE (Planck-Standard Model-dark energy scales). These two clues naturally lead to a solution with a supersymmetry-breaking brane-world. The train of thought from the clues to the solution is elucidated.

Gu, Je-An

2010-11-01

34

Cosmology with Ricci dark energy

NASA Astrophysics Data System (ADS)

We assume the cosmological dark sector to consist of pressureless matter and holographic dark energy with a cutoff length proportional to the Ricci scale. The requirement of separate energy-momentum conservation of the components is shown to establish a relation between the matter fraction and the (necessarily time-dependent) equation-of-state parameter of the dark energy. Focusing on intrinsically adiabatic pressure perturbations of the dark-energy component, the matter perturbations are found as linear combinations of the total energy-density perturbations of the cosmic medium and the relative (nonadiabatic) perturbations of the components. The resulting background dynamics is consistent with observations from supernovae of type Ia, baryonic acoustic oscillations and the differential age of old objects. The perturbation dynamics, on the other hand, is plagued by instabilities which excludes any phantom-type equation of state. The only stable configuration is singled out by a fixed relation between the present matter fraction ?m0 and the present value ?0 of the equation-of-state parameter of the dark energy. However, this instability-avoiding configuration is only marginally consistent with the observationally preferred background values of the mentioned parameters.

del Campo, Sergio; Fabris, Júlio C.; Herrera, Ramón; Zimdahl, Winfried

2013-06-01

35

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

36

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

37

Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields, the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multifield system in details with a general Lagrangian density p={sub i=1}{sup n}X{sub i}g(X{sub i}e{sup {lambda}{sub i}}{sup {phi}{sub i}}), where X{sub i}=-({nabla}{phi}{sub i}){sup 2}/2 is the kinetic energy of the ith field {phi}{sub i}, {lambda}{sub i} is a constant, and g is an arbitrary function in terms of Y{sub i}=X{sub i}e{sup {lambda}{sub i}}{sup {phi}{sub i}}. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from big-bang nucleosynthesis and the condition under which each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a thawing property of assisting scalar fields is crucial to determine the evolution of the field equation of state w{sub {phi}}, and (iii) the field equation of state today can be consistent with the observational bound w{sub {phi}}<-0.8 in the presence of multiple scalar fields.

Ohashi, Junko; Tsujikawa, Shinji [Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)

2009-11-15

38

NASA Astrophysics Data System (ADS)

I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a candidate and emphasizes why no other approach really solves the conceptual problems usually attributed to cosmological constant. Section 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract certain key ingredients which must be present in any viable solution. In the conventional approach, the equations of motion for matter fields are invariant under the shift of the matter Lagrangian by a constant while gravity breaks this symmetry. I argue that until the gravity is made to respect this symmetry, one cannot obtain a satisfactory solution to the cosmological constant problem. Hence cosmological constant problem essentially has to do with our understanding of the nature of gravity. Section 3 discusses such an alternative perspective on gravity in which the gravitational interaction—described in terms of a metric on a smooth spacetime—is an emergent, long wavelength phenomenon, and can be described in terms of an effective theory using an action associated with normalized vectors in the spacetime. This action is explicitly invariant under the shift of the matter energy momentum tensor T ab ? T ab + ? gab and any bulk cosmological constant can be gauged away. Extremizing this action leads to an equation determining the background geometry which gives Einstein’s theory at the lowest order with Lanczos-Lovelock type corrections. In this approach, the observed value of the cosmological constant has to arise from the energy fluctuations of degrees of freedom located in the boundary of a spacetime region.

Padmanabhan, T.

2008-02-01

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

LSST: a complementary probe of dark energy

The number of mass clusters and their distribution in redshift are very sensitive to the density of matter ?m and the equation of state of dark energy w. Using weak lens gravitational tomography one can detect clusters of dark matter, weigh them, image their projected mass distribution, and determine their 3-D location. The degeneracy curve in the ?m ? w

J. A. Tyson; D. M. Wittman; J. F. Hennawi; D. N. Spergelb

2003-01-01

51

Probing dark energy anisotropy

NASA Astrophysics Data System (ADS)

Wide area cosmological surveys enable investigation of whether dark energy properties are the same in different directions on the sky. Cosmic microwave background observations strongly restrict any dynamical effects from anisotropy, in an integrated sense. For more local constraints we compute limits from simulated distance measurements for various distributions of survey fields in a Bianchi I anisotropic universe. We then consider the effects of fitting for line of sight properties where isotropic dynamics is assumed (testing the accuracy through simulations) and compare sensitivities of observational probes for anisotropies, from astrophysical systematics as well as dark energy. We also point out some interesting features of anisotropic expansion in Bianchi I cosmology.

Appleby, Stephen A.; Linder, Eric V.

2013-01-01

52

Dipolar dark matter and dark energy

In previous work [L. Blanchet and A. Le Tiec, Phys. Rev. D 78, 024031 (2008)], a model of dark matter and dark energy based on the concept of gravitational polarization was investigated. This model was shown to recover the concordance cosmological scenario ({lambda}-cold dark matter) at cosmological scales, and the phenomenology of the modified Newtonian dynamics at galactic scales. In this article we prove that the model can be formulated with a simple and physically meaningful matter action in general relativity. We also provide alternative derivations of the main results of the model, and some details on the variation of the action.

Blanchet, Luc; Le Tiec, Alexandre [GReCO Institut d'Astrophysique de Paris-UMR 7095 du CNRS, Universite Pierre and Marie Curie, 98bis boulevard Arago, 75014 Paris (France)

2009-07-15

53

We review two mechanisms rooted in the infrared sector of QCD which, by exploiting the properties of the QCD ghost, as introduced by Veneziano, provide new insight on the cosmological dark energy problem, first, in the form of a Casimir-like energy from quantising QCD in a box, and second, in the form of additional, time-dependent, vacuum energy density in an expanding universe. Based on [1, 2].

Urban, Federico R.; Zhitnitsky, Ariel R. [Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1Z1 (Canada)

2010-08-30

54

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

55

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

56

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

57

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

58

We consider the possibility of getting accelerated expansion and w=-1 crossing in the context of a braneworld cosmological setup, endowed with a bulk energy-momentum tensor. For a given ansatz of the bulk content, we demonstrate that the bulk pressures dominate the dynamics at late times and can lead to accelerated expansion. We also analyze the constraints under which we can get a realistic profile for the effective equation of state and conclude that matter in the bulk has the effect of dark energy on the brane. Furthermore, we show that it is possible to simulate the behavior to a Chaplygin gas using nonexotic bulk matter.

Bogdanos, C.; Dimitriadis, A.; Tamvakis, K. [Physics Department, University of Ioannina, Ioannina GR451 10 (Greece)

2007-04-15

59

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

60

NASA Astrophysics Data System (ADS)

The problem of dark energy is briefly reviewed in both theoretical and observational aspects. In the theoretical aspect, dark energy scenarios are classified into symmetry, anthropic principle, tuning mechanism, modified gravity, quantum cosmology, holographic principle, back-reaction and phenomenological types. In the observational aspect, we introduce cosmic probes, dark energy related projects, observational constraints on theoretical models and model independent reconstructions.

Li, Miao; Li, Xiao-Dong; Wang, Shuang; Wang, Yi

2013-12-01

61

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

62

We examine a dark energy model where a scalar unparticle degree of freedom plays the role of quintessence. In particular, we study a model where the unparticle degree of freedom has a standard kinetic term and a simple mass potential, the evolution is slowly rolling and the field value is of the order of the unparticle energy scale ({lambda}{sub u}). We study how the evolution of w depends on the parameters B (a function of unparticle scaling dimension d{sub u}), the initial value of the field {phi}{sub i} (or equivalently, {lambda}{sub u}) and the present matter density {omega}{sub m0}. We use observational data from type Ia supernovae, baryon acoustic oscillations and the cosmic microwave background to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, unparticle dark energy model is very attractive, since unparticles (being bound states of fundamental fermions) are protected from radiative corrections. Further, coupling of unparticles to the standard model fields can be arbitrarily suppressed by raising the fundamental energy scale M{sub F}, making the unparticle dark energy model free of most of the problems that plague conventional scalar field quintessence models.

Dai, D.-C.; Stojkovic, Dejan [HEPCOS, Department of Physics, SUNY at Buffalo, Buffalo, New York 14260-1500 (United States); Dutta, Sourish [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2009-09-15

63

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

64

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

65

Dark energy and extended dark matter halos

NASA Astrophysics Data System (ADS)

The cosmological mean matter (dark and baryonic) density measured in the units of the critical density is ?m = 0.27. Independently, the local mean density is estimated to be ?loc = 0.08-0.23 from recent data on galaxy groups at redshifts up to z = 0.01-0.03 (as published by Crook et al. 2007, ApJ, 655, 790 and Makarov & Karachentsev 2011, MNRAS, 412, 2498). If the lower values of ?loc are reliable, as Makarov & Karachentsev and some other observers prefer, does this mean that the Local Universe of 100-300 Mpc across is an underdensity in the cosmic matter distribution? Or could it nevertheless be representative of the mean cosmic density or even be an overdensity due to the Local Supercluster therein. We focus on dark matter halos of groups of galaxies and check how much dark mass the invisible outer layers of the halos are able to host. The outer layers are usually devoid of bright galaxies and cannot be seen at large distances. The key factor which bounds the size of an isolated halo is the local antigravity produced by the omnipresent background of dark energy. A gravitationally bound halo does not extend beyond the zero-gravity surface where the gravity of matter and the antigravity of dark energy balance, thus defining a natural upper size of a system. We use our theory of local dynamical effects of dark energy to estimate the maximal sizes and masses of the extended dark halos. Using data from three recent catalogs of galaxy groups, we show that the calculated mass bounds conform with the assumption that a significant amount of dark matter is located in the invisible outer parts of the extended halos, sufficient to fill the gap between the observed and expected local matter density. Nearby groups of galaxies and the Virgo cluster have dark halos which seem to extend up to their zero-gravity surfaces. If the extended halo is a common feature of gravitationally bound systems on scales of galaxy groups and clusters, the Local Universe could be typical or even an overdense region, with a low density contrast ~1.

Chernin, A. D.; Teerikorpi, P.; Valtonen, M. J.; Dolgachev, V. P.; Domozhilova, L. M.; Byrd, G. G.

2012-03-01

66

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

67

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

68

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

69

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

70

NASA Astrophysics Data System (ADS)

We show that a very light scalar field experiencing quantum fluctuations during primordial inflation can explain the current cosmic acceleration. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the universe only recently. Assuming this scenario to be correct, and using supernovae data, the model predicts the energy scale of primordial inflation to be around a few TeV and suggests that it has lasted for an extremely long period. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale.

Ringeval, Christophe; Suyama, Teruaki; Takahashi, Tomo; Yamaguchi, Masahide; Yokoyama, Shuichiro

2014-03-01

71

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.

72

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.

73

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

74

Dipolar dark matter and dark energy

NASA Astrophysics Data System (ADS)

In previous work [L. Blanchet and A. Le Tiec, Phys. Rev. DPRVDAQ1550-7998 78, 024031 (2008)10.1103/PhysRevD.78.024031], a model of dark matter and dark energy based on the concept of gravitational polarization was investigated. This model was shown to recover the concordance cosmological scenario (?-cold dark matter) at cosmological scales, and the phenomenology of the modified Newtonian dynamics at galactic scales. In this article we prove that the model can be formulated with a simple and physically meaningful matter action in general relativity. We also provide alternative derivations of the main results of the model, and some details on the variation of the action.

Blanchet, Luc; Le Tiec, Alexandre

2009-07-01

75

Holographic dark-energy models

NASA Astrophysics Data System (ADS)

Different holographic dark-energy models are studied from a unifying point of view. We compare models for which the Hubble scale, the future event horizon or a quantity proportional to the Ricci scale are taken as the infrared cutoff length. We demonstrate that the mere definition of the holographic dark-energy density generally implies an interaction with the dark-matter component. We discuss the relation between the equation-of-state parameter and the energy density ratio of both components for each of the choices, as well as the possibility of noninteracting and scaling solutions. Parameter estimations for all three cutoff options are performed with the help of a Bayesian statistical analysis, using data from supernovae type Ia and the history of the Hubble parameter. The ?CDM model is the clear winner of the analysis. According to the Bayesian information criterion (BIC), all holographic models should be considered as ruled out, since the difference ?BIC to the corresponding ?CDM value is >10. According to the Akaike information criterion (AIC), however, we find ?AIC<2 for models with Hubble-scale and Ricci-scale cutoffs, indicating, that they may still be competitive. As we show for the example of the Ricci-scale case, also the use of certain priors, reducing the number of free parameters to that of the ?CDM model, may result in a competitive holographic model.

Del Campo, Sergio; Fabris, Júlio. C.; Herrera, Ramón; Zimdahl, Winfried

2011-06-01

76

Dark Energy: fiction or reality?

Is Dark Energy justified as an alternative to the cosmological constant LAMBDA in order to explain the acceleration of the cosmic expansion? It turns out that a straightforward dimensional analysis of Einstein equation provides us with clear evidences that the geometrical nature of LAMBDA is the only viable source to this phenomenon, in addition of the application of Ockham's razor principle. This contribution is primarily a review of the main stream in the interpretation of LAMBDA because it is at the origin of such a research program.

Triay, Roland [Centre de Physique Theorique, CNRS Luminy Case 907, 13288 Marseille Cedex 9 (France)

2010-06-15

77

Cosmological evolution of pilgrim dark energy

NASA Astrophysics Data System (ADS)

We study pilgrim dark energy model by taking IR cut-offs as particle and event horizons as well as conformal age of the universe. We derive evolution equations for fractional energy density and equation of state parameters for pilgrim dark energy. The phantom cosmic evolution is established in these scenarios which is well supported by the cosmological parameters such as deceleration parameter, statefinder parameters and phase space of ? ? and . We conclude that the consistent value of parameter ? is ?<0 in accordance with the current Planck and WMAP9 results.

Sharif, M.; Zubair, M.

2014-07-01

78

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

79

Dark mass creation during EWPT via Dark Energy interaction

NASA Astrophysics Data System (ADS)

We add Dark Matter-Dark Energy terms with a quintessence field interacting with a Dark Matter field to a Minimal Supersymmetry Model of the Electroweak (MSSM EW) Lagrangian previously used to calculate the magnetic field created during the Electroweak Phase Transition (EWPT). From the expectation value of the quintessence field, we estimate the Dark Matter mass for parameters used in previous work on Dark Matter-Dark Energy interactions.

Kisslinger, Leonard S.; Casper, Steven

2014-04-01

80

Dark energy physics expectations at DES

NASA Astrophysics Data System (ADS)

Giving rise to a new and exciting research field, observations of the last 13 years established the accelerated expansion of the Universe. This is a strong indication of new physics, either in the form of a new energy component of the Universe - dark energy - or of theories of gravity beyond general relativity. A powerful approach to this problem is the study of complementary cosmological probes in large optical galaxy surveys such as the Dark Energy Survey (DES). We present the expectations for dark energy physics based on the combination of four fundamental probes: galaxy clusters, weak lensing, large scale structure and supernovae. We show that DES data have constraining power to improve current measurements of the dark energy equation-of-state parameter by a factor of 3-5 and to distinguish between general relativity and modified gravity scenarios.

Soares-Santos, Marcelle; DES Collaboration

2012-07-01

81

Dark Matter, Dark Energy and General Relativity

NASA Astrophysics Data System (ADS)

From an earlier model of DM, we derived: An equation of state for DM. An equation governing the coupled distributions of baryonic and dark matters; galactic flat rotation curves; the Tully-Fisher relation; Milgrom's MOND relation; and a mechanism for the accelerated expansion of the Universe (J of Modern Physics, Dec. 2011). In the present paper, we add the derivation of: a mechanism for the formation of a super massive black hole at the center of every large galaxy; a rational for the finding (by Oort) that the Sun mass on the Solar Neighborhood scale is twice that found on the Solar System scale. Results are discussed; we conclude that general relativity may neither be applied to DM nor to the description of the Universe and its evolution. The correct description must be based not on General Relativity but on Newton's Gravitation, on the proposed model of DM, and on Einstein's relativity of 1905.

Leibovitz, Jacques

2013-04-01

82

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

83

NASA Astrophysics Data System (ADS)

A long-standing problem of theoretical physics is the exceptionally small value of the cosmological constant Lambda approx 10-120 measured in natural Planckian units. Here we derive this tiny number from a toroidal string cosmology based on closed strings. In this picture the dark energy arises from the correlation between momentum and winding modes that for short distances has an exponential fall-off with increasing values of the momenta. The freeze-out by the expansion of the background universe for these transplanckian modes may be interpreted as a frozen condensate of the closed-string modes in the three non-compactified spatial dimensions.

Frampton, Paul H.

2002-07-01

84

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.

85

Cosmological bounds on oscillating dark energy models

NASA Astrophysics Data System (ADS)

We study the cosmological constraints on the two purely phenomenological models of oscillating dark energy. In these oscillating models, the equation of state of dark energy varies periodically. The periodic equation of state may provide the natural way to unify the early acceleration (inflation) and the late time acceleration of the Universe. These models give the effective way to tackle the cosmic coincidence problem. We examine the observational constraints on the oscillatory models from the latest observational data including the gold sample of 182 SNe type Ia, the shift parameter, R, given by the WMAP and the BAO measurements from the SDSS.

Jain, Deepak; Dev, Abha; Alcaniz, J. S.

2007-11-01

86

Dark matter and dark energy: The critical questions

Stars account for only about 0.5% of the content of the Universe; the bulk of the Universe is optically dark. The dark side of the Universe is comprised of: at least 0.1% light neutrinos; 3.5% {+-} 1% baryons; 29% {+-} 4% cold dark matter; and 66% {+-} 6% dark energy. Now that we have characterized the dark side of the Universe, the challenge is to understand it. The critical questions are: (1) What form do the dark baryons take? (2) What is (are) the constituent(s) of the cold dark matter? (3) What is the nature of the mysterious dark energy that is causing the Universe to speed up.

Michael S. Turner

2002-11-19

87

NASA Astrophysics Data System (ADS)

Dark energy seems to dominate today's universe. The talk will begin by reviewing the astronomical evidence for cosmic acceleration. This cosmic acceleration implies either that the dark energy is the primary component of our universe or that general relativity is not valid on cosmological scales. The talk will focus on new probes of dark energy with a focus on measurements by the WFIRST mission and observations of the cosmic microwave background.

Spergel, David N.

2014-06-01

88

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

89

Generalized models of unification of dark matter and dark energy

NASA Astrophysics Data System (ADS)

A model of unification of dark matter and dark energy based on the modeling of the speed of sound as a function of the parameter of the equation of state is introduced. It is found that the model in which the speed of sound depends on the power of the parameter of the equation of state, cs2=?(-w)?, contains the generalized Chaplygin gas models as its subclass. An effective scalar field description of the model is obtained in a parametric form which in some cases can be translated into a closed form solution for the scalar field potential. A constraint on model parameters is obtained using the observational data on the Hubble parameter at different redshifts.

?aplar, Neven; Štefan?i?, Hrvoje

2013-01-01

90

Revival of the unified dark energy-dark matter model?

We consider the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and show that it admits an unique decomposition into dark energy and dark matter components once phantomlike dark energy is excluded. Within this framework, we study structure formation and show that difficulties associated to unphysical oscillations or blowup in the matter power spectrum can be circumvented. Furthermore, we show that the dominance of dark energy is related to the time when energy density fluctuations start deviating from the linear {delta}{approx}a behavior.

Bento, M.C.; Bertolami, O.; Sen, A.A. [Departamento de Fisica, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal); Departamento de Fisica and Centro de Fisica das Interaccoes Fundamentais, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal)

2004-10-15

91

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

92

Dark Energy Rules the Universe

The revolutionary discovery that the expansion of the universe is speeding up, not slowing down from gravity, means that 75 percent of our universe consists of mysterious dark energy. Berkeley Lab theoretical physicist Eric Linder delves into the mystery of dark energy as part of the Science in the Theatre lecture series on Nov. 24, 2008.

Eric Linder

2008-12-16

93

Dark Energy Rules the Universe

The revolutionary discovery that the expansion of the universe is speeding up, not slowing down from gravity, means that 75 percent of our universe consists of mysterious dark energy. Berkeley Lab theoretical physicist Eric Linder delves into the mystery of dark energy as part of the Science in the Theatre lecture series on Nov. 24, 2008.

94

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

95

Theoretical Models of Dark Energy

NASA Astrophysics Data System (ADS)

Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore, we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali-Gabadadze-Porrati (DGP) model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaître-Tolman-Bondi (LTB) model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.

Yoo, Jaewon; Watanabe, Yuki

2012-11-01

96

Non-linear dark energy clustering

NASA Astrophysics Data System (ADS)

We consider a dark energy fluid with arbitrary sound speed and equation of state and discuss the effect of its clustering on the cold dark matter distribution at the non-linear level. We write the continuity, Euler and Poisson equations for the system in the Newtonian approximation. Then, using the time renormalization group method to resum perturbative corrections at all orders, we compute the total clustering power spectrum and matter power spectrum. At the linear level, a sound speed of dark energy different from that of light modifies the power spectrum on observationally interesting scales, such as those relevant for baryonic acoustic oscillations. We show that the effect of varying the sound speed of dark energy on the non-linear corrections to the matter power spectrum is below the per cent level, and therefore these corrections can be well modelled by their counterpart in cosmological scenarios with smooth dark energy. We also show that the non-linear effects on the matter growth index can be as large as 10-15 per cent for small scales.

Anselmi, Stefano; Ballesteros, Guillermo; Pietroni, Massimo

2011-11-01

97

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

98

Cosmological bounds on the equation of state of dark matter

NASA Astrophysics Data System (ADS)

In this exploratory study, we investigate the bounds on the equation of state of dark matter. Modeling dark matter as a fluid component, we take into account both positive and negative fixed equations of state. Using CMB, supernovae Ia and large scale structure data we find constraints on the equation of state in a modified ?CDM cosmology. We obtain -1.50×10-6

Müller, Christian M.

2005-02-01

99

New generalized Chaplygin gas as a scheme for unification of dark energy and dark matter

We propose in this paper a new model for describing the unification of dark energy and dark matter. This new model is a further generalization of the generalized Chaplygin gas (GCG) model, thus dubbed new generalized Chaplygin gas (NGCG) model. The equation of state of the NGCG is given by p = - tilde A(a)\\/rhoalpha, where a is the scale

Xin Zhang; Feng-Quan Wu; Jingfei Zhang

2006-01-01

100

Inflation, dark matter, and dark energy in the string landscape.

We consider the conditions needed to unify the description of dark matter, dark energy, and inflation in the context of the string landscape. We find that incomplete decay of the inflaton field gives the possibility that a single field is responsible for all three phenomena. By contrast, unifying dark matter and dark energy into a single field, separate from the inflaton, appears rather difficult. PMID:17155379

Liddle, Andrew R; Ureña-López, L Arturo

2006-10-20

101

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.youtube.com/watch?v=T6IBoa4FxvY&feature=channel_page

102

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.youtube.com/watch?v=T6IBoa4FxvY&feature=channel_page

Linder, Eric

2008-01-01

103

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

104

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

105

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

106

NASA Astrophysics Data System (ADS)

By careful analysis of the data from the WMAP satellite, scientists were surprised to determine that about 70% of the matter in our universe is in some unknown form, and labeled it Dark Energy. Earlier, in 1998, two separate international groups of astronomers studying Ia supernovae were even more surprised to be forced to conclude that an amazing smooth transition occurred, from the expected slowing down of the expansion of our universe (due to normal positive gravitation) to an accelerating expansion of the universe that began at at a big bang age of the universe of about nine billion years. In 1918 Albert Einstein stated that his Lambda term in his theory of general relativity was ees,``the energy of empty space,'' and represented a negative pressure and thus a negative gravity force. However my 2004 ``Strong'' Magnetic Field model (SMF) for the origin of magnetic fields at Combination Time (Astro-ph0509223 and 0509222) in our big bang universe produces a unique topology for Superclusters, having almost all the mass, visible and invisible, i.e. from clusters of galaxies down to particles with mass, on the surface of an ellipsoid surrounding a growing very high vacuum. If I hypothesize, with Einstein, that there exists a constant ees force per unit volume, then, gradually, as the universe expands from Combination Time, two effects occur (a) the volume of the central high vacuum region increases, and (b) the density of positive gravity particles in the central region of each Supercluster in our universe decreases dramatically. Thus eventually Einstein's general relativity theory's repulsive gravity of the central very high vacuum region becomes larger than the positive gravitational attraction of all the clusters of galaxies, galaxies, quasars, stars and plasma on the Supercluster shell, and the observed accelerating expansion of our universe occurs. This assumes that our universe is made up mostly of such Superclusters. It is conceivable that the high vacuum region between Superclusters also plays a role in adding extra repulsive gravity force. Note that cosmologist Stephen Hawking comments on his website that ``There is no reason to rule out negative pressure. This is just tension.''

Greyber, Howard

2009-11-01

107

Dark Matter, Dark Energy and the Chaplygin Gas

We formulate a Zel'dovich-like approximation for the Chaplygin gas equation\\u000aof state P = -A\\/rho, and sketch how this model unifies dark matter with dark\\u000aenergy in a geometric setting reminiscent of M-theory.

Neven Bilic; Gary B. Tupper; Raoul D. Viollier

2002-01-01

108

Towards understanding the nature of dark energy

NASA Astrophysics Data System (ADS)

Dark energy turns out to be one of the most intriguing problems in modern cosmology. In this thesis, we study several projects related to unveiling the nature of dark energy. As a negative pressure component, the most important property of dark energy is its equation of state w. We study the prospect of constraining w through a future weak-lensing survey by combining two methods: the tomographic shear-shear correlations and the number counts of the shear-selected galaxy clusters. We calculate the covariance between the two observables and find it negligible. By forecasting the constraining power through the Fisher matrix formalism, we find that each method has serious parameter degeneracy, hut their combination results in appreciable complementarity. In order to use the abundance of galaxy clusters to constrain dark energy, the primary systematic effect is uncertainty in the mass-observable scaling relations. For the X-ray clusters, we calibrate the preheating model for the intracluster gas by comparing its predictions for the X-ray Luminosity-Temperature relations with those of two cluster samples observed at different redshifts. We find that the required entropy level increases with time, which indicates the time-dependence of feedback on the intracluster gas cannot be discarded. Instead of a missing unknown energy component, the dark energy problem may indicate that gravity deviates from GR on cosmological scales. We investigate the self-accelerating Dvali-Gabadadze-Porrati (DGP) model which explains the accelerated expansion of the universe from the perspective of modified gravity. By implementing the parameterized post-Friedmann (PPF) formalism into standard cosmological tools, we can calculate the growth of structure in DGP efficiently. This allows us to perform a thorough Markov Chain Monte Carlo analysis of the model, given the current observations of the anisotropies of the cosmic microwave background, magnitude of the supernovae and the Hubble constant. Our results show that this model cannot fit both the geometry and growth data simultaneously, with its best-fit nominally 5sigma poorer than that of the concordance ACDM model. If dark energy is not a cosmological constant, it clusters in space. The clustering properties are well described for the scalar field dark energy models. However, if w evolves across the phantom divide defined by w = -1, to avoid instabilities in the perturbations, the dark energy cannot be realized by a single scalar field, but a composite of multiple fields, the construction of which is cumbersome. We introduce a PPF description for these smooth dark energy models to avoid this difficulty. By comparing with true scalar field models, we find that the PPF description gives accurate results.

Fang, Wenjuan

2009-10-01

109

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

110

Asymptotic analysis of the Boltzmann equation for dark matter relics

NASA Astrophysics Data System (ADS)

This paper presents an asymptotic analysis of the Boltzmann equations (Riccati differential equations) that describe the physics of thermal dark-matter-relic abundances. Two different asymptotic techniques are used, boundary-layer theory, which makes use of asymptotic matching, and the delta expansion, which is a powerful technique for solving nonlinear differential equations. Two different Boltzmann equations are considered. The first is derived from general relativistic considerations and the second arises in dilatonic string cosmology. The global asymptotic analysis presented here is used to find the long-time behavior of the solutions to these equations. In the first case, the nature of the so-called freeze-out region and the post-freeze-out behavior is explored. In the second case, the effect of the dilaton on cold dark-matter abundances is calculated and it is shown that there is a large-time power-law fall off of the dark-matter abundance.

Bender, Carl M.; Sarkar, Sarben

2012-10-01

111

New interactions in the dark sector mediated by dark energy

Cosmological observations have revealed the existence of a dark matter sector, which is commonly assumed to be made up of one particle species only. However, this sector might be more complicated than we currently believe: there might be more than one dark matter species (for example, two components of cold dark matter or a mixture of hot and cold dark matter) and there may be new interactions between these particles. In this paper we study the possibility of multiple dark matter species and interactions mediated by a dark energy field. We study both the background and the perturbation evolution in these scenarios. We find that the background evolution of a system of multiple dark matter particles (with constant couplings) mimics a single fluid with a time-varying coupling parameter. However, this is no longer true on the perturbative level. We study the case of attractive and repulsive forces as well as a mixture of cold and hot dark matter particles.

Brookfield, Anthony W. [Astroparticle Theory and Cosmology Group, Department of Applied Mathematics, University of Sheffield, Hounsfield Road, Sheffield S3 7RH (United Kingdom); Astroparticle Theory and Cosmology Group, Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH (United Kingdom); Bruck, Carsten van de; Hall, Lisa M. H. [Astroparticle Theory and Cosmology Group, Department of Applied Mathematics, University of Sheffield, Hounsfield Road, Sheffield S3 7RH (United Kingdom)

2008-02-15

112

New interactions in the dark sector mediated by dark energy

NASA Astrophysics Data System (ADS)

Cosmological observations have revealed the existence of a dark matter sector, which is commonly assumed to be made up of one particle species only. However, this sector might be more complicated than we currently believe: there might be more than one dark matter species (for example, two components of cold dark matter or a mixture of hot and cold dark matter) and there may be new interactions between these particles. In this paper we study the possibility of multiple dark matter species and interactions mediated by a dark energy field. We study both the background and the perturbation evolution in these scenarios. We find that the background evolution of a system of multiple dark matter particles (with constant couplings) mimics a single fluid with a time-varying coupling parameter. However, this is no longer true on the perturbative level. We study the case of attractive and repulsive forces as well as a mixture of cold and hot dark matter particles.

Brookfield, Anthony W.; van de Bruck, Carsten; Hall, Lisa M. H.

2008-02-01

113

The Dark Energy Survey: Overview

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) will employ a new, 570-megapixel digital imager, the Dark Energy Camera, on the Blanco 4-meter telescope at CTIO to carry out 2 multi-band surveys over 525 nights: a wide-area survey of 5000 sq. deg. that will provide grizY measurements of ~200 million galaxies and a time-domain survey in griz over 30 sq. deg. that will yield well-measured light curves for ~4000 type Ia supernovae. The camera saw first light on the telescope in Sept. 2012 and has since been undergoing commissioning and science verification. The aim of the DES project is to probe dark energy using 4 techniques: supernovae, clusters, galaxy clustering (including baryon acoustic oscillations), and weak lensing. I will overview the project, highlighting the science program and setting the context for the other talks in this Special Session.

Frieman, Joshua; Dark Energy Survey Collaboration

2013-01-01

114

NASA Astrophysics Data System (ADS)

Event horizons and closed time-like curves cannot exist in the real world for the simple reason that they are inconsistent with quantum mechanics. Following ideas originated by Robert Laughlin, Pawel Mazur, Emil Mottola, David Santiago, and the speaker it is now possible to describe in some detail what happens physically when one approaches and crosses a region of space-time where classical general relativity predicts there should be an infinite red shift surface. This quantum critical physics provides a new perspective on a variety of enigmatic astrophysical phenomena including supernovae explosions, gamma ray bursts, positron emission, and dark matter.

Chapline, G.

115

Effective field theory of modified gravity with two scalar fields: Dark energy and dark matter

NASA Astrophysics Data System (ADS)

We present a framework for discussing the cosmology of dark energy and dark matter based on two scalar degrees of freedom. An effective field theory of cosmological perturbations is employed. A unitary gauge choice renders the dark energy field into the gravitational sector, for which we adopt a generic Lagrangian depending on three-dimensional geometrical scalar quantities arising in the Arnowitt-Deser-Misner decomposition. We add to this dark energy-associated gravitational sector a scalar field ? and its kinetic energy X as dark matter variables. Compared to the single-field case, we find that there are additional conditions to obey in order to keep the equations of motion for linear cosmological perturbations at second order. For such a second-order multifield theory, we derive conditions under which ghosts and Laplacian instabilities of the scalar and tensor perturbations are absent. We apply our general results to models with dark energy emerging in the framework of the Horndeski theory and dark matter described by a k-essence Lagrangian P(? ,X). We derive the effective coupling between such an imperfect-fluid dark matter and the gravitational sector under the quasistatic approximation on subhorizon scales. By considering the purely kinetic Lagrangian P(X) as a particular case, the formalism is verified to reproduce the gravitational coupling of a perfect-fluid dark matter.

Gergely, László Á.; Tsujikawa, Shinji

2014-03-01

116

Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints

The generalized Chaplygin gas (GCG) model allows for an unified description\\u000aof the recent accelerated expansion of the Universe and the evolution of energy\\u000adensity perturbations. This dark energy - dark matter unification is achieved\\u000athrough an exotic background fluid whose equation of state is given by $p = -\\u000aA\\/\\\\rho^{\\\\alpha}$, where $A$ is a positive constant and $0 <

M. C. Bento; O. Bertolami; A. A. Sen

2003-01-01

117

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

118

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

119

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

120

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

121

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

122

Cosmological dark energy and entanglement

Astrophysical observations of recent years indicate that the universe is composed of a large amount of dark energy (DE) responsible for an accelerated expansion of the universe. Here, we argue that DE can arise entirely from the standard principles of strong interaction physics and quantum entanglement. Good quantitative agreement of our estimate with the observation, without the need for any adjustable parameters, is encouraging.

Ghosh, Sanjay K.; Raha, Sibaji [Department of Physics, Bose Institute, 93/1 A.P.C. Road, Kolkata-700009, India Centre for Astroparticle Physics and Space Science, Bose Institute, Block EN, Sector V, Kolkata 700091 (India)

2011-09-23

123

Cluster probes of dark energy clustering

NASA Astrophysics Data System (ADS)

Cluster abundances are oddly insensitive to canonical early dark energy. Early dark energy with sound speed equal to the speed of light cannot be distinguished from a quintessence model with the equivalent expansion history for z<2 but negligible early dark energy density, despite the different early growth rate. However, cold early dark energy, with a sound speed much smaller than the speed of light, can give a detectable signature. Combining cluster abundances with cosmic microwave background (CMB) power spectra can determine the early dark energy fraction to 0.3% and distinguish a true sound speed of 0.1 from 1 at 99% confidence. We project constraints on early dark energy from the Euclid cluster survey, as well as the Dark Energy Survey, using both current and projected Planck CMB data, and assess the impact of cluster mass systematics. We also quantify the importance of dark energy perturbations, and the role of sound speed during a crossing of w=-1.

Appleby, Stephen A.; Linder, Eric V.; Weller, Jochen

2013-08-01

124

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

125

Analysis of generalized ghost version of pilgrim dark energy

NASA Astrophysics Data System (ADS)

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 ( ? 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 ? 2 and u. We also develop 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 ?CDM limit on the statefinder plane.

Sharif, M.; Jawad, Abdul

2014-05-01

126

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

127

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.

Trout, Aaron D.

2013-01-01

128

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

129

A parametric model for dark energy

NASA Astrophysics Data System (ADS)

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in cosmology but also in high energy physics theories. Following this approach, we explore in this Letter some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z over the entire cosmological evolution, i.e., z?[-1,?). This parameterization allows us to divide the parametric plane (w,w) in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. By assuming a flat universe, a statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution H(z) is performed to check the observational viability of the EoS parameterization here proposed.

Barboza, E. M.; Alcaniz, J. S.

2008-09-01

130

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

131

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.

Vikhlinin, Alexey

2010-01-01

132

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

133

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

134

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

135

Stable Dark Energy Stars:. AN Alternative to Black Holes?

NASA Astrophysics Data System (ADS)

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, w ? p/? < -1/3, to an exterior Schwarzschild vacuum solution at a junction interface, situated near to where the event horizon is expected to form. The motivation for implementing this generalization arises from the fact that recent observations have confirmed an accelerated cosmic expansion, for which dark energy is a possible candidate.

Lobo, Francisco S. N.

2008-09-01

136

Dissipative generalized Chaplygin gas as phantom dark energy

The generalized Chaplygin gas, characterized by the equation of state p=?A\\/??, has been considered as a model for dark energy due to its dark-energy-like evolution at late times. When dissipative processes are taken into account, within the framework of the standard Eckart theory of relativistic irreversible thermodynamics, cosmological analytical solutions are found. Using the truncated causal version of the Israel–Stewart

Norman Cruz; Samuel Lepe; Francisco Peña

2007-01-01

137

Inhomogeneous alternative to dark energy?

Recently, there have been suggestions that the apparent accelerated expansion of the universe is not caused by repulsive gravitation due to dark energy, but is rather a result of inhomogeneities in the distribution of matter. In this work, we investigate the behavior of a dust-dominated inhomogeneous Lemaître-Tolman-Bondi universe model, and confront it with various astrophysical observations. We find that such

Håvard Alnes; Morad Amarzguioui; Øyvind Grøn

2006-01-01

138

Dark matter and dark energy in dwarf galaxy systems

NASA Astrophysics Data System (ADS)

Quantitative estimates of themaximumallowed totalmasses and sizes of the dark-matter halos in groups and associations of dwarf galaxies—special types of metagalactic populations identified in recent astronomical observations with the Hubble Space Telescope—are presented. Dwarf-galaxy systems are formed of isolated dark-matter halos with a small number of dark galaxies embedded in them. Data on the sizes of these systems and the velocity dispersions of the embedded galaxies can be used to determine lower limits on the total dark-halo masses using the virial theorem. Upper limits follow from the conditions that the systems immersed in the cosmic dark-energy background be gravitationally bound. The median maximum masses are close to 1012 M ? for both groups and associations of dwarf galaxies, although the median virial masses for these two types of systems differ by approximately a factor of ten.

Chernin, A. D.; Teerikorpi, P.

2014-01-01

139

Dark energy anisotropic stress and large scale structure formation

We investigate the consequences of an imperfect dark energy fluid on the large scale structure. A phenomenological three parameter fluid description is used to study the effect of dark energy on the cosmic microwave background radiation (CMBR) and matter power spectrum. In addition to the equation of state and the sound speed, we allow a nonzero viscosity parameter for the fluid. Then anisotropic stress perturbations are generated in dark energy. In general, we find that this possibility is not excluded by the present day cosmological observations. In the simplest case when all of the three parameters are constant, we find that the observable effects of the anisotropic stress can be closely mimicked by varying the sound speed of perfect dark energy. However, now also negative values for the sound speed, as expected for adiabatic fluid model, are tolerable and in fact could explain the observed low quadrupole in the CMBR spectrum. We investigate also structure formation of imperfect fluid dark energy characterized by an evolving equation of state. In particular, we study models unifying dark energy with dark matter, such as the Chaplygin gas or the Cardassian expansion, with a shear perturbation included. This can stabilize the growth of inhomogeneities in these models, thus somewhat improving their compatibility with large scale structure observations.

Koivisto, Tomi [Helsinki Institute of Physics, FIN-00014 Helsinki (Finland); Mota, David F. [Institute of Theoretical Astrophysics, University of Oslo, Box 1029, 0315 Oslo (Norway)

2006-04-15

140

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

141

Two new diagnostics of dark energy

We introduce two new diagnostics of dark energy (DE). The first, Om, is a combination of the Hubble parameter and the cosmological redshift and provides a null test of dark energy being a cosmological constant {lambda}. Namely, if the value of Om(z) is the same at different redshifts, then DE{identical_to}{lambda}, exactly. The slope of Om(z) can differentiate between different models of dark energy even if the value of the matter density is not accurately known. For DE with an unevolving equation of state, a positive slope of Om(z) is suggestive of phantom (w<-1) while a negative slope indicates quintessence (w>-1). The second diagnostic--acceleration probe q--is the mean value of the deceleration parameter over a small redshift range. It can be used to determine the cosmological redshift at which the universe began to accelerate, again without reference to the current value of the matter density. We apply the Om and q diagnostics to the Union data set of type Ia supernovae combined with recent data from the cosmic microwave background (Wilkinson Microwave Anisotropy Probe 5) and baryon acoustic oscillations.

Sahni, Varun; Shafieloo, Arman [Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007 (India); Starobinsky, Alexei A. [Landau Institute for Theoretical Physics, Kosygina 2, Moscow 119334 (Russian Federation)

2008-11-15

142

Evolving dark energy with w not = -1.

Theories of evolving quintessence are constructed that generically lead to deviations from the w = -1 prediction of nonevolving dark energy. The small mass scale that governs evolution, m(phi) approximately = 10(-33) eV, is radiatively stable, and the "Why now?" problem is solved. These results rest on seesaw cosmology: Fundamental physics and cosmology can be broadly understood from only two mass scales, the weak scale nu and the Planck scale M. Requiring a scale of dark energy rho(DE)(1/4) governed by nu2/M and a radiatively stable evolution rate m(phi) given by nu4/M3 leads to a distinctive form for the equation of state w(z). Dark energy resides in the potential of a hidden axion field that is generated by a new QCD-like force that gets strong at the scale lambda approximately = nu2/M approximately = rho(DE)(1/4). The evolution rate is given by a second seesaw that leads to the axion mass m(phi) approximately = lambda2/f, with f approximately = M. PMID:16241641

Hall, Lawrence J; Nomura, Yasunori; Oliver, Steven J

2005-09-30

143

Hiding dark energy transitions at low redshift

We show that it is both observationally allowable and theoretically possible to have large fluctuations in the dark energy equation of state as long as they occur at ultralow redshifts z < or approx. 0.02. These fluctuations would masquerade as a local transition in the Hubble rate of a few percent or less and escape even future, high precision, high redshift measurements of the expansion history and structure. Scalar field models that exhibit this behavior have a sharp feature in the potential that the field traverses within a fraction of an e-fold of the present. The equation of state parameter can become arbitrarily large if a sharp dip or bump in the potential causes the kinetic and potential energy of the field to both be large and have opposite sign. While canonical scalar field models can decrease the expansion rate at low redshift, increasing the local expansion rate requires a noncanonical kinetic term for the scalar field.

Mortonson, Michael [Kavli Institute for Cosmological Physics and Department of Physics, University of Chicago, Chicago, Illinois 60637 (United States); Hu, Wayne [Kavli Institute for Cosmological Physics and Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church St, Ann Arbor, Michigan 48109-1040 (United States)

2009-09-15

144

Dark energy and fundamental physics

NASA Astrophysics Data System (ADS)

The acceleration of the expansion of the Universe which has been identified in recent years has deep connections with some of the most central issues in fundamental physics. At present, the most plausible explanation is some form of vacuum energy. The puzzle of vacuum energy is a central question which lies at the interface between quantum theory and general relativity. Solving it will presumably require to construct a quantum theory of gravity and a correspondingly consistent picture of spacetime. To account for the acceleration of the expansion, one may also think of more dynamical forms of energy, what is known as dark energy, or modifications of gravity. In what follows, we review the vacuum energy problem as well as the basic models for dark energy or modification of gravity. We emphasize the conceptual aspects rather than the techniques involved. We also discuss the difficulties encountered in each approach. This review is intended for astrophysicists or physicists not specialized in particle physics, who are interested in apprehending the issues at stake in fundamental physics.

Binétruy, P.

2013-10-01

145

Gauge conditions in combined dark energy and dark matter systems

When analyzing a system consisting of both dark matter and dark energy, an often used practice in the literature is to neglect the perturbations in the dark energy component. However, it has recently been argued, through the use of numerical simulations, that one cannot do so. In this work we show that by neglecting such perturbations one is implicitly making a choice of gauge. As such, one no longer has the freedom to choose, for example, a gauge comoving with the dark matter - in fact doing so will give erroneous, gauge dependent results. We obtain results consistent with the numerical simulations by using the formalism of cosmological perturbation theory, and thus without resorting to involved numerical calculations.

Christopherson, Adam J. [Astronomy Unit, School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London, E1 4NS (United Kingdom)

2010-10-15

146

Bistable dark solitons of a cubic-quintic Helmholtz equation

We provide a report on exact analytical bistable dark spatial solitons of a nonlinear Helmholtz equation with a cubic-quintic refractive-index model. Our analysis begins with an investigation of the modulational instability characteristics of Helmholtz plane waves. We then derive a dark soliton by mapping the desired asymptotic form onto a uniform background field and obtain a more general solution by deploying rotational invariance laws in the laboratory frame. The geometry of the new soliton is explored in detail, and a range of new physical predictions is uncovered. Particular attention is paid to the unified phenomena of arbitrary-angle off-axis propagation and nondegenerate bistability. Crucially, the corresponding solution of paraxial theory emerges in a simultaneous multiple limit. We conclude with a set of computer simulations that examine the role of Helmholtz dark solitons as robust attractors.

Christian, J. M.; McDonald, G. S.; Chamorro-Posada, P. [Joule Physics Laboratory, School of Computing, Science and Engineering, Materials and Physics Research Centre, University of Salford, Salford M5 4WT (United Kingdom); Departamento de Teoria de la Senal y Comunicaciones e Ingenieria Telematica, Universidad de Valladolid, ETSI Telecomunicacion, Campus Miguel Delibes s/n, 47011 Valladolid (Spain)

2010-05-15

147

Dark energy, matter creation and curvature

NASA Astrophysics Data System (ADS)

The most studied way to explain the current accelerated expansion of the universe is to assume the existence of dark energy; a new component that fills the universe, does not form clumps, currently dominates the evolution, and has a negative pressure. In this work I study an alternative model proposed by Lima et al. (Abramo and Lima in Class. Quantum Gravity 13:2953, 1996; Zimdahl in Phys. Rev. D 53:5483, 1996; Zimdahl and Pavón in Mon. Not. R. Astron. Soc. 266:872, 1994), which does not need an exotic equation of state, but assumes instead the existence of gravitational particle creation. Because this model fits the supernova observations as well as the ?CDM model, I perform in this work a thorough study of this model, considering an explicit spatial curvature. I found that in this scenario we can alleviate the cosmic coincidence problem, basically showing that these two components, dark matter and dark energy, are of the same nature, but they act at different scales. I also shown the inadequacy of some particle creation models, and I study a previously proposed new model that overcomes these difficulties.

Cárdenas, Víctor H.

2012-09-01

148

The equivalence between different Dark (matter) energy scenarios

NASA Astrophysics Data System (ADS)

We have shown that the phenomenological models with a cosmological constant of the type ?=?(? left( {{ddot R}/R} right)) and ?=3?H 2, where R is the scale factor of the universe and H is the Hubble constant, are equivalent to a quintessence model with a scalar (?) potential of the formV??-n, n= constant. The equation of state of the cosmic fluid is described by these parameters (?, ?, n) only. The equation of state of the cosmic fluid (dark energy) can be determined by any of these parameters. The actual amount of dark energy will define the equation of state of the cosmic fluid.All of the three forms can give rise to cosmic acceleration depending the amount of dark energy in the universe.

Arbab, Arbab I.

2004-05-01

149

Parametrizing dark sector perturbations via equations of state

NASA Astrophysics Data System (ADS)

The evolution of perturbations is a crucial part of the phenomenology of the dark sector cosmology. We advocate parametrizing these perturbations using equations of state for the entropy perturbation and the anisotropic stress. For small perturbations, these equations of state will be linear in the density, velocity and metric perturbations, and in principle these can be related back to the field content of the underlying model allowing for confrontation with observations. We illustrate our point by constructing gauge-invariant entropy perturbations for theories where the dark sector Lagrangian is a general function of a scalar field, its first and second derivatives, and the metric and its first derivative, L=L(?,???,?????,g??,??g??). As an example, we show how to apply this approach to the case of models of kinetic gravity braiding.

Battye, Richard A.; Pearson, Jonathan A.

2013-09-01

150

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 effect, providing a way to measure radial peculiar velocities of massive galaxy clusters. We show that cluster peculiar velocities have the potential to constrain several dark energy parameters. We compare three velocity statistics (the distribution of radial velocities, the mean pairwise streaming velocity, and the velocity correlation function) and analyze the relative merits of these statistics in constraining dark energy parameters. Of the three statistics, mean pairwise streaming velocity provides constraints that are least sensitive to velocity errors: the constraints on parameters degrade only by a factor of 2 when the random error is increased from 100 to 500 km/s. We also compare cluster velocities with other dark energy probes proposed in the Dark Energy Task Force report. For cluster velocity measurements with realistic priors, the eventual constraints on the dark energy density, the dark energy equation of state and its evolution are comparable to constraints from supernovae measurements, and better than cluster counts and baryon acoustic oscillations; adding velocity to other dark energy probes improves constraints on the figure of merit by more than a factor of 2. For upcoming Sunyaev-Zeldovich galaxy cluster surveys, even velocity measurements with errors as large as 1000 km/s will substantially improve the cosmological constraints compared to using the cluster number density alone.

Bhattacharya, Suman; Kosowsky, Arthur [Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, Pennsylvania 15260 (United States)

2008-04-15

151

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

152

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

153

Reconstruction of dark energy and expansion dynamics using Gaussian processes

NASA Astrophysics Data System (ADS)

An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space, as the errors found depend strongly on the parametrisation considered. We present a new non-parametric approach to reconstructing the history of the expansion rate and dark energy using Gaussian Processes, which is a fully Bayesian approach for smoothing data. We present a pedagogical introduction to Gaussian Processes, and discuss how it can be used to robustly differentiate data in a suitable way. Using this method we show that the Dark Energy Survey - Supernova Survey (DES) can accurately recover a slowly evolving equation of state to ?w = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimum error of ±0.025 at the sweet-spot at z ~ 0.16, provided the other parameters of the model are known. Errors on the expansion history are an order of magnitude smaller, yet make no assumptions about dark energy whatsoever. A code for calculating functions and their first three derivatives using Gaussian processes has been developed and is available for download.

Seikel, Marina; Clarkson, Chris; Smith, Mathew

2012-06-01

154

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

155

Interacting Ricci-like holographic dark energy

NASA Astrophysics Data System (ADS)

In a flat Friedmann-Lemaitre-Robertson-Walker background, a scheme of dark matter-dark energy interaction is studied considering a holographic Ricci-like model for the dark energy. Without giving a priori some specific model for the interaction function, we show that this function can experience a change of sign during the cosmic evolution. The parameters involved in the holographic model are adjusted with Supernova data and we obtained results compatible with the observable universe.

Arévalo, F.; Cifuentes, P.; Lepe, Samuel; Peña, Francisco

2014-05-01

156

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

157

Topology and Dark Energy: Testing Gravity in Voids

NASA Astrophysics Data System (ADS)

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.

Spolyar, Douglas; Sahlén, Martin; Silk, Joe

2013-12-01

158

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

159

Constraining the coupling constant between dark energy and dark matter

NASA Astrophysics Data System (ADS)

We have investigated constraints on the coupling between dark matter and the interacting Chaplygin gas. Our results indicate that the coupling constant c between these two entities can take arbitrary values, which can be either positive or negative, thus giving arbitrary freedom to the inter-conversion between Chaplygin gas and dark matter. Thus, our results indicate that the restriction 0< c<1 on the coupling constant occurs as a very special case. Our analysis also supports the existence of phantom energy under certain conditions on the coupling constant.

Jamil, Mubasher; Rashid, Muneer Ahmad

2009-03-01

160

A numerical approach is considered for spherically symmetric spacetimes that generalize Lemaitre-Tolman-Bondi dust solutions to nonzero pressure ('LTB spacetimes'). We introduce quasilocal (QL) variables that are covariant LTB objects satisfying evolution equations of Friedman-Lemaitre-Robertson-Walker (FLRW) cosmologies. We prove rigorously that relative deviations of the local covariant scalars from the QL scalars are nonlinear, gauge invariant and covariant perturbations on a FLRW formal background given by the QL scalars. The dynamics of LTB spacetimes is completely determined by the QL scalars and these exact perturbations. Since LTB spacetimes are compatible with a wide variety of ''equations of state,'' either single fluids or mixtures, a large number of known solutions with dark matter and dark energy sources in a FLRW framework (or with linear perturbations) can be readily examined under idealized but nontrivial inhomogeneous conditions. Coordinate choices and initial conditions are derived for a numerical treatment of the perturbation equations, allowing us to study nonlinear effects in a variety of phenomena, such as gravitational collapse, nonlocal effects, void formation, dark matter and dark energy couplings, and particle creation. In particular, the embedding of inhomogeneous regions can be performed by a smooth matching with a suitable FLRW solution, thus generalizing the Newtonian 'top hat' models that are widely used in astrophysical literature. As examples of the application of the formalism, we examine numerically the formation of a black hole in an expanding Chaplygin gas FLRW universe, as well as the evolution of density clumps and voids in an interactive mixture of cold dark matter and dark energy.

Sussman, Roberto A. [Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico (ICN-UNAM), A. P. 70--543, 04510 Mexico D. F. (Mexico)

2009-01-15

161

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

162

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

163

Probing Dark Energy with Constellation-X.

National Technical Information Service (NTIS)

Constellation-X (Con-X) will carry out two powerful and independent sets of tests of dark energy based on X-ray observations of galaxy clusters, providing comparable accuracy to other leading dark energy probes. The first group of tests will measure the a...

D. A. Rapetti S. W. Allen

2006-01-01

164

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

165

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

166

Examining the Evidence for Dynamical Dark Energy

NASA Astrophysics Data System (ADS)

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.

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

2012-10-01

167

Cosmological effects of a class of fluid dark energy models

We study the impact of a generalized Chaplygin gas as a candidate for dark energy on density perturbations and on cosmic microwave background (CMB) anisotropies. The generalized Chaplygin gas is a fluid component with an exotic equation of state p=-A\\/rhoalpha (a polytropic gas with negative constant and exponent). Such a component interpolates in time between dust and a cosmological constant,

Daniela Carturan; Fabio Finelli

2003-01-01

168

Kantowski-Sachs cosmological models with anisotropic dark energy

NASA Astrophysics Data System (ADS)

The exact solutions of the Einstein field equations for dark energy in Kantowski-Sachs metric under the assumption on the anisotropy of the fluid are obtained for exponential and power-law volumetric expansions. The isotropy of the fluid, space and expansion are examined.

Adhav, Kishor S.; Bansod, Abhijit S.; Wankhade, Rajesh P.; Ajmire, Hanumant G.

2011-08-01

169

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

170

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

171

Dark Energy and the Cosmological Constant: A Brief Introduction

ERIC Educational Resources Information Center

The recently observed acceleration of the expansion of the universe is a topic of intense interest. The favoured causes are the "cosmological constant" or "dark energy". The former, which appears in the Einstein equations as the term [lambda]g[subscript [mu]v], provides an extremely simple, well-defined mechanism for the acceleration. However,…

Harvey, Alex

2009-01-01

172

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

173

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

174

DBI models for the unification of dark matter and dark energy

We propose a model based on a DBI action for the unification of dark matter and dark energy. This is supported by the results of the study of its background behavior at early and late times, and reinforced by the analysis of the evolution of perturbations. We also perform a Bayesian analysis to set observational constraints on the parameters of the model using type Ia SN, CMB shift and BAO data. Finally, to complete the study we investigate its kinematics aspects, such as the effective equation of state parameter, acceleration parameter and transition redshift. Particularizing them for the best fit one appreciates that an effective phantom behavior is preferred.

Chimento, Luis P. [Departamento de Fisica, Universidad de Buenos Aires, 1428 Buenos Aires (Argentina); Lazkoz, Ruth; Sendra, Irene [Fisika Teorikoa, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea, 644 Posta Kutxatila, 48080 Bilbao (Spain)

2010-06-23

175

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

176

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

177

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

178

CMB lensing constraints on neutrinos and dark energy

NASA Astrophysics Data System (ADS)

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.

de Putter, Roland; Zahn, Oliver; Linder, Eric V.

2009-03-01

179

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

180

Constraining dark energy through the stability of cosmic structures

NASA Astrophysics Data System (ADS)

For a general dark-energy equation of state, we estimate the maximum possible radius of massive structures that are not destabilized by the acceleration of the cosmological expansion. A comparison with known stable structures constrains the equation of state. The robustness of the constraint can be enhanced through the accumulation of additional astrophysical data and a better understanding of the dynamics of bound cosmic structures.

Pavlidou, V.; Tetradis, N.; Tomaras, T. N.

2014-05-01

181

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

182

Precision Studies of Dark Energy with LSST

Starting around 2013, data from the Large Synoptic Survey Telescope (LSST) will be analyzed for a wide range of phenomena. By separately tracing the development of mass structure and rate of expansion of the universe, these data will address the physics of dark matter and dark energy, the possible existence of modified gravity on large scales, large extra dimensions, the neutrino mass, and possible self interaction of dark matter particles.

Tyson, J. Anthony [Department of Physics, University of California, Davis, CA 95616 (United States)

2006-11-17

183

Constraints on the anisotropy of dark energy

If the equation of state of dark energy is anisotropic there will be additional quadrupole anisotropy in the cosmic microwave background induced by the time-dependent anisotropic stress quantified in terms of {Delta}w. Assuming that the entire amplitude of the observed quadrupole is due to this anisotropy, we conservatively impose a limit of |{Delta}w|<2.1x10{sup -4} for any value of w{>=}-1 assuming that {Omega}{sub m}<0.5. This is considerably tighter than that which comes from supernovae. Stronger limits, up to a factor of 10, are possible for specific values of {Omega}{sub m} and w. Since we assume this component is uncorrelated with the stochastic component from inflation, we find that both the expectation value and the sample variance are increased. There is no improvement in the likelihood of an anomalously low quadrupole as suggested by previous work on an elliptical universe.

Appleby, Stephen; Battye, Richard [Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL (United Kingdom); Moss, Adam [Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 (Canada)

2010-04-15

184

Computing model independent perturbations in dark energy and modified gravity

NASA Astrophysics Data System (ADS)

We present a methodology for computing model independent perturbations in dark energy and modified gravity. This is done from the Lagrangian for perturbations, by showing how field content, symmetries, and physical principles are often sufficient ingredients for closing the set of perturbed fluid equations. The fluid equations close once ``equations of state for perturbations'' are identified: these are linear combinations of fluid and metric perturbations which construct gauge invariant entropy and anisotropic stress perturbations for broad classes of theories. Our main results are the proof of the equation of state for perturbations presented in a previous paper, and the development of the required calculational tools.

Battye, Richard A.; Pearson, Jonathan A.

2014-03-01

185

Unknowns and unknown unknowns: from dark sky to dark matter and dark energy

NASA Astrophysics Data System (ADS)

Answering well-known fundamental questions is usually regarded as the major goal of science. Discovery of other unknown and fundamental questions is, however, even more important. Recognition that "we didn't know anything" is the basic scientific driver for the next generation. Cosmology indeed enjoys such an exciting epoch. What is the composition of our universe ? This is one of the well-known fundamental questions that philosophers, astronomers and physicists have tried to answer for centuries. Around the end of the last century, cosmologists finally recognized that "We didn't know anything". Except for atoms that comprise slightly less than 5% of the universe, our universe is apparently dominated by unknown components; 23% is the known unknown (dark matter), and 72% is the unknown unknown (dark energy). In the course of answering a known fundamental question, we have discovered an unknown, even more fundamental, question: "What is dark matter? What is dark energy?" There are a variety of realistic particle physics models for dark matter, and its experimental detection may be within reach. On the other hand, it is fair to say that there is no widely accepted theoretical framework to describe the nature of dark energy. This is exactly why astronomical observations will play a key role in unveiling its nature. I will review our current understanding of the "dark sky", and then present on-going Japanese project, SuMIRe, to discover even more unexpected questions.

Suto, Yasushi

2010-07-01

186

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

187

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

188

Comparing holographic dark energy models with statefinder

NASA Astrophysics Data System (ADS)

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 CDM model in the and evolutions. In particular, the HDE model is highly degenerate with the 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 and diagnostics by using some quantities with higher order derivatives of the scale factor. It is shown that the statefinder diagnostic 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 CDM model can also be broken by this method. Especially for the HDE model, all the previous strong degeneracies appearing in the and 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 - plane is also made, in which the separations between the models (including the CDM model) can be directly measured in the light of the current values of the models.

Cui, Jing-Lei; Zhang, Jing-Fei

2014-04-01

189

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

190

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

191

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

192

Modified holographic dark energy in DGP brane world

NASA Astrophysics Data System (ADS)

In this Letter, the cosmological dynamics of a modified holographic dark energy which is derived from the UV/IR duality by considering the black hole mass in higher dimensions as UV cutoff, is investigated in Dvali-Gabadadze-Porrati (DGP) brane world model. We choose Hubble horizon and future event horizon as IR cutoff respectively. And the two branches of the DGP model are both taken into account. When Hubble horizon is considered as IR cutoff, the modified holographic dark energy (HDE) behaves like an effect dark energy that modification of gravity in pure DGP brane world model acts and it can drive the expansion of the universe speed up at late time in ?=-1 branch which in pure DGP model cannot undergo an accelerating phase. When future event horizon acts as IR cutoff, the equation of state parameter of the modified HDE can cross the phantom divide.

Liu, Dao-Jun; Wang, Hua; Yang, Bin

2010-10-01

193

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

194

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

195

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

196

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

197

Density Perturbations in Decaying Holographic Dark Energy Models

NASA Astrophysics Data System (ADS)

We study cosmological perturbations in the context of an interacting dark energy model, where the holographic dark energy with IR cutoff decays into the cold dark matter (CDM). For this purpose, we introduce three IR cutoffs of Hubble horizon, particle horizon, and future event horizon. Here we present small perturbations under the case that effective equation of state (EOS: ?eff) for the holographic energy density is determined to be the same negative constant as that for the CDM. Such new matter productions tend to dilute the density perturbations of CDM (matter contrast). For a decelerating universe of ?eff > -1/3, the matter contrast is growing as the universe evolves, while for an accelerating universe of ?eff < -1/3, the matter contrast is decaying, irrespective of the choice of IR cutoff. This shows clearly that the acceleration suppresses the growing of the density perturbations at the early universe.

Kim, Kyoung Yee; Lee, Hyung Won; Myung, Yun Soo

198

Estimating the uncorrelated dark energy evolution in the Planck era

NASA Astrophysics Data System (ADS)

The equation of state (EOS), w(z), is the most important parameter of dark energy. We reconstruct the evolution of this EOS in a model-independent way using the latest cosmic microwave background (CMB) data from Planck and other observations, such as type Ia supernovae, the baryonic acoustic oscillation measurements (SDSS, 6dF, BOSS, and WiggleZ), and the Hubble parameter value H(z). The results show that the EOS is consistent with the cosmological constant at the 2? confidence level, not preferring a dynamical dark energy. The uncorrelated EOS of dark energy constraints from Planck CMB data are much tighter than those from the WMAP 9-year CMB data.

Wang, F. Y.; Dai, Z. G.

2014-01-01

199

Probing dark energy through scale dependence

NASA Astrophysics Data System (ADS)

We consider the consequences of having no prior knowledge of the true dark energy model for the interpretation of cosmological observations. The magnitude of redshift-space distortions and weak-lensing shear is determined by the metric on the geodesics of which galaxies and light propagate. We show that, given precise enough observations, we can use these data to completely reconstruct the metric on our past light cone and therefore to measure the scale and time dependence of the anisotropic stress and the evolution of the gravitational potentials in a model-independent manner. Since both dark matter and dark energy affect the visible sector only through the gravitational field they produce, they are inseparable without a model for dark energy: galaxy bias cannot be measured and therefore the distribution of dark matter determined; the peculiar velocity of dark matter can be identified with that of the galaxies only when the equivalence principle holds. Given these limitations, we show how one can nonetheless build tests for classes of dark energy models which depend on making measurements at multiple scales at a particular redshift. They are null tests on the model-independent observables, do not require modeling evolution in time, and do not require any parametrization of the free functions of these models—such as the sound speed. We show that one in principle could rule out or constrain the whole class of the most general scalar-tensor theories even without assuming the quasistatic limit.

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

2013-12-01

200

Holographic dark energy in the DGP model

NASA Astrophysics Data System (ADS)

The braneworld model proposed by Dvali, Gabadadze, and Porrati leads to an accelerated universe without cosmological constant or any other form of dark energy. Nevertheless, we have investigated the consequences of this model when an holographic dark energy is included, taking the Hubble scale as IR cutoff. We have found that the holographic dark energy leads to an accelerated flat universe (de Sitter-like expansion) for the two branches: ?=±1, 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.

Cruz, Norman; Lepe, Samuel; Peña, Francisco; Avelino, Arturo

2012-09-01

201

Dark Energy and the Accelerating Universe

NASA Astrophysics Data System (ADS)

Ten years ago, the discovery that the expansion of the universe is accelerating put in place the last major building block of the present cosmological model, in which the universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy—for example, the quantum energy of the vacuum—or it may signal that general relativity (GR) breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.

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

2008-09-01

202

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

203

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

204

Testing and selecting dark energy models with lens redshift data

NASA Astrophysics Data System (ADS)

In this paper, we compare seven popular dark energy models under the assumption of a flat universe by using the latest observational data of gravitationally-lensed image separations observed in the Cosmic Lens All-Sky Survey (CLASS), the PMN-NVSS Extragalactic Lens Survey (PANELS), the Sloan Digital Sky Survey (SDSS) and other surveys, which are (nearly) complete for the image separation range 0''.3????7''. We combine the 29 lens redshift data with the cosmic microwave background (CMB) observation from the Wilkinson Microwave Anisotropy Probe (WMAP7) results, the baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) Data Release. The model comparison statistic, the Bayesian information criterion is also applied to assess the worth of the models. This statistic favors models that give a good fit with fewer parameters. Based on this analysis, we find that the simplest cosmological constant model that has only one free parameter is still preferred by the current data. For the other dynamical dark energy models, we find that some of them, such as the Ricci dark energy model, the Affine equation-of-state dark energy, and the generalized Chaplygin gas, can provide good fits to the current data. The Dvali-Gabadadze-Porrati model is the only one-parameter model that can give a rather good fit but also nest ? while the three-parameter model, namely, the interactive dark energy, is clearly disfavored by the data, as it is unable to provide a good fit.

Cao, Shuo; Zhu, Zong-Hong; Zhao, Ren

2011-07-01

205

Impact of dark energy perturbations on the growth index

NASA Astrophysics Data System (ADS)

We show that in clustering dark energy models the growth index of linear matter perturbations, ?, can be much lower than in ?CDM or smooth quintessence models and presents a strong variation with redshift. We find that the impact of dark energy perturbations on ? is enhanced if the dark energy equation of state has a large and rapid decay at low redshift. We study four different models with these features and show that we may have 0.33dark energy and modified gravity models, finding that some f(R) and clustering dark energy models can present similar values for ?.

Batista, Ronaldo C.

2014-06-01

206

Dark Energy Models in Plane Symmetric Space-Time with Time Varying ? Term

NASA Astrophysics Data System (ADS)

This paper deals with the Einstein's field equations for the space-time described by an inhomogeneous plane symmetric metric in presence of dark energy with time varying cosmological term ?. The dark energy is given by either the quintessence or Chaplygin gas. It is shown that the models are isotropic and the deceleration parameter of the models is constant.

Shen, M.

2013-01-01

207

Constraining Dark Matter and Dark Energy Models using Astrophysical Surveys

NASA Astrophysics Data System (ADS)

This thesis addresses astrophysical probes to constrain dark matter (DM) and dark energy models. Primordial black holes (PBHs) remain one of the few DM candidates within the Standard Model of Particle Physics. This thesis presents a new probe of this PBH DM, using the microlensing of the source stars monitored by the already existing Kepler satellite. With its photometric precision and the large projected cross section of the nearby stars, it is found that previous constraints on PBH DM could theoretically be extended by two orders of magnitude. Correcting a well-known microlensing formula, a limb-darkening analysis is included, and a new approximation is calculated for future star selection. A preliminary prediction is calculated for the planned Wide-Field Infrared Survey Telescope. A preliminary study of the first two years of publicly available Kepler data is presented. The investigation yields many new sources of background error not predicted in the theoretical calculations, such as stellar flares and comets in the field of view. Since no PBH candidates are detected, an efficiency of detection is therefore calculated by running a Monte Carlo with fake limb-darkened finite-source microlensing events. It is found that with just the first 8 quarters of data, a full order of magnitude of the PBH mass range can be already constrained. Finally, one of the astrophysical probes of dark energy is also addressed - specifically, the baryon acoustic oscillations (BAO) measurement in the gas distribution, as detected in quasar absorption lines. This unique measurement of dark energy at intermediate redshifts is being measured by current telescope surveys. The last part of this thesis therefore focuses on understanding the systematic effects in such a detection. Since the bias between the underlying dark matter distribution and the measured gas flux distribution is based on gas physics, hydrodynamic simulations are used to understand the evolution of neutral hydrogen over time. It is found the location of the peak is a dependable measurement, but more robust simulations will have to be run for a full understanding of the shape and size of the peak.

Cieplak, Agnieszka M.

208

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

209

Dark MaGICC: the effect of dark energy on disc galaxy formation. Cosmology does matter

NASA Astrophysics Data System (ADS)

We present the Dark MaGICC (Making Galaxies in a Cosmological Context) project, which aims to investigate the effect of dark energy (DE) modelling on disc galaxy formation via hydrodynamical cosmological simulations. Dark MaGICC includes four dynamical DE scenarios with time varying equations of state, one with a self-interacting Ratra-Peebles model. In each scenario, we simulate three disc galaxies with high resolution using smoothed particle hydrodynamics. The baryonic physics model is the same used in the MaGICC project, and we varied only the background cosmology. We find that the DE parametrization has a surprisingly important impact on galaxy evolution and on structural properties of galaxies at z = 0, in striking contrast with predictions from pure N-body simulations. The different background evolutions can (depending on the behaviour of the DE equation of state) either enhance or quench star formation with respect to a ? cold dark matter model, at a level similar to the variation of the stellar feedback parametrization, with strong effects on the final galaxy rotation curves. While overall stellar feedback is still the driving force in shaping galaxies, we show that the effect of the DE parametrization plays a larger role than previously thought, especially at lower redshifts. For this reason, the influence of DE parametrization on galaxy formation must be taken into account, especially in the era of precision cosmology.

Penzo, C.; Macciò, A. V.; Casarini, L.; Stinson, G. S.; Wadsley, J.

2014-07-01

210

Dark energy as double N-flation - observational predictions

NASA Astrophysics Data System (ADS)

We propose a simple model for dark energy useful for comparison with observations. It is based on the idea that dark energy and inflation should be caused by the same physical process. As motivation, we note that Linde's simple chaotic inflation ? produces values of ns= 0.967 and r= 0.13, which are consistent with the Wilkinson Microwave Anisotropy Probe (WMAP) 1? error bars. We therefore propose ? with m1˜ 10-5 and m2? 10-60, where c= 1 =? and the reduced Planck mass is set to unity. The field ?1 drives inflation and has damped by now (?1, 0= 0), while ?2 is currently rolling down its potential to produce dark energy. Using this model, we derive the formula ?w(z) ?w(z) + 1 =?w0(H0/H(z))2 via the slow-roll approximation. Our numerical results from exact and self-consistent solution of the equations of motion for ?2 and the Friedmann equations support this formula, and it should hold for any slow-roll dark energy. Our potential can be easily realized in N-flation models with many fields, and is easily falsifiable by upcoming experiments - for example, if Linde's chaotic inflation is ruled out. But if r values consistent with Linde's chaotic inflation are detected then one should take this model seriously indeed.

Gott, J. Richard; Slepian, Zachary

2011-09-01

211

Cosmological model of the interaction between dark matter and dark energy

NASA Astrophysics Data System (ADS)

We tested a cosmological model for the interaction between dark matter and dark energy with a dynamic equation of state wDE(z) = w0 + w1z/ (1 + z), using type Ia supernovae (SNe Ia), Hubble parameter data, baryonic acoustic oscillation (BAO) measurements, and cosmic microwave background (CMB) observations. This interacting cosmological model has not been studied before. The best-fit parameters with 1? uncertainties are ? = - 0.022 ± 0.006, ?0DM = 0.213 ± 0.008, w0 = - 1.210 ± 0.033 and w1 = 0.872 ± 0.072 with ?2min/d.o.f. = 0.990. At the 1? confidence level, we find ? < 0, which means that the energy transfer prefer ably occurring from dark matter to dark energy. We also find that the SNe Ia data disagree with the combined CMB, BAO, and Hubble parameter data. The evolution of ?DM/?DE indicates that this interacting model is a good approach to solve the coincidence problem, because ?DE decreases with scale factor a. The transition redshift is ztr = 0.63 ± 0.07 in this model.

Wang, J. S.; Wang, F. Y.

2014-04-01

212

Effective dark energy models and dark energy models with bounce in frames of F( T) gravity

NASA Astrophysics Data System (ADS)

Various cosmological models in frames of F( T) gravity are considered. The general scheme of constructing effective dark energy models with various evolution is presented. It is showed that these models in principle are compatible with ?CDM model. The dynamics of universe governed by F( T) gravity can mimics ?CDM evolution in past but declines from it in a future. We also construct some dark energy models with the "real" (non-effective) equation-of-state parameter w such that w?-1. It is showed that in F( T) gravity the Universe filled phantom field not necessarily ends its existence in singularity. There are two possible mechanisms permitting the final singularity. Firstly due to the nonlinear dependence between energy density and H 2 ( H is the Hubble parameter) the universe can expands not so fast as in the general relativity and in fact Little Rip regime take place instead Big Rip. We also considered the models with possible bounce in future. In these models the universe expansion can mimics the dynamics with future singularity but due to bounce in future universe begin contracts.

Astashenok, Artyom V.

2014-05-01

213

Clustering, angular size, and dark energy

The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder distance which is characterized by a smoothness parameter {alpha}(z) and a power index {gamma}, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the {alpha} parameter is totally unconstrained by this sample of angular-diameter data.

Santos, R. C.; Lima, J. A. S. [Departamento de Astronomia, Universidade de Sao Paulo, 05508-900 Sao Paulo, SP (Brazil)

2008-04-15

214

Clustering, angular size, and dark energy

NASA Astrophysics Data System (ADS)

The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder distance which is characterized by a smoothness parameter ?(z) and a power index ?, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the ? parameter is totally unconstrained by this sample of angular-diameter data.

Santos, R. C.; Lima, J. A. S.

2008-04-01

215

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

216

Establishing the Robustness of Cosmological Tests of General Relativity to Dark Energy Perturbations

NASA Astrophysics Data System (ADS)

Cosmological tests to distinguish between dark energy and modifications to gravity are a promising route to obtain clues on the origin of cosmic acceleration. We studied the robustness of these tests to the presence of dark energy density, velocity, and anisotropic stress perturbations. I will present our results on the growth index parameter and the modified growth parameters that enter the perturbed Einstein equations. In all cases I show that models of dark energy with perturbations can be distinguished from modified gravity models. This distinction is possible either because dark energy perturbations have a less significant effect on the observable, or the effects occur at different scales than those due to modified gravity. Notably, dispersion in the value of the growth index parameter obtained for the dark energy models remains small making it the most robust method to distinguish between the two scenarios. In summary, we found that the currently proposed cosmological tests to distinguish between dark energy and modified gravity are robust to dark energy perturbations even for extreme cases. This is certainly the case even for dark energy models with equations of state of dark energy that fall well outside of current cosmological constraints.

Dossett, Jason; Ishak, M.

2014-01-01

217

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

218

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

219

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

220

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.

221

Weyl's Scale Invariance: Inflation, Dark Matter and Dark Energy Connections

NASA Astrophysics Data System (ADS)

We show that the standard model extended to include Weyl's symmetry as a local symmetry and an additional neutral scalar singlet can account for the puzzles pertaining to inflation, dark matter, and dark energy of the universe. The distinct signature of the Weylon is that it has either no or very feeble, if any, coupling to ordinary matter, a feature that can serve as the unambiguous identifying signal of the Weylon, once it is produced at the LHC and/or the ILC. We also point out that some other extensions of the standard model implementing scale invariance appearing in the recent literature after our work are nothing but disguised or mutilated versions of our model. We invite the physics community to consult our comments in hep-th/0702080, arXiv:0704.1836 [hep-ph], and arXiv:0712.2487 [hep-ph] for more details. To our knowledge, the special combination of a singlet scalar and the Higgs doublet multiplied by a scalar curvature has never been presented before our work hep-th/0403039. Our theory has rekindled interest in the ``discarded'' Weyl scale invariance. We await with eagerness the harvest this idea will reap at the ongoing and future high energy facilities.

Nishino, Hitoshi; Rajpoot, Subhash

2009-04-01

222

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

223

QCD Ghost Dark Energy in RS II Braneworld with Bulk-Brane Interaction

NASA Astrophysics Data System (ADS)

We investigate the QCD ghost model of dark energy in the framework of RS II braneworld. We assume there is an energy flow between the brane and bulk, and hence the continuity equation for the ghost dark energy is violated, while it is still preserved for the dark matter on the brane. We find that with the brane-bulk interaction, the equation of state parameter of ghost dark energy on the brane, can cross the phantom line w D =-1 at the present time, which confirms by some cosmological evidences. This result is in contrast to the standard cosmology where w D of ghost dark energy never cross the phantom line and the universe enters a de Sitter phase at the late time.

Yousefi, Leila; Sheykhi, Ahmad

2014-05-01

224

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

225

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

226

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

227

Testable dark energy predictions from current data

Given a class of dark energy models, constraints from one set of cosmic acceleration observables make predictions for other observables. Here we present the allowed ranges for the expansion rate H(z), distances D(z), and the linear growth function G(z) (as well as other, derived growth observables) from the current combination of cosmological measurements of supernovae, the cosmic microwave background, baryon acoustic oscillations, and the Hubble constant. With a cosmological constant as the dark energy and assuming near-minimal neutrino masses, the growth function is already predicted to better than 2% precision at any redshift, with or without spatial curvature. Direct measurements of growth that match this precision offer the opportunity to stringently test and potentially rule out a cosmological constant. While predictions in the broader class of quintessence models are weaker, it is remarkable that they are typically only a factor of 2-3 less precise than forecasted predictions for future space-based supernovae and Planck CMB measurements. In particular, measurements of growth at any redshift, or the Hubble constant H{sub 0}, that exceed {Lambda}CDM predictions by substantially more than 2% would rule out not only a cosmological constant but also the whole quintessence class, with or without curvature and early dark energy. Barring additional systematic errors hiding in the data, such a discovery would require more exotic explanations of cosmic acceleration such as phantom dark energy, dark energy clustering, or modifications of gravity.

Mortonson, Michael J. [Center for Cosmology and AstroParticle Physics, Ohio State University, Columbus, Ohio 43210 (United States); Department of Physics, University of Chicago, Chicago, Illinois 60637 (United States); Kavli Institute for Cosmological Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States); Hu, Wayne [Kavli Institute for Cosmological Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church St, Ann Arbor, Michigan 48109-1040 (United States)

2010-03-15

228

Dark Matter and Dark Energy: The Critical Questions.

National Technical Information Service (NTIS)

Stars account for only about 0.5% of the content of the Universe; the bulk of the Universe is optically dark. The dark side of the Universe is comprised of: at least 0.1% light neutrinos; 3:5% plus or minus 1% baryons; 29% plus or minus 4% cold dark matte...

M. S. Turner

2002-01-01

229

Dark energy physics expectations at DES

Giving rise to a new and exciting research field, observations of the last 13 years established the accelerated expansion of the Universe. This is a strong indication of new physics, either in the form of a new energy component of the Universe – dark energy – or of theories of gravity beyond general relativity. A powerful approach to this problem

Marcelle Soares-Santos

2012-01-01

230

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

231

Novel methods for probing dark energy

NASA Astrophysics Data System (ADS)

Our knowledge about the evolution of the universe has improved considerably in recent times. We learn about its earliest epochs from cosmic microwave background (CMB) observations while other experiments, such as supernovae (SNe) surveys, shed light on its recent behavior. Surprisingly, the combined analysis of the various probes available provides evidence against the simplest theoretical model. Furthermore, observations seem to favor the least expected alternative: a universe dominated by an unidentified component, the so-called dark energy , which would explain the observed late-time accelerated expansion. Our understanding of the nature of dark energy is severely limited by how little we know about its equation of state parameter w , defined as the ratio between its pressure and its energy density. The purpose of this thesis is to present advances in the study of w , in particular about its possible time-dependence. The first part discusses a new experimental tool to characterize w phenomenologically. The method relies on measurements of the Integrated Sachs- Wolfe (ISW) effect---a process that reflects the interaction between CMB photons and the gravitational potentials the photons traverse before reaching us. To do this, a new way of parametrizing w is employed, in order to separate the information contained in CMB observations from the information supplemented by ISW observations. Forecasted accuracy of this method is calculated for expected short-term (5 yr.) and long-term (10 yr.) data, and compared to the accuracy of SNe and CMB methods. In the second part a model-independent method to determine the time-variation of w is presented. The aim is to alleviate the biasing and model-dependence problems frequently encountered in the analysis of observations that assumea particular form of w a priori . The solution proposed is to let data guide the reconstruction of w by means of an optimization procedure that dictates the shape of a piecewise constant approximation of w . Since we are unable to decide, on theoretical grounds, how many segments to employ in our parametrization, model selection in terms of the Bayesian information crite rion is used. The results of such a reconstruction of w are presented for a readily available SNe dataset.

Attolini, Christian Stephan-Otto

232

HETDEX: Measuring Dark Energy at High Redshift

NASA Astrophysics Data System (ADS)

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is a blind spectroscopic survey to map the evolution of dark energy using Lyman-alpha emitting galaxies as tracers. The survey instrument, VIRUS, consists of 75 IFUs distributed across the 22-arcmin field of the upgraded 9.2-m HET. Each 50x50 sq. arcsec IFU is made up of 448 1.5-arcsec fibers, and feeds a pair of spectrographs with a fixed bandpass of 350-550 nm and resolving power R 700. Each exposure gathers 33,600 spectra. The baseline survey will deliver spectra of 0.8M LAEs in a 9 cubic Gpc volume with 1.9 < z < 3.5, and 1M [OII] emitters with z < 0.48. We expect to measure both the Hubble parameter and angular diameter distance to better than 1%. HETDEX will provide a unique window on the evolution of dark energy.

Gebhardt, Karl; Hill, G.; Komatsu, E.; Drory, N.; DePoy, D.; Ciardullo, R.; Gronwall, C.; Fabricius, M.; Wisotzki, L.; HETDEX Collaboration

2012-01-01

233

Generalized Chaplygin gas as a unified scenario of dark matter\\/energy: observational constraints

Although various cosmological observations congruously suggest that our\\u000auniverse is dominated by two dark components, the cold dark matter without\\u000apressure and the dark energy with negative pressure, the nature and origin of\\u000athese components is yet unknow. The generalized Chaplygin gas (gCg),\\u000aparametrized by an equation of state, $p = -A\\/\\\\rho_{\\\\rm gCg}^{\\\\alpha}$, was\\u000arecently proposed to be a candidate

Zong-Hong Zhu

2004-01-01

234

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

235

Might Dark Matter and Energy be Intrinsic Properties of Space?

NASA Astrophysics Data System (ADS)

It is shown that if a volume element V, of space is assumed to have intrinsic energy E, then basic principles of mechanics, thermodynamics and special relativity lead to the equation of state: E= pV, where p is the pressure. When this equation of state is incorporated in the Einstein equations it leads to the prediction that the orbital speed of matter circling a visible galaxy embedded in a spherical galactic halo should be relativistic, in disagreement with observations. However, it also leads directly to the interesting notion that the inertial mass of such a medium can be understood as a resistance to being compressed via Lorentz contraction. It is then shown that the mathematical structure of thermodynamics suggests another plausible definition of pressure if we allow for the possibility that the intrinsic energy of spacetime may not be described by the same work-energy relationship as ordinary matter. This additional possibility leads to the equation of state: E=- pV. While both of these equations of state describe forms of energy that are quite unlike ordinary energy, neither alone is able to account for observed rotational velocity curves of matter orbiting visible galaxies. Therefore, the possibility that space has two distinct components of energy is investigated. This results in a plausible, two-component equation of state in which the former equation of state is tentatively identified as the “dark matter” (DM) component, the latter as the “dark energy” (DE) component. The effective equation of state of space, accounting for the presence of both components, may then be written in the form: p= w ?, where ? is the total energy density, p the total pressure, and w represents the fractional excess of DM over DE (and therefore satisfies: -1? w?+1). Given the wide range of possible spacetime properties implied by this equation it appears to be a viable candidate for explaining observations presently attributed to the presence of both DM and DE. Specifically, the static, spherically symmetric solution of Einstein’s field equations, neglecting effects of ordinary matter, predicts the inverse r 2 distribution of intrinsic space energy required to explain observed constant rotational velocity curves for matter in circular orbits around visible galaxies embedded within spherically symmetric galactic halos. The proposed equation of state is also capable of describing regions of space undergoing accelerated expansion as regions where DE is dominant (i.e., w<-1/3).

Bruner, Ronald F.

2009-09-01

236

DESTINY, The Dark Energy Space Telescope

NASA Technical Reports Server (NTRS)

We have proposed the development of a low-cost space telescope, Destiny, as a concept for the NASA/DOE Joint Dark Energy Mission. Destiny is a 1.65m space telescope, featuring a near-infrared (0.85-1.7m) survey camera/spectrometer with a moderate flat-field field of view (FOV). Destiny will probe the properties of dark energy by obtaining a Hubble diagram based on Type Ia supernovae and a large-scale mass power spectrum derived from weak lensing distortions of field galaxies as a function of redshift.

Pasquale, Bert A.; Woodruff, Robert A.; Benford, Dominic J.; Lauer, Tod

2007-01-01

237

Dark Energy: A Crisis for Fundamental Physics

Astrophysical observations provide robust evidence that our current picture of fundamental physics is incomplete. The discovery in 1998 that the expansion of the Universe is accelerating (apparently due to gravitational repulsion between regions of empty space!) presents us with a profound challenge, at the interface between gravity and quantum mechanics. This "Dark Energy" problem is arguably the most pressing open question in modern fundamental physics. The first talk will describe why the Dark Energy problem constitutes a crisis, with wide-reaching ramifications. One consequence is that we should probe our understanding of gravity at all accessible scales, and the second talk will present experiments and observations that are exploring this issue.

Stubbs, Christopher (Harvard) [Harvard

2010-04-12

238

Dark energy properties from large future galaxy surveys

NASA Astrophysics Data System (ADS)

We perform a detailed forecast on how well a Euclid-like survey will be able to constrain dark energy and neutrino parameters from a combination of its cosmic shear power spectrum, galaxy power spectrum, and cluster mass function measurements. We find that the combination of these three probes vastly improves the survey's potential to measure the time evolution of dark energy. In terms of a dark energy figure-of-merit defined as (?(wp)?(wa))-1, we find a value of 690 for Euclid-like data combined with Planck-like measurements of the cosmic microwave background anisotropies in a 10-dimensional cosmological parameter space, assuming a ?CDM fiducial cosmology. For the more commonly used 7-parameter model, we find a figure-of-merit of 1900 for the same data combination. We consider also the survey's potential to measure dark energy perturbations in models wherein the dark energy is parameterised as a fluid with a nonstandard non-adiabatic sound speed, and find that in an optimistic scenario in which w0 deviates from -1 by as much as is currently observationally allowed, models with hat cs2 = 10-6 and hat cs2 = 1 can be distinguished from one another at more than 2? significance. We emphasise that constraints on the dark energy sound speed from cluster measurements are strongly dependent on the modelling of the cluster mass function; significantly weaker sensitivities ensue if we modify our model to include fewer features of nonlinear dark energy clustering. Finally, we find that the sum of neutrino masses can be measured with a 1? precision of 0.015 eV, even in complex cosmological models in which the dark energy equation of state varies with time. The 1? sensitivity to the effective number of relativistic species Neffml is approximately 0.03, meaning that the small deviation of 0.046 from 3 in the standard value of Neffml due to non-instantaneous decoupling and finite temperature effects can be probed with 1? precision for the first time.

Basse, Tobias; Eggers Bjælde, Ole; Hamann, Jan; Hannestad, Steen; Wong, Yvonne Y. Y.

2014-05-01

239

Evolution of dark energy perturbations in scalar-tensor cosmologies

We solve analytically and numerically the generalized Einstein equations in scalar-tensor cosmologies to obtain the evolution of dark energy and matter linear perturbations. We compare our results with the corresponding results for minimally coupled quintessence perturbations. We find that scalar-tensor dark energy density perturbations are amplified by a factor of about 10{sup 4} compared to minimally coupled quintessence perturbations on scales less than about 1000 h{sup -1} Mpc (sub-Hubble scales). On these scales dark energy perturbations constitute a fraction of about 10% compared to matter density perturbations. Scalar-tensor dark energy density perturbations are anticorrelated with matter linear perturbations on sub-Hubble scales. This anticorrelation of matter with negative pressure perturbations induces a mild amplification of matter perturbations by about 10% on sub-Hubble scales. The evolution of scalar field perturbations on sub-Hubble scales is scale independent and therefore corresponds to a vanishing effective speed of sound (c{sub s{Phi}=}0). We briefly discuss the observational implications of our results, which may include predictions for galaxy and cluster halo profiles that are modified compared to {Lambda}CDM. The observed properties of these profiles are known to be in some tension with the predictions of {Lambda}CDM.

Bueno Sanchez, J. C.; Perivolaropoulos, L. [Department of Physics, University of Ioannina, Ioannina (Greece)

2010-05-15

240

LRS Bianchi type I models with anisotropic dark energy and constant deceleration parameter

Locally rotationally symmetric Bianchi type I cosmological models are examined in the presence of dynamically anisotropic\\u000a dark energy and perfect fluid. We assume that the dark energy (DE) is minimally interacting, has dynamical energy density,\\u000a anisotropic equation of state parameter (EoS). The conservation of the energy-momentum tensor of the DE is assumed to consist\\u000a of two separately additive conserved parts.

Özgür Akarsu; Can Battal K?l?nç

2010-01-01

241

Unification of dark matter and dark energy: the inhomogeneous Chaplygin gas

We extend the world model of Kamenshchik et al. to large perturbations by formulating a Zeldovich-like approximation. We sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M-theory.

Neven Bili?

2002-01-01

242

NASA Astrophysics Data System (ADS)

The new class of higher-dimensional cosmological model of the early universe filled with dark energy in the form of wet dark fluid, within the framework of the f ( R, T) theory of gravity (Harko et al., Phys. Rev. D 84, 024020 (2011)) is considered. A new equation of state for the dark- energy component of the universe is used. It is modeled on the equation of state , which can be describing a liquid, for example, water. A cosmological model with an appropriate choice of the function f ( T) is constructed. The physical behavior of the model is studied. The well-known astrophysical phenomena, namely the Hubble parameter, lookback time, proper distance, luminosity distance, distance modulus, angular diameter, jerk parameter and cosmic snap with redshift and statefinder parameters are discussed.

Samanta, G. C.; Jaiswal, S.; Biswal, S. K.

2014-03-01

243

Phantom dark energy from nonlocal infrared modifications of general relativity

NASA Astrophysics Data System (ADS)

We discuss the cosmological consequences of a model based on a nonlocal infrared modification of Einstein equations. We find that the model generates a dynamical dark energy that can account for the presently observed value of ?DE, without introducing a cosmological constant. Tuning a free mass parameter m to a value m ?0.67H0, we reproduce the observed value ?DE?0.68. This leaves us with no free parameter, and we then get a pure prediction for the equation of state parameter of dark energy. Writing wDE(a)=w0+(1-a)wa, we find w0?-1.04 and wa?-0.02, consistent with the Planck data and on the phantom side. We also argue that nonlocal equations of the type that we propose must be understood as purely classical effective equations, such as those derived in semiclassical gravity for the in-in matrix elements of the metric. As such, any apparent ghost instability in such equations only affects the classical dynamics, but there is no propagating degree of freedom associated to the ghost and no issue of ghost-induced quantum vacuum decay.

Maggiore, Michele

2014-02-01

244

Neutrino dark energy-revisiting the stability issue

A coupling between a light scalar field and neutrinos has been widely discussed as a mechanism for linking (time varying) neutrino masses and the present energy density and equation of state of dark energy. However, it has been pointed out that the viability of this scenario in the non-relativistic neutrino regime is threatened by the strong growth of hydrodynamic perturbations associated with a negative adiabatic sound speed squared. In this paper we revisit the stability issue in the framework of linear perturbation theory in a model independent way. The criterion for the stability of a model is translated into a constraint on the scalar-neutrino coupling, which depends on the ratio of the energy densities in neutrinos and cold dark matter. We illustrate our results by providing meaningful examples for both stable and unstable models.

Bjaelde, Ole Eggers; Hannestad, Steen [Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C (Denmark); Brookfield, Anthony W; Van de Bruck, Carsten [Department of Applied Mathematics, Astro-Particle Theory and Cosmology Group, Hounsfield Road, Hicks Building, University of Sheffield, Sheffield S3 7RH (United Kingdom); Mota, David F [Institute for Theoretical Physics, University of Heidelberg, D-69120 Heidelberg (Germany); Schrempp, Lily [Deutsches Elektron-Synchroton DESY, Hamburg, Notkestrasse 85, 22607 Hamburg (Germany); Tocchini-Valentini, Domenico, E-mail: oeb@phys.au.dk, E-mail: php04awb@sheffield.ac.uk, E-mail: C.vandebruck@sheffield.ac.uk, E-mail: sth@phys.au.dk, E-mail: d.mota@thphys.uni-heidelberg.de, E-mail: lily.schrempp@desy.de, E-mail: dtv@skysrv.pha.jhu.edu [Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)

2008-01-15

245

Dark energy from mass varying neutrinos

NASA Astrophysics Data System (ADS)

We show that mass varying neutrinos (MaVaNs) can behave as a negative pressure fluid which could be the origin of the cosmic acceleration. We derive a model independent relation between the neutrino mass and the equation of state parameter of the neutrino dark energy, which is applicable for general theories of mass varying particles. The neutrino mass depends on the local neutrino density and the observed neutrino mass can exceed the cosmological bound on a constant neutrino mass. We discuss microscopic realizations of the MaVaN acceleration scenario, which involve a sterile neutrino. We consider naturalness constraints for mass varying particles, and find that both eV cut-offs and eV mass particles are needed to avoid fine-tuning. In microscopic realizations of this scenario with a sterile neutrino, these considerations give the sterile neutrino a maximum mass today of order an eV, which could be detectable at MiniBooNE. Because the sterile neutrino was much heavier at earlier times, constraints from big bang nucleosynthesis on additional states are not problematic. We consider regions of high neutrino density and find that the most likely place today to find neutrino masses which are significantly different from the neutrino masses in our solar system is in a supernova. The possibility of different neutrino mass in different regions of the galaxy and the local group could be significant for Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology of and the constraints on the 'acceleron', the scalar field which is responsible for the varying neutrino mass, and briefly discuss neutrino density dependent variations in other constants, such as the fine structure constant.

Fardon, Rob; Nelson, Ann E.; Weiner, Neal

2004-10-01

246

An introduction to the dark energy problem

NASA Astrophysics Data System (ADS)

In this work we review briefly the origin and history of the cosmological constant and its recent reincarnation in the form of the dark energy component of the universe. We also comment on the fundamental problems associated to its existence and magnitude which require an urgent solution for the sake of the internal consistency of theoretical physics.

Dobado, Antonio; Maroto, Antonio L.

2009-04-01

247

Dark Energy and the Hierarchy Problem.

National Technical Information Service (NTIS)

The well-known hierarchy between the Planck scale (approx. 10(sup 19)GeV) and the TeV scale, namely a ratio of (approx) 10(sup 16) between the two, is coincidentally repeated in a inverted order between the TeV scale and the dark energy scale at (approx) ...

P. Chien

2006-01-01

248

Cooling the Dark Energy Camera Instrument.

National Technical Information Service (NTIS)

DECam, camera for the Dark Energy Survey (DES), is undergoing general design and component testing. For an overview see DePoy, et al in these proceedings. For a description of the imager, see Cease, et al in these proceedings. The CCD instrument will be m...

A. Stefanik B. Onal H. Cease R. L. Schmitt S. Kuhlmann

2008-01-01

249

Dark Energy Survey Data Management System.

National Technical Information Service (NTIS)

The Dark Energy Survey (DES) collaboration will study cosmic acceleration with a 5000 deg2 griZY survey in the southern sky over 525 nights from 2011-2016. The DES data management (DESDM) system will be used to process and archive these data and the resul...

C. Beldica G. E. Daues J. Mohr J. A. Darnell M. Jarvis

2008-01-01

250

Stringy Model of Cosmological Dark Energy

NASA Astrophysics Data System (ADS)

A string field theory (SFT) nonlocal model of the cosmological dark energy providing w<-1 is briefly surveyed. We summarize recent developments and open problems, as well as point out some theoretical issues related with others applications of the SFT nonlocal models in cosmology, in particular, in inflation and cosmological singularity.

Aref'eva, Irina Ya.

2007-11-01

251

Stringy Model of Cosmological Dark Energy

A string field theory (SFT) nonlocal model of the cosmological dark energy providing w<-1 is briefly surveyed. We summarize recent developments and open problems, as well as point out some theoretical issues related with others applications of the SFT nonlocal models in cosmology, in particular, in inflation and cosmological singularity.

Aref'eva, Irina Ya. [Steklov Mathematical Institute of Russian Academy of Sciences, Gubkin st., 8, 119991, Moscow (Russian Federation)

2007-11-20

252

A geometric measure of dark energy with pairs of galaxies.

Observations indicate that the expansion of the Universe is accelerating, which is attributed to a ‘dark energy’ component that opposes gravity. There is a purely geometric test of the expansion of the Universe (the Alcock–Paczynski test), which would provide an independent way of investigating the abundance (?(X)) and equation of state (W(X)) of dark energy. It is based on an analysis of the geometrical distortions expected from comparing the real-space and redshift-space shape of distant cosmic structures, but it has proved difficult to implement. Here we report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows us to determine that the Universe is flat. By alternately fixing its spatial geometry at ?(k)?0 and the dark energy equation-of-state parameter at W(X)?-1, and using the results of baryon acoustic oscillations, we can establish at the 68.3% confidence level that and -0.85>W(X)>-1.12 and 0.60

Marinoni, Christian; Buzzi, Adeline

2010-11-25

253

Entanglement in holographic dark energy models

NASA Astrophysics Data System (ADS)

We study a process of equilibration of holographic dark energy (HDE) with the cosmic horizon around the dark-energy dominated epoch. This process is characterized by a huge amount of information conveyed across the horizon, filling thereby a large gap in entropy between the system on the brink of experiencing a sudden collapse to a black hole and the black hole itself. At the same time, even in the absence of interaction between dark matter and dark energy, such a process marks a strong jump in the entanglement entropy, measuring the quantum-mechanical correlations between the horizon and its interior. Although the effective quantum field theory (QFT) with a peculiar relationship between the UV and IR cutoffs, a framework underlying all HDE models, may formally account for such a huge shift in the number of distinct quantum states, we show that the scope of such a framework becomes tremendously restricted, devoid virtually any application in other cosmological epochs or particle-physics phenomena. The problem of negative entropies for the non-phantom stuff is also discussed.

Horvat, R.

2010-10-01

254

Higher signal harmonics, LISA's angular resolution, and dark energy

It is generally believed that the angular resolution of the Laser Interferometer Space Antenna (LISA) for binary supermassive black holes (SMBH) will not be good enough to identify the host galaxy or galaxy cluster. This conclusion, based on using only the dominant harmonic of the binary SMBH signal, changes substantially when higher signal harmonics are included in assessing the parameter estimation problem. We show that in a subset of the source parameter space the angular resolution increases by more than a factor of 10, thereby making it possible for LISA to identify the host galaxy/galaxy cluster. Thus, LISA's observation of certain binary SMBH coalescence events could constrain the dark energy equation of state to within a few percent, comparable to the level expected from other dark energy missions.

Arun, K. G. [LAL, Universite Paris-Sud, IN2P3/CNRS, Orsay (France); GReCO, Institut d'Astrophysique de Paris-C.N.R.S., Paris (France); Iyer, Bala R. [Raman Research Institute, Bangalore, 560 080 (India); Sathyaprakash, B. S.; Broeck, Chris van den [School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff, CF24 3YB (United Kingdom); Sinha, Siddhartha [Raman Research Institute, Bangalore, 560 080 (India); Department of Physics, Indian Institute of Science, Bangalore, 560 012 (India)

2007-11-15

255

Viscous dark energy and phantom field in an anisotropic universe

NASA Astrophysics Data System (ADS)

In this paper we have investigated the general form of viscous and non-viscous dark energy equation of state (EoS) parameter in the scope of anisotropic Bianchi type I space-time. We show that the presence of bulk viscosity causes transition of ? de from quintessence to phantom but the phantom state is an unstable state (as expected) and EoS of DE tends to -1 at late time. Then we show this phantomic description of the viscous dark energy and reconstruct the potential of the phantom scalar field. It is found that bulk viscosity pushes the universe to a darker region. We have also shown that at late time q˜- ? de .

Amirhashchi, Hassan; Pradhan, Anirudh

2014-05-01

256

CONSTRAINING PERTURBATIVE EARLY DARK ENERGY WITH CURRENT OBSERVATIONS

In this work, 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 constrain these models using current observations. We consider EDE models in which the dark energy equation of state is at least w{sub m} {approx_gt} -0.1 at early times, which could lead to an EDE density of up to {Omega}{sub DE}(z{sub CMB})= 0.03{Omega}{sub m}(z{sub CMB}). Our analysis shows that marginalizing over the non-DE parameters such as {Omega}{sub m}, H{sub 0}, andn{sub s} , current CMB observations alone can constrain the scale factor of transition from EDE to late-time dark energy to a{sub t} {approx_gt} 0.44 and width of transition to {Delta}{sub t} {approx_lt} 0.37. The equation of state at present is somewhat weakly constrained to w{sub 0} {approx_lt} -0.6, if we allow H{sub 0} < 60 km s{sup -1} Mpc{sup -1}. Taken together with other observations, such as SNe, Hubble Space Telescope, and Sloan Digital Sky Survey luminous red galaxies, w{sub 0} is constrained much more tightly to w{sub 0} {approx_lt} -0.9, while redshift of transition and width of transition are also tightly constrained to a{sub t} {approx_lt} 0.19 and{Delta}{sub t} {approx_lt} 0.21. The evolution of the equation of state for EDE models is thus tightly constrained to {Lambda}CDM-like behavior at low redshifts. Incorrectly assuming dark energy perturbations to be negligible leads to different constraints on the equation of state parameters-w{sub 0} {approx_lt} -0.8, a{sub t} {approx_lt} 0.33, and{Delta}{sub t} {approx_lt} 0.31, thus highlighting the necessity of self-consistently including dark energy perturbations in the analysis. If we allow the spatial curvature to be a free parameter, then the constraints are relaxed to w{sub 0} {approx_lt} -0.77, a{sub t} {approx_lt} 0.35, and{Delta}{sub t} {approx_lt} 0.35 with -0.014 < {Omega}{sub {kappa}} < 0.031 for CMB + other observations. For perturbed EDE models, the 2{sigma} lower limit on {sigma}{sub 8} ({sigma}{sub 8} {>=} 0.59) is much lower than that in {Lambda}CDM ({sigma}{sub 8} {>=} 0.72), thus raising the interesting possibility of discriminating EDE from {Lambda}CDM using future observations such as halo mass functions or the Sunyaev-Zeldovich power spectrum.

Alam, Ujjaini [ISR-1, ISR Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2010-05-10

257

Dark energy model selection with current and future data

NASA Astrophysics Data System (ADS)

The main goal of the next generation of weak-lensing probes is to constrain cosmological parameters by measuring the mass distribution and geometry of the low-redshift Universe and thus to test the concordance model of cosmology. A future all-sky tomographic cosmic shear survey with design properties similar to Euclid has the potential to provide the statistical accuracy required to distinguish between different dark energy models. In order to assess the model selection capability of such a probe, we consider the dark energy equation-of-state parameter w0. We forecast the Bayes factor of future observations, in the light of current information from Planck, by computing the predictive posterior odds distribution. We find that Euclid is unlikely to overturn current model selection results, and that the future data are likely to be compatible with a cosmological constant model. This result holds for a wide range of priors.

Debono, Ivan

2014-08-01

258

Distance measurements from supernovae and dark energy constraints

Constraints on dark energy from current observational data are sensitive to how distances are measured from Type Ia supernova (SN Ia) data. We find that flux averaging of SNe Ia can be used to test the presence of unknown systematic uncertainties, and yield more robust distance measurements from SNe Ia. We have applied this approach to the nearby+SDSS+ESSENCE+SNLS+HST set of 288 SNe Ia, and the 'Constitution' set of 397 SNe Ia. Combining the SN Ia data with cosmic microwave background anisotropy data from Wilkinson Microwave Anisotropy Probe 5 yr observations, the Sloan Digital Sky Survey baryon acoustic oscillation measurements, the data of 69 gamma-ray bursts (GRBs) , and the Hubble constant measurement from the Hubble Space Telescope project SHOES, we measure the dark energy density function X(z){identical_to}{rho}{sub X}(z)/{rho}{sub X}(0) as a free function of redshift (assumed to be a constant at z>1 or z>1.5). Without the flux averaging of SNe Ia, the combined data using the Constitution set of SNe Ia seem to indicate a deviation from a cosmological constant at {approx}95% confidence level at 0 < or apporx. z < or approx. 0.8; they are consistent with a cosmological constant at {approx}68% confidence level when SNe Ia are flux averaged. The combined data using the nearby+SDSS+ESSENCE+SNLS+HST data set of SNe Ia are consistent with a cosmological constant at 68% confidence level with or without flux averaging of SNe Ia, and give dark energy constraints that are significantly more stringent than that using the Constitution set of SNe Ia. Assuming a flat Universe, dark energy is detected at >98% confidence level for z{<=}0.75 using the combined data with 288 SNe Ia from nearby+SDSS+ESSENCE+SNLS+HST, independent of the assumptions about X(z{>=}1). We quantify dark energy constraints without assuming a flat Universe using the dark energy figure of merit for both X(z) and a dark energy equation-of-state linear in the cosmic scale factor.

Wang Yun [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W Brooks Street, Norman, Oklahoma 73019 (United States)

2009-12-15

259

Holographic Dark Energy (DE) Cosmological Models with Quintessence in Bianchi Type-V Space Time

NASA Astrophysics Data System (ADS)

In this paper, author studied homogeneous and anisotropic Bianchi type-V universe filled with matter and holographic dark energy (DE) components. The exact solutions to the corresponding Einstein’s field equations are obtained for exponential and power-law volumetric expansion. The holographic dark energy (DE) EoS parameter behaves like constant, i.e. ? ? =-1, which is mathematically equivalent to cosmological constant ( ?) for exponential expansion of the model, whereas the holographic dark energy (DE) EoS parameter behaves like quintessence for power-law expansion of the model. A correspondence between the holographic dark energy (DE) models with the quintessence dark energy (DE) is also established. Quintessence potential and dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe. The statefinder diagnostic pair { r, s} is adopted to characterize different phases of the universe.

Samanta, G. C.

2013-12-01

260

Reconstruction of Dark Energy Using Supernova and Other Datasets

We explore different methods of extracting information about dark energy from the presently available data. Supernova data is of primary importance to the study of dark energy, and over the last decade, many different reconstruction methods have been applied to this data. In this talk, we summarize some methods of model independent reconstruction of dark energy from supernova data. We

Ujjaini Alam; Varun Sahni; Alexei A. Starobinsky

2008-01-01

261

Cosmological consequences of a Chaplygin gas dark energy

A combination of recent observational results has given rise to what is currently known as the dark energy problem. Although several possible candidates have been extensively discussed in the literature, the nature of this dark energy component is not well understood at present. In this paper we investigate some cosmological implications of another dark energy candidate: an exotic fluid known

Abha Dev; J. S. Alcaniz; Deepak Jain

2003-01-01

262

Symmetron Dark Energy in Laboratory Experiments

NASA Astrophysics Data System (ADS)

The symmetron scalar field is a matter-coupled dark energy candidate which effectively decouples from matter in high-density regions through a symmetry restoration. We consider a previously unexplored regime, in which the vacuum mass ?˜2.4×10-3eV of the symmetron is near the dark energy scale, and the matter coupling parameter M˜1TeV is just beyond standard model energies. Such a field will give rise to a fifth force at submillimeter distances which can be probed by short-range gravity experiments. We show that a torsion pendulum experiment such as Eöt-Wash can exclude symmetrons in this regime for all self-couplings ??7.5.

Upadhye, Amol

2013-01-01

263

Symmetron dark energy in laboratory experiments.

The symmetron scalar field is a matter-coupled dark energy candidate which effectively decouples from matter in high-density regions through a symmetry restoration. We consider a previously unexplored regime, in which the vacuum mass ?~2.4×10(-3) eV of the symmetron is near the dark energy scale, and the matter coupling parameter M~1 TeV is just beyond standard model energies. Such a field will give rise to a fifth force at submillimeter distances which can be probed by short-range gravity experiments. We show that a torsion pendulum experiment such as Eöt-Wash can exclude symmetrons in this regime for all self-couplings ? is < or approximately equal to 7.5. PMID:23373910

Upadhye, Amol

2013-01-18

264

Inflationary and dark energy regimes in 2+1 dimensions

NASA Astrophysics Data System (ADS)

In this work we investigate the behavior of three-dimensional (3D) cosmological models. The simulation of inflationary and dark-energy-dominated eras are among the possible results in these 3D formulations; taking as starting point the results obtained by Cornish and Frankel. Motivated by those results, we investigate, first, the inflationary case where we consider a two-constituent cosmological fluid: the scalar field represents the hypothetical inflaton which is in gravitational interaction with a matter/radiation contribution. For the description of an old universe, it is possible to simulate its evolution starting with a matter dominated universe that faces a decelerated/accelerated transition due to the presence of the additional constituent (simulated by the scalar field or ruled by an exotic equation of state) that plays the role of dark energy. We obtain, through numerical analysis, the evolution in time of the scale factor, the acceleration, the energy densities, and the hydrostatic pressure of the constituents. The alternative scalar cosmology proposed by Cornish and Frankel is also under investigation in this work. In this case an inflationary model can be constructed when another non-polytropic equation of state (the van der Waals equation) is used to simulate the behavior of an early 3D universe.

Christmann, M. H.; Devecchi, F. P.; Kremer, G. M.; Zanetti, C. M.

2006-02-01

265

NASA Astrophysics Data System (ADS)

In this work the collapsing process of a spherically symmetric star, made of dust cloud, in the background of dark energy is studied for two different gravity theories separately, i.e., DGP Brane gravity and Loop Quantum gravity. Two types of dark energy fluids, namely, Modified Chaplygin gas and Generalised Cosmic Chaplygin gas are considered for each model. Graphs are drawn to characterize the nature and the probable outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different gravity theories. It is found that in case of dark matter, there is a great possibility of collapse and consequent formation of Black hole. In case of dark energy possibility of collapse is far lesser compared to the other cases, due to the large negative pressure of dark energy component. There is an increase in mass of the cloud in case of dark matter collapse due to matter accumulation. The mass decreases considerably in case of dark energy due to dark energy accretion on the cloud. In case of collapse with a combination of dark energy and dark matter, it is found that in the absence of interaction there is a far better possibility of formation of black hole in DGP brane model compared to Loop quantum cosmology model.

Rudra, Prabir; Biswas, Ritabrata; Debnath, Ujjal

2012-12-01

266

Current cosmological observations show a strong signature of the existence of a dark energy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state wX=p\\/rho=-1), which, however, raises several theoretical difficulties. This has led to models for a dark energy component that evolves with time. We discuss certain questions

T. Padmanabhan; T. Roy Choudhury

2003-01-01

267

Interacting dark energy in f(R) gravity

NASA Astrophysics Data System (ADS)

The field equations in f(R) gravity derived from the Palatini variational principle and formulated in the Einstein conformal frame yield a cosmological term which varies with time. Moreover, they break the conservation of the energy-momentum tensor for matter, generating the interaction between matter and dark energy. Unlike phenomenological models of interacting dark energy, f(R) gravity derives such an interaction from a covariant Lagrangian which is a function of a relativistically invariant quantity (the curvature scalar R). We derive the expressions for the quantities describing this interaction in terms of an arbitrary function f(R), and examine how the simplest phenomenological models of a variable cosmological constant are related to f(R) gravity. Particularly, we show that ?c2=H2(1-2q) for a flat, homogeneous and isotropic, pressureless universe. For the Lagrangian of form R-1/R, which is the simplest way of introducing current cosmic acceleration in f(R) gravity, the predicted matter-dark energy interaction rate changes significantly in time, and its current value is relatively weak (on the order of 1% of H0), in agreement with astronomical observations.

Pop?awski, Nikodem J.

2006-10-01

268

Galaxy clusters and structure formation in quintessence versus phantom dark energy universe

NASA Astrophysics Data System (ADS)

A self-gravitating gas in the Newtonian limit is studied in the presence of dark energy with a linear and constant equation of state. Entropy extremization associates to the isothermal Boltzmann distribution an effective density that includes "dark energy particles"—which either strengthen or weaken mutual gravitational attraction in the case of quintessence or phantom dark energy, respectively—that satisfy a linear equation of state. Stability is studied for microcanonical (fixed-energy) and canonical (fixed-temperature) ensembles. Compared to the previously studied cosmological constant case, in the present work it is found that quintessence increases the instability domain under gravitational collapse, while phantom dark energy decreases it. Thus, structures are more easily formed in a quintessence- rather than in a phantom-dominated universe. Assuming that galaxy clusters are spherical, nearly isothermal, and in hydrostatic equilibrium, we find that dark energy with a linear and constant equation of state (for fixed radius, mass, and temperature) steepens their total density profile. In the case of a cosmological constant, this effect accounts for a 1.5% increase in the density contrast, that is, the center to edge density ratio of the cluster. We also propose a method to constrain phantom dark energy.

Roupas, Zacharias; Axenides, Minos; Georgiou, George; Saridakis, Emmanuel N.

2014-04-01

269

Dark energy from approximate U(1 symmetry

NASA Astrophysics Data System (ADS)

The PLANCK observation strengthens the argument that the observed acceleration of the Universe is dominated by the invisible component of dark energy. We address how this extremely small DE density can be obtained in an ultraviolet complete theory. From two mass scales, the grand unification scale MG and the Higgs boson mass, we parametrize the scale of dark energy (DE). To naturally generate an extremely small DE term, we introduce an almost flat DE potential of a pseudo-Goldstone boson of an approximate global symmetry U(1 originating from some discrete symmetries allowed in an ultraviolet complete theory, as e.g. obtained in string theory constructions. For the DE potential to be extremely shallow, the pseudo-Goldstone boson is required not to couple to the QCD anomaly. This fixes uniquely the nonrenormalizable term generating the potential suppressed by MG7 in supergravity models.

Kim, Jihn E.; Nilles, Hans Peter

2014-03-01

270

Self-Interacting Holographic Dark Energy

NASA Astrophysics Data System (ADS)

We investigate a spatially flat Friedmann-Robertson-Walker (FRW) universe where dark matter exchanges energy with a self-interacting holographic dark energy (SIHDE). Using the ?2-statistical method on the Hubble function, we obtain a critical redshift that seems to be consistent with both BAO and CMB data. We calculate the theoretical distance modulus for confronting with the observational data of SNe Ia for small redshift z ? 0.1 and large redshift 0.1 ? z ? 1.5. The model gets accelerated faster than the ?CDM one and it can be a good candidate to alleviate the coincidence problem. We also examine the age crisis at high redshift associated with the old quasar APM 08279+5255.

Chimento, Luis P.; Forte, Mónica; Richarte, Martín G.

2013-01-01

271

NASA Astrophysics Data System (ADS)

Recent observations on Type-Ia supernovae and low density (? m =0.3) measurement of matter including dark matter suggest that the present-day universe consists mainly of repulsive-gravity type ‘exotic matter’ with negative-pressure often said ‘dark energy’ (? x =0.7). But the nature of dark energy is mysterious and its puzzling questions, such as why, how, where and when about the dark energy, are intriguing. In the present paper the authors attempt to answer these questions while making an effort to reveal the genesis of dark energy and suggest that ‘the cosmological nuclear binding energy liberated during primordial nucleo-synthesis remains trapped for a 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=1 for 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 . When dark energy is released free at Z=80, w=-2/3 . But as on present day at Z=0 when the radiation-strength-fraction ( ?), has diminished to ??0, the w=-1+?1/3=-1 . This, almost solves the dark-energy mystery of negative pressure and repulsive-gravity. The proposed theory makes several estimates/predictions which agree reasonably well with the astrophysical constraints and observations. Though there are many candidate-theories, the proposed model of this paper presents an entirely new approach (cosmological nuclear energy) as a possible candidate for dark energy.

Gupta, R. C.; Pradhan, Anirudh

2010-04-01

272

Dark energy simulacrum in nonlinear electrodynamics

Quasiconstant external fields in nonlinear electromagnetism generate a global contribution proportional to g{sup {mu}{nu}}in the energy-momentum tensor, thus a simulacrum of dark energy. To provide a thorough understanding of the origin and strength of its effects, we undertake a complete theoretical and numerical study of the energy-momentum tensor T{sup {mu}{nu}}for nonlinear electromagnetism. The Euler-Heisenberg nonlinearity due to quantum fluctuations of spinor and scalar matter fields is considered and contrasted with the properties of classical nonlinear Born-Infeld electromagnetism. We address modifications of charged particle kinematics by strong background fields.

Labun, Lance; Rafelski, Johann [Department of Physics, University of Arizona, Tucson, Arizona, 85721 (Unites States) and Ludwig-Maximillians-Universitaet Muenchen, 85748 Garching (Germany)

2010-03-15

273

Present and future evidence for evolving dark energy

We compute the Bayesian evidences for one- and two-parameter models of evolving dark energy, and compare them to the evidence for a cosmological constant, using current data from Type Ia supernova, baryon acoustic oscillations, and the cosmic microwave background. We use only distance information, ignoring dark energy perturbations. We find that, under various priors on the dark energy parameters, {lambda}CDM is currently favored as compared to the dark energy models. We consider the parameter constraints that arise under Bayesian model averaging, and discuss the implication of our results for future dark energy projects seeking to detect dark energy evolution. The model selection approach complements and extends the figure-of-merit approach of the Dark Energy Task Force in assessing future experiments, and suggests a significantly-modified interpretation of that statistic.

Liddle, Andrew R.; Mukherjee, Pia; Parkinson, David [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom); Wang Yun [Department of Physics and Astronomy, University of Oklahoma, Norman, Oklahoma 73019 (United States)

2006-12-15

274

Investigating clustering dark energy with 3D weak cosmic shear

NASA Astrophysics Data System (ADS)

As observational evidence increasingly consolidates the case for a cosmological constant ? being the source of the Universe's accelerated expansion, the question whether, and if so, how well, future experiments could detect deviations from this standard scenario is raised with urgency. Assuming a dark energy component different from a cosmological constant, the observable effects in general include gravitational clustering described by the fluid's (rest-frame) speed of sound cs. We employ 3D weak cosmic shear, a proposed method to take advantage of the full 3D information inherent to the cosmic shear field, to explore the capability of future surveys to detect dark energy clustering and the signature of an enhanced amplitude of the matter power spectrum on large scales. For this purpose, we present adequate numerical methods facilitating 3D weak cosmic shear calculations. We find that the possible constraints heavily depend on the dark energy equation of state w. If w is not very close to -1, constraining the squared sound speed c2s within an order of magnitude seems possible with a combination of Euclid and Planck data.

Ayaita, Youness; Schäfer, Björn Malte; Weber, Maik

2012-06-01

275

Probing dark energy dynamics from current and future cosmological observations

We report the constraints on the dark energy equation-of-state w(z) using the latest 'Constitution' SNe sample combined with the WMAP5 and Sloan Digital Sky Survey data. Assuming a flat Universe, and utilizing the localized principal component analysis and the model selection criteria, we find that the {Lambda}CDM model is generally consistent with the current data, yet there exists a weak hint of the possible dynamics of dark energy. In particular, a model predicting w(z)<-1 at z is an element of [0.25,0.5) and w(z)>-1 at z is an element of [0.5,0.75), which means that w(z) crosses -1 in the range of z is an element of [0.25,0.75), is mildly favored at 95% confidence level. Given the best fit model for current data as a fiducial model, we make future forecast from the joint data sets of Joint Dark Energy Mission, Planck, and Large Synoptic Survey Telescope, and we find that the future surveys can reduce the error bars on the w bins by roughly a factor of 10 for a 5-w-bin model.

Zhao Gongbo [Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Road, Portsmouth, PO1 3FX (United Kingdom); Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6 (Canada); Zhang Xinmin [Theoretical Physics Division, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918-4, Beijing 100049 (China); Theoretical Physics Center for Science Facilities (TPCSF), Chinese Academy of Sciences (China)

2010-02-15

276

General Astrophysics with TPF: Not Just Dark Energy

NASA Technical Reports Server (NTRS)

Besides searching for Earth-LIke Planets, TPF can study Jupiters, Neptunes, and all sorts of exotic planets. It can image debris-disks, YSO disks, AGN disks, maybe even AGB disks. And you are probably aware that a large optical space telescope like TPF-C or TPF-O can be a fantastic tool for studying the equation of state of the Dark Energy. I will review some of the future science of TPF-C, TPF-I and TPF-O, focusing on the applications of TPF to the study of objects in our Galaxy: especially circumstellar disks and planets other than exo-Earths.

Kuchner, Marc

2006-01-01

277

Viscous Dark Energy in f(T) Gravity

NASA Astrophysics Data System (ADS)

We study the bulk viscosity taking dust matter in the generalized teleparallel gravity. We consider different dark energy (DE) models in this scenario along with a time-dependent viscous model to construct the viscous equation of state (EoS) parameter for these DE models. We discuss the graphical representation of this parameter to investigate the viscosity effects on the accelerating expansion of the universe. It is mentioned here that the behavior of the universe depends upon the viscous coefficients showing the transition from decelerating to accelerating phase. It leads to the crossing of phantom divide line and becomes phantom dominated for specific ranges of these coefficients.

Sharif, M.; Rani, Shamaila

2013-09-01

278

Observational constraints on holographic tachyonic dark energy in interaction with dark matter

NASA Astrophysics Data System (ADS)

We discuss an interacting tachyonic dark energy model in the context of the holographic principle. The potential of the holographic tachyon field in interaction with dark matter is constructed. The model results are compared with CMB shift parameter, baryonic acoustic oscilations, lookback time and the Constitution supernovae sample. The coupling constant of the model is compatible with zero, but dark energy is not given by a cosmological constant.

Micheletti, Sandro M. R.

2010-05-01

279

Limits on dark radiation, early dark energy, and relativistic degrees of freedom

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number N{sub eff} over the standard value of 3.046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We find that while early dark energy does not alter the current constraints on N{sub eff}, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on N{sub eff}, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Y{sub p} and the scalar perturbation index n{sub s}. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy.

Calabrese, Erminia; Melchiorri, Alessandro [Physics Department and INFN, Universita' di Roma ''La Sapienza'', Ple Aldo Moro 2, 00185, Rome (Italy); Huterer, Dragan [Department of Physics, University of Michigan, 450 Church St, Ann Arbor, Michigan 48109 (United States); Linder, Eric V. [Berkeley Lab and University of California, Berkeley, California 94720, USA. (United States); Institute for the Early Universe WCU, Ewha Womans University, Seoul (Korea, Republic of); Pagano, Luca [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena California 91109 (United States)

2011-06-15

280

Holographic field theory models of dark energy in interaction with dark matter

NASA Astrophysics Data System (ADS)

We discuss two Lagrangian interacting dark energy models in the context of the holographic principle. The potentials of the interacting fields are constructed. The models are compared with CMB distance information, baryonic acoustic oscillations, lookback time and the Constitution supernovae sample. For both models, the results are consistent with a nonvanishing interaction in the dark sector of the Universe and the sign of coupling is consistent with dark energy decaying into dark matter, alleviating the coincidence problem—with more than 3 standard deviations of confidence for one of them. However, this is because the noninteracting holographic dark energy model is a bad fit to the combination of data sets used in this work as compared to the cosmological constant with cold dark matter model, so that one needs to introduce the interaction in order to improve this model.

Micheletti, Sandro M. R.

2012-06-01

281

This program is concerned with developing and verifying the validityof observational methods for constraining the properties of dark matter and dark energy in the universe. Excellent progress has been made in comparing observational projects involving weak gravitational lensing using both ground and space-based instruments, in further constraining the nature of dark matter via precise measures of its distribution in clusters of galaxies using strong gravitational lensing, in demonstrating the possible limitations of using distant supernovae in future dark energy missions, and in investigating the requirement for ground-based surveys of baryonic acoustic oscillations.

Ellis, Richard, S.

2008-02-01

282

We study a unification model for dark energy, dark matter, and inflation with a single scalar field with noncanonical kinetic term. In this model, the kinetic term of the Lagrangian accounts for the dark matter and dark energy, and at early epochs, a quadratic potential accounts for slow roll inflation. The present work is an extension to the work by Bose and Majumdar [Phys. Rev. D 79, 103517 (2009).] with a more general kinetic term that was proposed by Chimento in Phys. Rev. D 69, 123517 (2004). We demonstrate that the model is viable at the background and linear perturbation levels.

De-Santiago, Josue; Cervantes-Cota, Jorge L. [Departamento de Fisica, Instituto Nacional de Investigaciones Nucleares (Mexico); Berkeley Center for Cosmological Physics, LBNL and University of California, Berkeley, California 94720, USA and Departamento de Fisica, Instituto Nacional de Investigaciones Nucleares (Mexico)

2011-03-15

283

Triple unification of inflation, dark matter, and dark energy using a single field

We construct an explicit scenario whereby the same material driving inflation in the early universe can comprise dark matter in the present universe, using a simple quadratic potential. Following inflation and preheating, the density of inflaton/dark matter particles is reduced to the observed level by a period of thermal inflation, of a duration already invoked in the literature for other reasons. Within the context of the string landscape, one can further argue for a nonzero vacuum energy of this field, thus unifying inflation, dark matter, and dark energy into a single fundamental field.

Liddle, Andrew R. [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom); Pahud, Cedric [Astronomy Centre, University of Sussex, Brighton BN1 9QH (United Kingdom); Institut d'Astrophysique de Paris, UMR 7095-CNRS, Universite Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris (France); Urena-Lopez, L. Arturo [Instituto de Fisica de la Universidad de Guanajuato, C.P. 37150, Leon, Guanajuato (Mexico)

2008-06-15

284

In this paper, we investigate the integrated Sachs-Wolfe effect in the quintessence cold dark matter model with constant equation of state and constant speed of sound in dark energy rest frame, including dark energy perturbation and its anisotropic stress. Comparing with the {Lambda}CDM model, we find that the integrated Sachs-Wolfe (ISW)-power spectrums are affected by different background evolutions and dark energy perturbation. As we change the speed of sound from 1 to 0 in the quintessence cold dark matter model with given state parameters, it is found that the inclusion of dark energy anisotropic stress makes the variation of magnitude of the ISW source uncertain due to the anticorrelation between the speed of sound and the ratio of dark energy density perturbation contrast to dark matter density perturbation contrast in the ISW-source term. Thus, the magnitude of the ISW-source term is governed by the competition between the alterant multiple of (1+3/2xc-circumflex{sub s}{sup 2}) and that of {delta}{sub de}/{delta}{sub m} with the variation of c-circumflex{sub s}{sup 2}.

Wang, Y. T.; Xu, L. X.; Gui, Y. X. [School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian, Liaoning 116024 (China)

2010-10-15

285

Dark Energy: As A Fact of Matter

NASA Astrophysics Data System (ADS)

At the current time Dark Energy has been attributed to a property of space itself, ie. the vacuum energy of space. This explanation does not account for the inflationary expansion of the early universe nor does it explain the flatness of the universe. If however, Dark Energy is considered to be a property of matter, these issues are resolved. The Space Production Model of Gravity proposes that matter is converted to and emits space. The amount of space emitted per unit time is proportionate to mass by the formula: 4?(2Gm/c^2)^2(c) Where: G = gravitational constant m = mass c = speed of light Using this formula and the observed Hubble constant one can calculate the mass of the universe to be 5.441x10^53kg. One can also calculate the size of the universe at a given age. 1.36x10^20 cubic meters of new space is produced per planck time. Therefore one planck time after the Big Bang the radius of the universe was 3164 kilometers; after 1 second 88515 light years; and after 13.7 billion years 67 billion light years. The ``Flatness'' problem is also resolved. More massive gravitationally bound objects will produce more volume per unit time than less massive objects. This means that the rate of expansion of the universe is dependent upon the local mass density, the higher the density the greater the rate of expansion. Over time this produces a universe that is extremely homogenous. If the Space Production Model is true, the current Dark Energy survey should reveal that more massive gravitationally bound objects such as galaxy super-clusters recede slightly faster than much less massive lone galaxies at the same distance.

Bowen, Richard

2013-04-01

286

Dynamics of Dirac-Born-Infeld dark energy interacting with dark matter

NASA Astrophysics Data System (ADS)

We study the dynamics of Dirac-Born-Infeld (DBI) dark energy interacting with dark matter. The DBI dark energy model considered here has a scalar field with a nonstandard kinetic energy term, and has potential and brane tension that are power-law functions. The new feature considered here is an interaction between the DBI dark energy and dark matter through a phenomenological interaction between the DBI scalar field and the dark matter fluid. We analyze two different types of interactions between the DBI scalar field and the dark matter fluid. In particular we study the phase-space diagrams of and look for critical points of the phase space that are both stable and lead to accelerated, late-time expansion. In general we find that the interaction between the two dark components does not appear to give rise to late-time accelerated expansion. However, the interaction can make the critical points in the phase space of the system stable. Whether such stabilization occurs or not depends on the form of the interaction between the two dark components.

Kaeonikhom, Chakkrit; Singleton, Douglas; Sushkov, Sergey V.; Yongram, N.

2012-12-01

287

Binary mixture of anisotropic dark energy and perfect fluid in Kantowski-Sach universe

NASA Astrophysics Data System (ADS)

The Kantowski-Sachs cosmological models with binary mixture of perfect fluid and anisotropic dark energy have been studied. In order to obtain a unique solution, it is assumed that the energy conservation equation of the perfect fluid (PF) and dark energy (DE) vanishes separately together with a special law for the mean Hubble parameter which yields a constant value of the deceleration parameter. To have a general description of an anisotropic dark-energy component, a phenomenological parameterization of dark energy in terms of its equation of state (EoS) ? ( de) and two skewness parameters ( ?, ?) have been introduced. It has been found out that the anisotropic distribution of dark energy leads to the present accelerated expansion of the universe. A cosmological scenario with the deceleration parameter having the desired property of signature flip has been developed. The geometrical and physical parameters of the models are studied. The analysis of the models reveals that the present acceleration, isotropy of the universe turn out to be natural consequences of dark energy.

Adhav, K. S.

2011-11-01

288

Reconciling dark energy models with f(R) theories

Higher-order theories of gravity have recently attracted a lot of interest as alternative candidates to explain the observed cosmic acceleration without the need of introducing any scalar field. A critical ingredient is the choice of the function f(R) of the Ricci scalar curvature entering the gravity Lagrangian and determining the dynamics of the Universe. We describe an efficient procedure to reconstruct f(R) from the Hubble parameter H depending on the redshift z. Using the metric formulation of f(R) theories, we derive a third order linear differential equation for f[R(z)] which can be numerically solved after setting the boundary conditions on the basis of physical considerations. Since H(z) can be reconstructed from the astrophysical data, the method we present makes it possible to determine, in principle, what is the f(R) theory which best reproduces the observed cosmological dynamics. Moreover, the method allows to reconcile dark energy models with f(R) theories finding out what is the expression of f(R) which leads to the same H(z) of the given quintessence model. As interesting examples, we consider 'quiessence' (dark energy with constant equation of state) and the Chaplygin gas.

Capozziello, S.; Cardone, V.F.; Troisi, A. [Dipartimento di Fisica 'E.R. Caianiello', Universita di Salerno, Baronissi (Salerno), Italy and INFN, Sez. di Napoli, Gruppo Coll. di Salerno, via S. Allende, 84081 - Baronissi, Salerno (Italy)

2005-02-15

289

Dark energy properties in DBI theory

The Dirac-Born-Infeld (DBI) action from string theory provides several new classes of dark energy behavior beyond quintessence due to its relativistic kinematics. We constrain parameters of natural potentials and brane tensions with cosmological observations as well as showing how to design these functions for a desired expansion history. We enlarge the attractor solutions, including new ways of obtaining cosmological constant behavior, to the case of generalized DBI theory with multiple branes. An interesting novel signature of DBI attractors is that the sound speed is driven to zero, unlike for quintessence where it is the speed of light.

Ahn, Changrim; Kim, Chanju [Institute for the Early Universe and Department of Physics, Ewha Womans University, Seoul 120-750 (Korea, Republic of); Linder, Eric V. [Institute for the Early Universe and Department of Physics, Ewha Womans University, Seoul 120-750 (Korea, Republic of); Berkeley Center for Cosmological Physics and Berkeley Lab, University of California, Berkeley, California 94720 (United States)

2009-12-15

290

Inflation and new agegraphic dark energy

In the note, we extend the discussion of the new agegraphic dark energy (NADE) model to include the inflation stage. Usually, in the inflation models, for convenience the conformal time {eta} is set to be zero at the end of inflation. This is incompatible with the NADE model since {eta}=0 indicates the divergence of NADE. To avoid the difficulty, we can redefine the conformal time as {eta}+{delta}. However, we find that the positive constant {delta} must be so large that NADE cannot become dominated at present time.

Sun Chengyi [Institute of Modern Physics, Northwest University, Xian, 710069 (China); Yue Ruihong [Faculty of Science, Ningbo University, Ningbo 315211 (China)

2011-05-15

291

The Dark Energy Survey CCD imager design

The Dark Energy Survey is planning to use a 3 sq. deg. camera that houses a {approx} 0.5m diameter focal plane of 62 2kx4k CCDs. The camera vessel including the optical window cell, focal plate, focal plate mounts, cooling system and thermal controls is described. As part of the development of the mechanical and cooling design, a full scale prototype camera vessel has been constructed and is now being used for multi-CCD readout tests. Results from this prototype camera are described.

Cease, H.; DePoy, D.; Diehl, H.T.; Estrada, J.; Flaugher, B.; Guarino, V.; Kuk, K.; Kuhlmann, S.; Schultz, K.; Schmitt, R.L.; Stefanik, A.; /Fermilab /Ohio State U. /Argonne

2008-06-01

292

Interacting new agegraphic dark energy in nonflat Brans-Dicke cosmology

We construct a cosmological model of late acceleration based on the new agegraphic dark energy model in the framework of Brans-Dicke cosmology where the new agegraphic energy density {rho}{sub D}=3n{sup 2}m{sub p}{sup 2}/{eta}{sup 2} is replaced with {rho}{sub D}=3n{sup 2{phi}2}/(4{omega}{eta}{sup 2}). We show that the combination of the Brans-Dicke field and agegraphic dark energy can accommodate a w{sub D}=-1 crossing for the equation of state of noninteracting dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of w{sub D} to the phantom regime can be more easily accounted for than when we resort to the Einstein field equations. In the limiting case {alpha}=0 ({omega}{yields}{infinity}), all previous results of the new agegraphic dark energy in Einstein gravity are restored.

Sheykhi, Ahmad [Department of Physics, Shahid Bahonar University, P.O. Box 76175, Kerman (Iran, Islamic Republic of); Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), Maragha (Iran, Islamic Republic of)

2010-01-15

293

Dark energy from primordial inflationary quantum fluctuations.

We show that current cosmic acceleration can be explained by an almost massless scalar field experiencing quantum fluctuations during primordial inflation. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the Universe only recently. By using supernovae data, completed with baryonic acoustic oscillations from galaxy surveys and cosmic microwave background anisotropies, we infer the energy scale of primordial inflation to be around a few TeV, which implies a negligible tensor-to-scalar ratio of the primordial fluctuations. Moreover, our model suggests that inflation lasted for an extremely long period. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale. PMID:20867625

Ringeval, Christophe; Suyama, Teruaki; Takahashi, Tomo; Yamaguchi, Masahide; Yokoyama, Shuichiro

2010-09-17

294

Exotic-singularity-driven dark energy

NASA Astrophysics Data System (ADS)

We discuss various types of exotic (non-standard) singularities in the Universe: a Big-Rip (BR or type I), a Sudden Future Singularity (SFS or type II), a Generalized Sudden Future Singularity, a Finite Scale Factor singularity (FSF or type III), a Big-Separation (BS or type IV) and a w-singularity. They are characterized by violation of all or some of the energy conditions which results in a blow-up of all or some of the physical quantities: the scale factor, the energy density, the pressure, and the barotropic index. We relate the emergence of these singularities with physical theories (superstring, brane, higher-order gravity, loop quantum cosmology). We show how the models involving exotic singularities may serve as dark energy by applying the observational data. In particular, we show that some of these exotic singularities (though being of a weak type according to relativistic definitions) may occur in the near future of the universe.

Dabrowski, Mariusz P.; Denkiewicz, Tomasz

2010-06-01

295

Comparison of perturbations in fluid and scalar field models of dark energy

We compare perturbations in a fluid model of dark energy with those in a scalar field. As compared to the {lambda}CDM model, large scale matter power spectrum is suppressed in fluid model as well as in a generic quintessence dark energy model. To check the efficacy of fluid description of dark energy in emulating a scalar field, we consider a potential which gives the same background evolution as a fluid with a constant equation of state. We show that for sub-Hubble scales, a fluid model effectively emulates a scalar field model. At larger scales, where dark energy perturbations may play a significant role, the fluid analogy breaks down and the evolution of matter density contrast depends on individual scalar field models.

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

2009-06-15

296

Exploring parameter constraints on quintessential dark energy: The Albrecht-Skordis model

NASA Astrophysics Data System (ADS)

We consider the effect of future dark energy experiments on “Albrecht-Skordis” (AS) models of scalar field dark energy using the Monte Carlo Markov chain method. We deal with the issues of parametrization of these models, and have included spatial curvature as a parameter, finding it to be important. We use the Dark Energy Task Force (DETF) simulated data to represent future experiments and report our results in the form of likelihood contours in the chosen parameter space. Simulated data is produced for cases where the background cosmology has a cosmological constant, as well as cases where the dark energy is provided by the AS model. The latter helps us demonstrate the power of DETF Stage 4 data in the context of this specific model. Though the AS model can produce equations of state functions very different from what is possible with the w0-wa parametrization used by the DETF, our results are consistent with those reported by the DETF.

Barnard, Michael; Abrahamse, Augusta; Albrecht, Andreas; Bozek, Brandon; Yashar, Mark

2008-05-01

297

Dissipative or conservative cosmology with dark energy?

All evolutional paths for all admissible initial conditions of FRW cosmological models with dissipative dust fluid (described by dark matter, baryonic matter and dark energy) are analyzed using dynamical system approach. With that approach, one is able to see how generic the class of solutions leading to the desired property-acceleration-is. The theory of dynamical systems also offers a possibility of investigating all possible solutions and their stability with tools of Newtonian mechanics of a particle moving in a one-dimensional potential which is parameterized by the cosmological scale factor. We demonstrate that flat cosmology with bulk viscosity can be treated as a conservative system with a potential function of the Chaplygin gas type. We characterize the class of dark energy models that admit late time de Sitter attractor solution in terms of the potential function of corresponding conservative system. We argue that inclusion of dissipation effects makes the model more realistic because of its structural stability. We also confront viscous models with SNIa observations. The best fitted models are obtained by minimizing the {chi}{sup 2} function which is illustrated by residuals and {chi}{sup 2} levels in the space of model independent parameters. The general conclusion is that SNIa data supports the viscous model without the cosmological constant. The obtained values of {chi}{sup 2} statistic are comparable for both the viscous model and {lambda}CDM model. The Bayesian information criteria are used to compare the models with different power-law parameterization of viscous effects. Our result of this analysis shows that SNIa data supports viscous cosmology more than the {lambda}CDM model if the coefficient in viscosity parameterization is fixed. The Bayes factor is also used to obtain the posterior probability of the model.

Szydlowski, Marek [Astronomical Observatory, Jagiellonian University, Orla 171, 30-244 Krakow (Poland); Marc Kac Complex Systems Research Center, Jagiellonian University, Reymonta 4, 30-059 Krakow (Poland)], E-mail: uoszydlo@cyf-kr.edu.pl; Hrycyna, Orest [Department of Theoretical Physics, Faculty of Philosophy, John Paul II Catholic University of Lublin, Al. Raclawickie 14, 20-950 Lublin (Poland)], E-mail: hrycyna@kul.lublin.pl

2007-12-15

298

Dark matter and dark energy from the solution of the strong CP problem.

The Peccei-Quinn (PQ) solution of the strong CP problem requires the existence of axions, which are viable candidates for dark matter. If the Nambu-Goldstone potential of the PQ model is replaced by a potential V(|Phi|) admitting a tracker solution, the scalar field |Phi| can account for dark energy, while the phase of Phi yields axion dark matter. If V is a supergravity (SUGRA) potential, the model essentially depends on a single parameter, the energy scale Lambda. Once we set Lambda approximately equal to 10(10) GeV at the quark-hadron transition, |Phi| naturally passes through values suitable to solve the strong CP problem, later growing to values providing fair amounts of dark matter and dark energy. PMID:15447249

Mainini, Roberto; Bonometto, Silvio A

2004-09-17

299

Testing the interaction between dark energy and dark matter with Planck data

NASA Astrophysics Data System (ADS)

Interacting dark energy and dark matter is used to go beyond the standard cosmology. We base our arguments on Planck data and conclude that an interaction is compatible with the observations and can provide a strong argument towards consistency of different values of cosmological parameters.

Costa, André A.; Xu, Xiao-Dong; Wang, Bin; Ferreira, Elisa G. M.; Abdalla, E.

2014-05-01

300

Coincidences of Dark Energy with Dark Matter: Clues for a Simple Alternative?

A rare coincidence of scales in standard particle physics is needed to explain why Lambda or the negative pressure of cosmological dark energy (DE) coincides with the positive pressure P0 of random motion of dark matter (DM) in bright galaxies. Recently Zlosnik and coworkers proposed to modify the Einstein curvature by adding nonlinear pressure from a medium flowing with a

HongSheng Zhao

2007-01-01

301

Chaplygin gas cosmology—unification of dark matter and dark energy

The models that unify dark matter and dark energy based upon the Chaplygin gas fail owing to the suppression of structure formation by the adiabatic speed of sound. Including string theory effects, in particular the Kalb–Ramond field, we show how nonadiabatic perturbations allow a successful structure formation.

Neven Bili?; Gary B. Tupper; Raoul D. Viollier

2007-01-01

302

WMAP five-year data constraints on the unified model of dark energy and dark matter

We derive constraints on the parameter space of the unified model of dark energy and dark matter, the generalized Chaplygin gas, from the amplitudes and positions of the first few peaks and first trough of the cosmic microwave background radiation power spectrum, using the latest WMAP five-year data.

Barreiro, T. [Departamento de Matematica, Universidade Lusofona de Humanidades e Tecnologias, Avenida Campo Grande, 376, 1749-024 Lisboa (Portugal); Bertolami, O.; Torres, P. [Departamento de Fisica, Instituto Superior Tecnico, Avenida Rovisco Pais 1, 1049-001 Lisboa (Portugal)

2008-08-15

303

Will multiple probes of dark energy find modified gravity?

One of the most pressing issues in cosmology is whether general relativity (GR) plus a dark sector is the underlying physical theory or whether a modified gravity model is needed. Upcoming dark energy experiments designed to probe dark energy with multiple methods can address this question by comparing the results of the different methods in constraining dark energy parameters. Disagreement would signal the breakdown of the assumed model (GR plus dark energy). We study the power of this consistency test by projecting constraints in the w{sub 0}-w{sub a} plane from the four different techniques of the Dark Energy Survey in the event that the underlying true model is modified gravity. We find that the standard technique of looking for overlap has some shortcomings, and we propose an alternative, more powerful Multidimensional Consistency Test. We introduce the methodology for projecting whether a given experiment will be able to use this test to distinguish a modified gravity model from GR.

Shapiro, Charles [Institute of Cosmology and Gravitation, Portsmouth, PO1 3FX (United Kingdom); Dodelson, Scott [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Kavli Institute for Cosmological Physics, Chicago, Illinois 60637 (United States); Hoyle, Ben [Institut de Ciencies del Cosmos, Barcelona (Spain); Samushia, Lado [Institute of Cosmology and Gravitation, Portsmouth, PO1 3FX (United Kingdom); National Abastumani Astrophysical Observatory, Ilia State University, 2A Kazbegi Ave, GE-0160 Tbilisi (Georgia); Flaugher, Brenna [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States)

2010-08-15

304

The Dark Energy Survey & Camera (DECam)

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) is a next generation optical survey aimed at understanding the expansion rate of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the survey, the DES Collaboration is building the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera that will be mounted at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. CCD production has finished, yielding roughly twice the required 62 2kx4k detectors. The construction of DECam is finished. Integration and commissioning on a "telescope simulator" of the major hardware and software components, except for the optics has concluded at Fermilab. Final assembly of the optical corrector is underway at University College, London. Most components have already been received at CTIO, where installation has started. "First-light" will be in May or June 2012. This oral presentation will describe DES and the technical design and status of DECam.

Diehl, H. Thomas; Dark Energy Survey Collaboration

2012-01-01

305

The Dark Energy Camera readout system

NASA Astrophysics Data System (ADS)

The Dark Energy Camera (DECam) was developed for use by the Dark Energy Survey (DES). The camera will be installed in the Blanco 4M telescope at the Cerro Tololo Inter-American Observatory (CTIO) and be ready for observations in the second half of 2012. The focal plane consists of 62 2×4K and 12 2×2K fully depleted CCDs. The camera provides a 3 sq. degree view and the survey will cover a 5000 sq. degree area. The camera cage and corrector have already been installed. The development of the electronics to readout the focal plane was a collaborative effort by multiple institutions in the United States and in Spain. The goal of the electronics is to provide readout at 250 kpixels/second with less than 15erms noise. Integration of these efforts and initial testing took place at Fermi National Accelerator Laboratory. DECam currently resides at CTIO and further testing has occurred in the Coudé room of the Blanco. In this paper, we describe the development of the readout system, test results and the lessons learned.

Shaw, Theresa; Ballester, Otger; Cardiel-Sas, Laia; Castilla, Javier; Chappa, Steve; de Vicente, Juan; Holm, Scott; Huffman, Dave; Kozlovsky, Mark; Martínez, Gustavo; Moore, Todd; Olsen, Jamieson; Simaitis, Vaidas; Stuermer, Walter

2012-07-01

306

HUBBLE PARAMETER MEASUREMENT CONSTRAINTS ON DARK ENERGY

We use 21 Hubble parameter versus redshift data points from Simon et al., Gaztanaga et al., Stern et al., and Moresco et al. to place constraints on model parameters of constant and time-evolving dark energy cosmologies. The inclusion of the eight new measurements results in H(z) constraints more restrictive than those derived by Chen and Ratra. These constraints are now almost as restrictive as those that follow from current Type Ia supernova (SNIa) apparent magnitude versus redshift data, which now more carefully account for systematic uncertainties. This is a remarkable result. We emphasize, however, that SNIa data have been studied for a longer time than the H(z) data, possibly resulting in a better estimate of potential systematic errors in the SNIa case. A joint analysis of the H(z), baryon acoustic oscillation peak length scale, and SNIa data favors a spatially flat cosmological model currently dominated by a time-independent cosmological constant but does not exclude slowly evolving dark energy.

Farooq, Omer; Mania, Data; Ratra, Bharat, E-mail: omer@phys.ksu.edu, E-mail: mania@phys.ksu.edu, E-mail: ratra@phys.ksu.edu [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)] [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)

2013-02-20

307

Final state and thermodynamics of a dark energy universe

As it follows from the classical analysis, the typical final state of a dark energy universe where a dominant energy condition is violated is a finite-time, sudden future singularity (a big rip). For a number of dark energy universes (including scalar phantom and effective phantom theories as well as specific quintessence models) we demonstrate that quantum effects play the dominant

Shin'ichi Nojiri; Sergei D. Odintsov

2004-01-01

308

Dynamical evolution of quintessence dark energy in collapsing dark matter halos

In this paper, we analyze the dynamical evolution of quintessence dark energy induced by the collapse of dark matter halos. Different from other previous studies, we develop a numerical strategy which allows us to calculate the dark energy evolution for the entire history of the spherical collapse of dark matter halos, without the need of separate treatments for linear, quasilinear, and nonlinear stages of the halo formation. It is found that the dark energy perturbations evolve with redshifts, and their specific behaviors depend on the quintessence potential as well as the collapsing process. The overall energy density perturbation is at the level of 10{sup -6} for cluster-sized halos. The perturbation amplitude decreases with the decrease of the halo mass. At a given redshift, the dark energy perturbation changes with the radius to the halo center, and can be either positive or negative depending on the contrast of {partial_derivative}{sub t}{phi}, {partial_derivative}{sub r}{phi}, and {phi} with respect to the background, where {phi} is the quintessence field. For shells where the contrast of {partial_derivative}{sub r}{phi} is dominant, the dark energy perturbation is positive and can be as high as about 10{sup -5}.

Wang Qiao; Fan Zuhui [Department of Astronomy, School of Physics, Peking University, Beijing, 100871 (China)

2009-06-15

309

NASA Astrophysics Data System (ADS)

In this paper, we propose new efficient and accurate numerical methods for computing dark solitons and review some existing numerical methods for bright and/or dark solitons in the nonlinear Schrödinger equation (NLSE), and compare them numerically in terms of accuracy and efficiency. We begin with a review of dark and bright solitons of NLSE with defocusing and focusing cubic nonlinearities, respectively. For computing dark solitons, to overcome the nonzero and/or non-rest (or highly oscillatory) phase background at far field, we design efficient and accurate numerical methods based on accurate and simple artificial boundary conditions or a proper transformation to rest the highly oscillatory phase background. Stability and conservation laws of these numerical methods are analyzed. For computing interactions between dark and bright solitons, we compare the efficiency and accuracy of the above numerical methods and different existing numerical methods for computing bright solitons of NLSE, and identify the most efficient and accurate numerical methods for computing dark and bright solitons as well as their interactions in NLSE. These numerical methods are applied to study numerically the stability and interactions of dark and bright solitons in NLSE. Finally, they are extended to solve NLSE with general nonlinearity and/or external potential and coupled NLSEs with vector solitons.

Bao, Weizhu; Tang, Qinglin; Xu, Zhiguo

2013-02-01

310

Interacting Generalized Dark Energy and Reconstruction of Scalar Field Models

NASA Astrophysics Data System (ADS)

In this paper, we consider the interacting generalized dark energy with cold dark matter and analyze the behavior of evolution parameter via dark energy and interacting parameters. It is found that the evolution parameter crosses the phantom divide line in most of the cases of integration constants. We also establish the correspondence of scalar field models (quintessence, k-essence and dilaton) with this dark energy model in which scalar fields show the increasing behavior. The scalar potential corresponds to attractor solutions in quintessence case.

Sharif, M.; Jawad, Abdul

2013-12-01

311

Supernovae constraints on models of dark energy reexamined

We use the Type Ia Supernova gold sample data of Riess et al in order to constrain three models of dark energy. We study the Cardassian model, the Dvali-Turner gravity modified model, and the generalized Chaplygin gas model of dark energy-dark matter unification. In our best-fit analysis for these three dark energy proposals we consider the flat model and the nonflat model priors. We also discuss the degeneracy of the models with the XCDM model through the computation of the so-called jerk parameter.

Bento, M.C.; Santos, N.M.C.; Sen, A.A. [Departamento de Fisica and Centro de Fisica Teorica de Particulas, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal); Bertolami, O. [Departamento de Fisica, Instituto Superior Tecnico, Avenida Rovisco Pais, 1049-001 Lisbon (Portugal)

2005-03-15

312

No evidence for dark energy dynamics from a global analysis of cosmological data

We use a variant of principal component analysis to investigate the possible temporal evolution of the dark energy equation of state, w(z). We constrain w(z) in multiple redshift bins, utilizing the most recent data from type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, the integrated Sachs-Wolfe effect, galaxy clustering, and weak lensing data. Unlike other recent analyses, we find no significant evidence for evolving dark energy; the data remain completely consistent with a cosmological constant. We also study the extent to which the time evolution of the equation of state would be constrained by a combination of current- and future-generation surveys, such as Planck and the Joint Dark Energy Mission.

Serra, Paolo; Cooray, Asantha [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Holz, Daniel E. [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Melchiorri, Alessandro [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department and Sezione INFN, University of Rome, 'La Sapienza', Piazzale Aldo Moro 2, 00185 Rome (Italy); Pandolfi, Stefania [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department and International Centre for Relativistic Astrophysics, University of Rome, 'La Sapienza', Piazzale Aldo Moro 2, 00185 Rome (Italy); Sarkar, Devdeep [Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697 (United States); Physics Department, University of Michigan, Ann Arbor, Michigan 48109 (United States)

2009-12-15

313

The Hobby-Eberly Telescope Dark Energy Experiment

NASA Astrophysics Data System (ADS)

We describe a project (HETDEX) to measure the evolution of dark energy out to z~4 with high precision. The galaxy power spectrum contains the baryonic oscillations seen in the CMB, and these features remain in the linear regime at high redshift. The separation of these peaks in the power spectrum is a standard ruler imprinted on both the angular and redshift space distribution of galaxies, providing direct constraints on the local Hubble constant H(z) and the angular diameter distance DA(z), both of which are related to the dark energy equation of state parameter w(z). We propose the use of Ly-? emitting galaxies as the tracer of the power spectrum, and describe the Visible IFU Replicable Ultra-cheap Spectrograph (VIRUS), capable of undertaking such a survey. VIRUS will be a very wide field integral field spectrograph of a new replicated design, consisting of more than a hundred individual spectrograph units. The VIRUS instrument mounted on a new wide field corrector on the Hobby-Eberly Telescope (HET) will allow 1 million Ly-? emitting galaxies to be mapped over 1.8 < z < 3.8 in 200 square degrees of sky, a volume 10 times that of the Sloan Digital Sky Survey (SDSS) in 100 nights of operation. This survey of large scale structure is sufficient to measure the power spectrum to 2-3% accuracy and constrain the positions of the baryonic acoustical peaks on the matter spectrum to <= 1%. This in turn provides sufficient accuracy on H(z) and DA(z) to challenge the level of dark energy constraint expected from the SNAP satellite at a fraction of the cost, while providing unique constraints at z>2. The baryonic acoustical peaks method is largely free of systematic biases and provides an independent test of results found by other methods.

Hill, Gary J.; Gebhardt, Karl; Komatsu, Eiichiro; MacQueen, Phillip J.

2004-12-01

314

Observational constraints on dark energy cosmological model parameters

NASA Astrophysics Data System (ADS)

The expansion rate of the Universe changes with time, initially slowing (decelerating) when the universe was matter dominated, because of the mutual gravitational attraction of all the matter in it, and more recently speeding up (accelerating). A number of cosmological observations now strongly support the idea that the Universe is spatially flat (provided the dark energy density is at least approximately time independent) and is currently undergoing an accelerated cosmological expansion. A majority of cosmologists consider "dark energy" to be the cause of this observed accelerated cosmological expansion. The "standard" model of cosmology is the spatially-flat ?CDM model. Although most predictions of the ?CDM model are reasonably consistent with measurements, the ?CDM model has some curious features. To overcome these difficulties, different Dark Energy models have been proposed. Two of these models, the XCDM parametrization and the slow rolling scalar field model ?-CDM, along with "standard" ?CDM, with the generalization of XCDM and ?-CDM in non-flat spatial geometries are considered here and observational data are used to constrain their parameter sets. In this thesis, we start with a overview of the general theory of relativity, Friedmann's equations, and distance measures in cosmology. In the following chapters we explain how we can constrain the three above mentioned cosmological models using three data sets: measurements of the Hubble parameter H(z), Supernova (SN) apparent magnitudes, and the baryonic acoustic oscillations (BAO) peak length scale, as functions of redshift z. We then discuss constraints on the deceleration-acceleration transition redshift zda using unbinned and binned H(z) data. Finally, we incorporate the spatial curvature in the XCDM and ?-CDM model and determine observational constraints on the parameters of these expanded models.

Farooq, Muhammad Omer

315

CONSTRAINTS ON DARK ENERGY FROM BARYON ACOUSTIC PEAK AND GALAXY CLUSTER GAS MASS MEASUREMENTS

We use baryon acoustic peak measurements by Eisenstein et al. and Percival et al., together with the Wilkinson Microwave Anisotropy Probe (WMAP) measurement of the apparent acoustic horizon angle, and galaxy cluster gas mass fraction measurements of Allen et al., to constrain a slowly rolling scalar field dark energy model, phiCDM, in which dark energy's energy density changes in time. We also compare our phiCDM results with those derived for two more common dark energy models: the time-independent cosmological constant model, LAMBDACDM, and the XCDM parameterization of dark energy's equation of state. For time-independent dark energy, the Percival et al. measurements effectively constrain spatial curvature and favor a close to the spatially flat model, mostly due to the WMAP cosmic microwave background prior used in the analysis. In a spatially flat model the Percival et al. data less effectively constrain time-varying dark energy. The joint baryon acoustic peak and galaxy cluster gas mass constraints on the phiCDM model are consistent with but tighter than those derived from other data. A time-independent cosmological constant in a spatially flat model provides a good fit to the joint data, while the alpha parameter in the inverse power-law potential phiCDM model is constrained to be less than about 4 at 3sigma confidence level.

Samushia, Lado; Ratra, Bharat, E-mail: lado@phys.ksu.ed, E-mail: ratra@phys.ksu.ed [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)

2009-10-01

316

Dark energy from Casimir energy on noncommutative extra dimensions

We study the possibility that dark energy is a manifestation of the Casimir energy on extra dimensions with the topology of S{sup 2}. We consider our universe to be M{sup 4}xS{sup 2} and modify the geometry by introducing noncommutativity on the extra dimensions only, i.e. replacing S{sup 2} with the fuzzy version S{sub F}{sup 2}. We find the energy density as a function of the size of the representation M+1 of the algebra of S{sub F}{sup 2}, and we calculate its value for the M+1=2 case. The value of the energy density turns out to be positive, i.e. provides dark energy, and the size of the extra dimensions agrees with the experimental limit. We also recover the correct commutative limit as the noncommutative parameter goes to zero.

Fabi, S.; Harms, B.; Karatheodoris, G. [Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487-0324 (United States)

2006-10-15

317

Dark energy as double N-flation - observational predictions

We propose a simple model for dark energy useful for comparison with observations. It is based on the idea that dark energy and inflation should be caused by the same physical process. As motivation, we note that Linde's simple chaotic inflation ? produces values of ns= 0.967 and r= 0.13, which are consistent with the Wilkinson Microwave Anisotropy Probe (WMAP)

J. Richard Gott; Zachary Slepian

2011-01-01

318

Constraints on interacting Dark Energy models from galaxy rotation curves

Interacting Dark Energy models have been introduced as a possible alternative to the standard ?CDM concordance cosmological scenario in order to ease the fine-tuning problems of the cosmological constant. However, the interaction of the Dark Energy field with other massive particles in the universe induces also an effective modification of structure formation processes, leading to a different dynamical behavior of

Marco Baldi; Paolo Salucci

2012-01-01

319

Status of the Dark Energy Survey Camera (DECam) Project.

National Technical Information Service (NTIS)

The Dark Energy Survey Collaboration has completed construction of the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be used to perform the 5000 sq. deg. D...

2012-01-01

320

New Approach to Testing Dark Energy Models by Observations.

National Technical Information Service (NTIS)

We propose a new approach to the consistency test of dark energy models with observations. To test a category of dark energy models, we suggest introducing a characteristic Q(z) that in general varies with the redshift z but in those models plays the role...

L. E. Strigari

2013-01-01

321

On the Higgs-like quintessence for dark energy

NASA Astrophysics Data System (ADS)

We propose a dynamical (quintessence) model of dark energy in the current Universe with a renormalizable (Higgs-like) scalar potential. We prove the viability of our model (after fine-tuning) for the certain range of the average scalar curvature values, and study the cosmological signatures distinguishing our model from the standard description of dark energy in terms of a cosmological constant.

Ketov, Sergei V.; Watanabe, Natsuki

2014-07-01

322

Advanced Dark Energy Physics Telescope (ADEPT). Final Report.

National Technical Information Service (NTIS)

In 2006, we proposed to NASA a detailed concept study of ADEPT (the Advanced Dark Energy Physics Telescope), a potential space mission to reliably measure the time-evolution of dark energy by conducting the largest effective volume survey of the universe ...

C. L. Bennett

2009-01-01

323

CONSTRAINING DARK ENERGY WITH GAMMA-RAY BURSTS

We use the measurement of gamma-ray burst (GRB) distances to constrain dark energy cosmological model parameters. We employ two methods for analyzing GRB data-fitting luminosity relation of GRBs in each cosmology and using distance measures computed from binned GRB data. Current GRB data alone cannot tightly constrain cosmological parameters and allow for a wide range of dark energy models.

Samushia, Lado; Ratra, Bharat, E-mail: lado@phys.ksu.ed, E-mail: ratra@phys.ksu.ed [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, KS 66506 (United States)

2010-05-10

324

Nuclear recoil energy spectrum of finite-sized dark matter

NASA Astrophysics Data System (ADS)

Research has shown that direct dark matter detection experiments can distinguish between pointlike and finite-sized dark-matter candidates, both of which are of theoretical interests. In particular, there is an additional form factor in the typical cross section of finite-sized dark matter, causing the nuclear recoil energy spectrum of finite-sized dark matter to decrease more rapidly with the recoil energy than that of pointlike dark matter. Since the spectrum of finite-sized dark matter peaks below 1 keV, which is the common experimental threshold, and falls off rapidly at higher energies, detector with sub-keV threshold is necessary. The current goal of TEXONO-CDEX research program, on the studies of low energy neutrino and dark matter physics at Kuo-Sheng Reactor Neutrino Laboratory and China Jin-Ping Underground Laboratory, is to open the sub-keV detector window with germanium detectors. This work derives a model-independent, theoretical prediction of the nuclear recoil energy spectrum of finite-sized dark matter and is working toward using the predicted spectrum to analyze the experimental data of TEXONO-CDEX, in hope to substantiate or rule out dark matter candidates.

Chen, Anffany

2012-10-01

325

Dark energy camera installation at CTIO: overview

NASA Astrophysics Data System (ADS)

The Dark Energy Camera (DECam) has been installed on the V. M. Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. This major upgrade to the facility has required numerous modifications to the telescope and improvements in observatory infrastructure. The telescope prime focus assembly has been entirely replaced, and the f/8 secondary change procedure radically changed. The heavier instrument means that telescope balance has been significantly modified. The telescope control system has been upgraded. NOAO has established a data transport system to efficiently move DECam's output to the NCSA for processing. The observatory has integrated the DECam highpressure, two-phase cryogenic cooling system into its operations and converted the Coudé room into an environmentally-controlled instrument handling facility incorporating a high quality cleanroom. New procedures to ensure the safety of personnel and equipment have been introduced.

Abbott, Timothy M.; Muñoz, Freddy; Walker, Alistair R.; Smith, Chris; Montane, Andrés.; Gregory, Brooke; Tighe, Roberto; Schurter, Patricio; van der Bliek, Nicole S.; Schumacher, German

2012-09-01

326

Calibration Monitor for Dark Energy Experiments

The goal of this program was to design, build, test, and characterize a flight qualified calibration source and monitor for a Dark Energy related experiment: ACCESS - 'Absolute Color Calibration Experiment for Standard Stars'. This calibration source, the On-board Calibration Monitor (OCM), is a key component of our ACCESS spectrophotometric calibration program. The OCM will be flown as part of the ACCESS sub-orbital rocket payload in addition to monitoring instrument sensitivity on the ground. The objective of the OCM is to minimize systematic errors associated with any potential changes in the ACCESS instrument sensitivity. Importantly, the OCM will be used to monitor instrument sensitivity immediately after astronomical observations while the instrument payload is parachuting to the ground. Through monitoring, we can detect, track, characterize, and thus correct for any changes in instrument senstivity over the proposed 5-year duration of the assembled and calibrated instrument.

Kaiser, M. E.

2009-11-23

327

Neutrino dark energy in grand unified theories

We studied a left-right symmetric model that can accommodate the neutrino dark energy ({nu}DE) proposal. The type-III seesaw mechanism is implemented to give masses to the neutrinos. After explaining the model, we study the consistency of the model by minimizing the scalar potential and obtaining the conditions for the required vacuum expectation values of the different scalar fields. This model is then embedded in an SO(10) grand unified theory and the allowed symmetry breaking scales are determined by the condition of the gauge coupling unification. Although SU(2){sub R} breaking is required to be high, its Abelian subgroup U(1){sub R} is broken in the TeV range, which can then give the required neutrino masses and predicts new gauge bosons that could be detected at LHC. The neutrino masses are studied in detail in this model, which shows that at least 3 singlet fermions are required.

Bhatt, Jitesh R.; Sarkar, Utpal; Singh, Santosh K. [Physical Research Laboratory, Ahmedabad 380009 (India); Gu, P.-H. [Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34014 Trieste (Italy)

2009-10-01

328

Dark Energy Survey: understanding the acceleration of the Universe

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) is a next generation optical survey aimed at understanding the expansion rate of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. Using a new 570 Megapixel CCD camera mounted on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, the survey will image over 5000 square-degrees of the southern galactic cap with 5 filters (g, r, i, z, Y) in 5 years and improve the constraints on the evolution of the equation of state of Dark Energy by a factor of 3 - 5. After achieving first light on Sept. 12, 2012, the DES is moving forward at amazing speed. Commissioning was carried out in Sept-Oct., 2012, followed by Science Verification that was successfully completed by February 2013. During the Science Verification, DES has been focusing on three patches of the sky, imaging a couple of hundred square degrees at full DES depth. Of the three patches, one is along the southern celestial equatorial region, overlapping with many existing optical surveys for better calibration, and the other two are overlapping with the South Pole Telescope (SPT) Sunyaev-Zel'dovich survey in the southern sky. The official survey will start in Sept. 2013 and a lot of exciting science is just ahead.

Hao, Jiangang

2013-04-01

329

Linear and nonlinear instabilities in unified dark energy models

We revisit the paradigm of unified dark energy discussing in detail the averaging problem in this type of scenario, highlighting the need for a full nonlinear treatment. We also address the question of if and how models with one or several dark fluids can be observationally distinguished. Simpler and physically clearer derivations of some key results, most notably on the relation between the generalized Chaplygin gas and the standard ({lambda}CDM) 'concordance' model and on a Jeans-type small-scale instability of some coupled dark energy/dark matter models are presented.

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 Fisica do Porto, Rua do Campo Alegre 687, 4169-007 Porto (Portugal); Centro de Astrofisica, Universidade do Porto, Rua das Estrelas s/n, 4150-762 Porto (Portugal); Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA (United Kingdom)

2008-03-15

330

Baryon acoustic oscillation intensity mapping of dark energy.

The expansion of the Universe appears to be accelerating, and the mysterious antigravity agent of this acceleration has been called "dark energy." To measure the dynamics of dark energy, baryon acoustic oscillations (BAO) can be used. Previous discussions of the BAO dark energy test have focused on direct measurements of redshifts of as many as 10(9) individual galaxies, by observing the 21 cm line or by detecting optical emission. Here we show how the study of acoustic oscillation in the 21 cm brightness can be accomplished by economical three-dimensional intensity mapping. If our estimates gain acceptance they may be the starting point for a new class of dark energy experiments dedicated to large angular scale mapping of the radio sky, shedding light on dark energy. PMID:18352692

Chang, Tzu-Ching; Pen, Ue-Li; Peterson, Jeffrey B; McDonald, Patrick

2008-03-01

331

The CHASE laboratory search for chameleon dark energy

A scalar field is a favorite candidate for the particle responsible for dark energy. However, few theoretical means exist that can simultaneously explain the observed acceleration of the Universe and evade tests of gravity. The chameleon mechanism, whereby the properties of a particle depend upon the local environment, is one possible avenue. We present the results of the Chameleon Afterglow Search (CHASE) experiment, a laboratory probe for chameleon dark energy. CHASE marks a significant improvement other searches for chameleons both in terms of its sensitivity to the photon/chameleon coupling as well as its sensitivity to the classes of chameleon dark energy models and standard power-law models. Since chameleon dark energy is virtually indistinguishable from a cosmological constant, CHASE tests dark energy models in a manner not accessible to astronomical surveys.

Steffen, Jason H.; /Fermilab

2010-11-01

332

Improved Dark Energy Constratints from approx. 100 New CfA Supernova Type Ia Light Curves.

National Technical Information Service (NTIS)

We combine the CfA3 supernovae Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. to form the Constituti...

A. Rest M. Hicken P. Challis P. L. Kelly R. P. Kirshner S. Blondin S. Jha W. M. Wood-Vasey

2009-01-01

333

We generalize the small scale dynamics of the Universe by taking into account models with an equation of state which evolves with time, and provide a complete formulation of the cluster virialization attempting to address the nonlinear regime of structure formation. In the context of the current dark energy models, we find that galaxy clusters appear to form at z{approx}1-2, in agreement with previous studies. Also, we investigate thoroughly the evolution of spherical matter perturbations, as the latter decouple from the background expansion and start to 'turn around' and finally collapse. Within this framework, we find that the concentration parameter depends on the choice of the considered dark energy (homogeneous or clustered). In particular, if the distribution of the dark energy is clustered then we produce more concentrated structures with respect to the homogeneous dark energy. Finally, comparing the predicted concentration parameter with the observed concentration parameter, measured for four massive galaxy clusters, we find that the scenario which contains a pure homogeneous dark energy is unable to reproduce the data. The situation becomes somewhat better in the case of an inhomogeneous (clustered) dark energy.

Basilakos, Spyros; Sanchez, Juan Carlos Bueno; Perivolaropoulos, Leandros [Academy of Athens, Research Center for Astronomy and Applied Mathematics, Soranou Efesiou 4, 11527 Athens (Greece); Department of Physics, University of Ioannina, 45110 Ioannina (Greece)

2009-08-15

334

Holographic Dark Energy Model Characterized by the Conformal-Age Length

NASA Astrophysics Data System (ADS)

A holographic dark energy model characterized by the conformal-age-like length scale L = (1)/(a4(t))\\int 0^tdt^' a3(t^') is motivated from the four-dimensional space-time volume at cosmic time t in the flat Friedmann-Robertson-Walker (FRW) universe. It is shown that when the background constituent with constant equation of state wm dominates the universe in the early time, the fractional energy density of the dark energy scales as ? de˜= (9)/(4)(3+wm)2 d2a^2 with the equation of state given by w de˜=-(2)/(3)+wm. The value of wm is taken to be wm?-1 during inflation, wm = ? in radiation-dominated epoch and wm = 0 in matter-dominated epoch, respectively. When the model parameter d takes the normal value at order one, the fractional density of dark energy is naturally negligible in the early universe, ?de ?1 at a ?1. With such an analytic feature, the model can be regarded as a single-parameter model like the ?CDM model, so that the present fractional energy density ?de(a = 1) can solely be determined by solving the differential equation of ?de once d is given. We further extend the model to the general case in which both matter and radiation are present. The scenario involving possible interaction between the dark energy and the background constituent is also discussed.

Huang, Zhuo-Peng; Wu, Yue-Liang

2012-06-01

335

Why is the running vacuum energy more benign than the holographic Ricci dark energy?

NASA Astrophysics Data System (ADS)

A very interesting generalization of the running vacuum energy density has been recently advanced [S. Basilakos, D. Polarski, and J. Sola, Phys. Rev. D 86, 043010 (2012)]. The Friedmann equation of this model looks pretty much similar to that of a homogeneous and isotropic universe filled with an holographic Ricci dark energy (HRDE) component. Despite the analogy between these two models, it turns out that one of them, generalization of the running vacuum energy, is singularity-free in the future while the other, HRDE, is not. Indeed, a universe filled with an HRDE component can hit, for example, a big rip singularity. We clarify this issue by solving analytically the Friedmann equation for both models and analyzing the role played by the local conservation of the energy density of the different components when filling the universe. In addition, not everything is bad news about the HRDE. In fact, we point out that in some particular cases the HRDE, when endowed with a negative cosmological constant and in the absence of an explicit dark matter component, can mimic dark matter and explain the late-time cosmic acceleration of the universe through an asymptotically de Sitter universe.

Bouhmadi-López, Mariam; Tavakoli, Yaser

2013-01-01

336

Joint galaxy-lensing observables and the dark energy

Deep multicolor galaxy surveys with photometric redshifts will provide a large number of two-point correlation observables: galaxy-galaxy angular correlations, galaxy-shear cross correlations, and shear-shear correlations between all redshifts. These observables can potentially enable a joint determination of the dark-energy-dependent evolution of the dark matter and distances as well as the relationship between galaxies and dark matter halos. With recent cosmic

Wayne Hu; Bhuvnesh Jain

2004-01-01

337

Pilgrim dark energy in f(T,T G ) cosmology

NASA Astrophysics Data System (ADS)

We work on the reconstruction scenario of pilgrim dark energy" (PDE) in $f(T, T_G)$. In PDE model it is assumed that a repulsive force that is accelerating the Universe is phantom type with $(w_{DE}<-1)$ and it so strong that prevents formation of the black hole. We construct the $f(T, T_G)$ models and correspondingly evaluate equation of state parameter for various choices of scale factor. Also, we assume polynomial form of $f(T, T_G)$ in terms of cosmic time and reconstruct $H$ and $w_{DE}$ in this manner. Through discussion, it is concluded that PDE shows aggressive phantom-like behaviour for$s=-2$ in $f(T, T_G)$ gravity.

Chattopadhyay, Surajit; Jawad, Abdul; Momeni, Davood; Myrzakulov, Ratbay

2014-07-01

338

Mega-masers, Dark Energy and the Hubble Constant

Powerful water maser emission (water mega-masers) can be found in accretion disks in the nuclei of some galaxies. Besides providing a measure of the mass at the nucleus, such mega-masers can be used to determine the distance to the host galaxy, based on a kinematic model. We will explain the importance of determining the Hubble Constant to high accuracy for constraining the equation of state of Dark Energy and describe the Mega-maser Cosmology Project that has the goal of determining the Hubble Constant to better than 3%. Time permitting, we will also present the scientific capabilities of the current and future NRAO facilities: ALMA, EVLA, VLBA and GBT, for addressing key astrophysical problems

Lo, Fred K. Y.

2007-10-15

339

The WiggleZ Project: AAOmega and Dark Energy

NASA Astrophysics Data System (ADS)

We describe the `WiggleZ' spectroscopic survey of 400,000 star-forming galaxies selected from a combination of GALEX ultra-violet and SDSS + RCS2 optical imaging. The fundamental goal is a detection of the baryonic acoustic oscillations in galaxy clustering at high-redshift (0.5 < z < 1) and a precise measurement of the equation of state of dark energy from this purely geometric and robust method. The survey has already started on the 3.9m Anglo-Australian Telescope using the AAOmega spectrograph, and planned to complete during 2009. The WWW page for the survey can be found at astronomy.swin.edu.au/wigglez.

Glazebrook, K.; Blake, C.; Couch, W.; Forbes, D.; Drinkwater, M.; Jurek, R.; Pimbblet, K.; Madore, B.; Martin, C.; Small, T.; Forster, K.; Colless, M.; Sharp, R.; Croom, S.; Woods, D.; Pracy, M.; Gilbank, D.; Yee, H.; Gladders, M.

2007-12-01

340

Mega-masers, Dark Energy and the Hubble Constant

Powerful water maser emission (water mega-masers) can be found in accretion disks in the nuclei of some galaxies. Besides providing a measure of the mass at the nucleus, such mega-masers can be used to determine the distance to the host galaxy, based on a kinematic model. We will explain the importance of determining the Hubble Constant to high accuracy for constraining the equation of state of Dark Energy and describe the Mega-maser Cosmology Project that has the goal of determining the Hubble Constant to better than 3%. Time permitting, we will also present the scientific capabilities of the current and future NRAO facilities: ALMA, EVLA, VLBA and GBT, for addressing key astrophysical problems

341

Interacting holographic dark energy models: a general approach

NASA Astrophysics Data System (ADS)

Dark energy models inspired by the cosmological holographic principle are studied in homogeneous isotropic spacetime with a general choice for the dark energy density ?d=3(? H2+?dot{H}). Special choices of the parameters enable us to obtain three different holographic models, including the holographic Ricci dark energy (RDE) model. Effect of interaction between dark matter and dark energy on the dynamics of those models are investigated for different popular forms of interaction. It is found that crossing of phantom divide can be avoided in RDE models for ?>0.5 irrespective of the presence of interaction. A choice of ?=1 and ?=2/3 leads to a varying ?-like model introducing an IR cutoff length ? -1/2. It is concluded that among the popular choices an interaction of the form Q?H? m suits the best in avoiding the coincidence problem in this model.

Som, S.; Sil, A.

2014-04-01

342

Controlling for the Effects of Baryons in Cosmic Shear Constraints on Dark Energy

NASA Astrophysics Data System (ADS)

The uncertainty introduced by the effects of baryons on the power spectrum of the convergence field is a significant theoretical error limiting forthcoming gravitational lensing surveys. A proposed method to account for baryonic effects is to include parameters that characterize dark matter halos, and to fit lensing data to these halo parameters concurrently with the cosmological parameters. We test this proposal by using this technique to model convergence power spectrum predictions from a set of cosmological simulations. We estimate biases in dark energy parameters that would be incurred if one were to fit the spectra predicted by the simulations either with no model for baryons, or with the proposed method. Neglecting baryonic effects leads to biases in dark energy parameters that are several times the statistical errors of surveys like the Dark Energy Survey. The proposed method to correct for baryonic effects renders the residual biases in dark energy equation of state parameters smaller than the statistical errors. These results suggest that this mitigation method may be applied to analyze convergence spectra from a survey like the Dark Energy Survey. For significantly larger surveys, such as the LSST or Euclid, the biases introduced by baryonic effects remain significant.

Zentner, Andrew

2013-04-01

343

Entropy and universality of the Cardy-Verlinde formula in a dark energy universe

We study the entropy of a FRW universe filled with dark energy (cosmological\\u000aconstant, quintessence or phantom). For general or time-dependent equation of\\u000astate $p=w\\\\rho$ the entropy is expressed in terms of energy, Casimir energy,\\u000aand $w$. The correspondent expression reminds one about 2d CFT entropy only for\\u000aconformal matter. At the same time, the cosmological Cardy-Verlinde formula\\u000arelating three

Iver Brevik; Sergei D. Odintsov; Luciano Vanzo; Edifici Nexus

2004-01-01

344

Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter

The problem of explaining the acceleration of the expansion of the universe\\u000aand the observational and theoretical difficulties associated with dark matter\\u000aand dark energy are discussed. The possibility that Einstein gravity does not\\u000acorrectly describe the large-scale structure of the universe is considered and\\u000aan alternative gravity theory is proposed as a possible resolution to the\\u000aproblems.

J. W. Moffat

2004-01-01

345

Hydrodynamical simulations of galaxy clusters in dark energy cosmologies - I. General properties

NASA Astrophysics Data System (ADS)

We investigate the influence of dark energy on structure formation, within five different cosmological models, namely a concordance ? cold dark matter model, two models with dynamical dark energy, viewed as a quintessence scalar field [using a Ratra and Peebles (RP) and a supergravity (SUGRA) potential form], and two extended quintessence models (EQp and EQn), where the quintessence scalar field interacts non-minimally with gravity (scalar-tensor theories). We adopted for all models the normalization of the matter power spectrum ?8 to match the cosmic microwave background data. In the models with dynamical dark energy and quintessence, we describe the equation of state with w0?-0.9, still within the range allowed by observations. For each model, we have performed hydrodynamical simulations in a cosmological box of (300 Mpc h-1)3 including baryons and allowing for cooling and star formation. The contemporary presence of evolving dark energy and baryon physics allows us to investigate the interplay between the different background cosmology and the evolution of the luminous matter. Since cluster baryon fraction can be used to constrain other cosmological parameters such as ?m, we also analyse how dark energy influences the baryon content of galaxy clusters. We find that in models with dynamical dark energy, the evolving cosmological background leads to different star formation rates and different formation histories of galaxy clusters, but the baryon physics is not affected in a relevant way. We investigate several proxies of the cluster mass function based on X-ray observables like temperature, luminosity, Mgas and Ygas. We conclude that the X-ray temperature and Mgas functions are better diagnostic to disentangle the growth of structures among different dark energy models. We also evaluate the cosmological volumes needed to distinguish the dark energy models here investigated using the cluster number counts (in terms of the mass function and the X-ray luminosity and temperature functions). Relaxed, massive clusters, when studied in regions sufficiently far from the centre, are built up in a very similar way despite the different dark energy models here considered. We confirm that the overall baryon fraction is almost independent of the dark energy models at a few per cent level. The same is true for the gas fraction. This evidence reinforces the use of galaxy clusters as cosmological probe of the matter and energy content of the Universe.

De Boni, C.; Dolag, K.; Ettori, S.; Moscardini, L.; Pettorino, V.; Baccigalupi, C.

2011-08-01

346

Chemical potential and the nature of dark energy: The case of a phantom field

The influence of a possible nonzero chemical potential {mu} on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state, p={omega}{rho} ({omega}<0, constant). The entropy condition, S{>=}0, implies that the possible values of {omega} are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For {mu}>0, the {omega} parameter must be greater than -1 (vacuum is forbidden) while for {mu}<0 not only the vacuum but even a phantomlike behavior ({omega}<-1) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, {mu}/T={mu}{sub 0}/T{sub 0}. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons {mu} is always negative and the extended Wien's law allows only a dark component with {omega}<-1/2, which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for {mu}<0. However, fermionic particles with {mu}>0 are permitted only if -1<{omega}<-1/2. The thermodynamics and statistical arguments constrain the equation-of-state parameter to be {omega}<-1/2, a result surprisingly close to the maximal value required to accelerate a Friedmann-Robertson-Walker-type universe dominated by matter and dark energy ({omega} < or approx. -10/21)

Lima, J. A. S.; Pereira, S. H. [Departamento de Astronomia, Universidade de Sao Paulo, Rua do Matao, 1226 - 05508-900, Sao Paulo, Sao Paulo (Brazil)

2008-10-15

347

The abundance and distribution of collapsed objects such as galaxy clusters will become an important tool to investigate the nature of dark energy and dark matter. Number counts of very massive objects are sensitive not only to the equation of state of dark energy, which parametrizes the smooth component of its pressure, but also to the sound speed of dark energy, which determines the amount of pressure in inhomogeneous and collapsed structures. Since the evolution of these structures must be followed well into the nonlinear regime, and a fully relativistic framework for this regime does not exist yet, we compare two approximate schemes: the widely used spherical collapse model and the pseudo-Newtonian approach. We show that both approximation schemes convey identical equations for the density contrast, when the pressure perturbation of dark energy is parametrized in terms of an effective sound speed. We also make a comparison of these approximate approaches to general relativity in the linearized regime, which lends some support to the approximations.

Abramo, L. R.; Batista, R. C. [Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo (Brazil); Liberato, L.; Rosenfeld, R. [Instituto de Fisica Teorica, Universidade Estadual Paulista, R. Pamplona 145, 01405-900, Sao Paulo (Brazil)

2009-01-15

348

Reconstruction of Scalar Field Dark Energy Models in Kaluza—Klein Universe

NASA Astrophysics Data System (ADS)

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the flat Kaluza—Klein universe. We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum. It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times. We also develop its correspondence with some scalar field dark energy models. It is interesting to mention here that all the results are consistent with the present observations.

Sharif, M.; Abdul, Jawad

2013-08-01

349

Comment on Ricci dark energy in Chern-Simons modified gravity

NASA Astrophysics Data System (ADS)

We revisit the Ricci dark energy in Chern-Simons modified gravity. As far as the cosmological evolution, this is nothing but the Ricci dark energy with a minimally coupled scalar without potential which means that the role of Chern-Simons term is suppressed. Using the equation of state parameter, this model is similar to the modified Chaplygin gas model only when two are around the de Sitter universe deriving by the cosmological constant in the future. However, the two past evolutions are different.

Myung, Yun Soo

2013-08-01

350

NEW LIMITS ON EARLY DARK ENERGY FROM THE SOUTH POLE TELESCOPE

We present new limits on early dark energy (EDE) from the cosmic microwave background (CMB) using data from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite on large angular scales and South Pole Telescope on small angular scales. We find a strong upper limit on the EDE density of {Omega}{sub e} < 0.018 at 95% confidence, a factor of three improvement over WMAP data alone. We show that adding lower-redshift probes of the expansion rate to the CMB data improves constraints on the dark energy equation of state, but not the EDE density. We also explain how small-scale CMB temperature anisotropy constrains EDE.

Reichardt, C. L. [Department of Physics, University of California, Berkeley, CA 94720 (United States); De Putter, R. [IFIC, Universidad de Valencia-CSIC, Valencia 46071 (Spain); Zahn, O. [Berkeley Center for Cosmological Physics, Department of Physics, University of California, and Lawrence Berkeley National Labs, Berkeley, CA 94720 (United States); Hou, Z., E-mail: cr@bolo.berkeley.edu [Department of Physics, University of California, Davis, CA 95616 (United States)

2012-04-10

351

Comparative analysis of model-independent methods for exploring the nature of dark energy

NASA Astrophysics Data System (ADS)

We make a comparative analysis of the various independent methods proposed in the literature for studying the nature of dark energy, using four different mocks of SnIa data. In particular, we explore a generic principal components analysis approach, the genetic algorithms, a series of approximations like Padé power law approximants, and various expansions in orthogonal polynomials, as well as cosmography, and compare them with the usual fit to a model with a constant dark energy equation of state w. We find that, depending on the mock data, some methods are more efficient than others at distinguishing the underlying model, although there is no universally better method.

Nesseris, Savvas; García-Bellido, Juan

2013-09-01

352

Scalar field dark energy perturbations and their scale dependence

We estimate the amplitude of perturbation in dark energy at different length scales for a quintessence model with an exponential potential. It is shown that on length scales much smaller than Hubble radius, perturbation in dark energy is negligible in comparison to that in dark matter. However, on scales comparable to the Hubble radius ({lambda}{sub p}>1000 Mpc) the perturbation in dark energy in general cannot be neglected. As compared to the {lambda}CDM model, the large-scale matter power spectrum is suppressed in a generic quintessence dark energy model. We show that on scales {lambda}{sub p}<1000 Mpc, this suppression is primarily due to different background evolution compared to the {lambda}CDM model. However, on much larger scales perturbation in dark energy can affect the matter power spectrum significantly. Hence this analysis can act as a discriminator between the {lambda}CDM model and other generic dark energy models with w{sub de}{ne}-1.

Unnikrishnan, Sanil; Seshadri, T. R. [Department of Physics and Astrophysics, University of Delhi, Delhi 110007 (India); Jassal, H. K. [Harish-Chandra Research Institute, Chhatnag Road, Jhunsi, Allahabad 211 019 (India)

2008-12-15

353

On the spherical collapse model in dark energy cosmologies

We study the spherical collapse model in dark energy cosmologies, in which\\u000adark energy is modelled as a minimally coupled scalar field. We first follow\\u000athe standard assumption that dark energy does not cluster on the scales of\\u000ainterest. Investigating four different popular potentials in detail, we show\\u000athat the predictions of the spherical collapse model depend on the potential

David F. Mota; Carsten van de Bruck

2004-01-01

354

The distinguishability of interacting dark energy from modified gravity

NASA Astrophysics Data System (ADS)

We study the observational viability of coupled quintessence models with their expansion and growth histories matched to modified gravity cosmologies. We find that for a Dvali-Gabadadze-Porrati model which has been fitted to observations, the matched interacting dark energy models are observationally disfavoured. We also study the distinguishability of interacting dark energy models matched to scalar-tensor theory cosmologies and show that it is not always possible to find a physical interacting dark energy model which shares their expansion and growth histories.

Clemson, Timothy; Koyama, Kazuya

2013-01-01

355

Neutrino dark energy with more than one neutrino species

The mass varying neutrino scenario is a model that successfully explains the origin of dark energy while at the same time solves the coincidence problem. The model is, however, heavily constrained by its stability towards the formation of neutrino bound states when the neutrinos become nonrelativistic. We discuss these constraints and find that natural, adiabatic, stable models with the right amount of dark energy today do not exist. Second, we explain why using the lightest neutrino, which is still relativistic, as an explanation for dark energy does not work because of a feedback mechanism from the heavier neutrinos.

Bjaelde, Ole Eggers; Hannestad, Steen [Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark and Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States); Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C (Denmark)

2010-03-15

356

We characterize the full family of soliton solutions sitting over a background plane wave and ruled by the cubic-quintic nonlinear Schroedinger equation in the regime where a quintic focusing term represents a saturation of the cubic defocusing nonlinearity. We discuss the existence and properties of solitons in terms of catastrophe theory and fully characterize bistability and instabilities of the dark-antidark pairs, revealing mechanisms of decay of antidark solitons into dispersive shock waves.

Crosta, M. [Dept. of Physics, Sapienza University of Rome, I-00185, Rome (Italy); Fratalocchi, A. [PRIMALIGHT, Faculty of Electrical Engineering, Applied Mathematics and Computational Science, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900 (Saudi Arabia); Trillo, S. [Dipartimento di Ingegneria, Universita di Ferrara, Via Saragat 1, I-44122 Ferrara (Italy)

2011-12-15

357

Model of dark matter and dark energy based on gravitational polarization

A model of dark matter and dark energy based on the concept of gravitational polarization is investigated. We propose an action in standard general relativity for describing, at some effective or phenomenological level, the dynamics of a dipolar medium, i.e. one endowed with a dipole moment vector, and polarizable in a gravitational field. Using first-order cosmological perturbations, we show that the dipolar fluid is undistinguishable from standard dark energy (a cosmological constant {lambda}) plus standard dark matter (a pressureless perfect fluid), and therefore benefits from the successes of the {lambda}-cold-dark-matter scenario at cosmological scales. Invoking an argument of 'weak clusterization' of the mass distribution of dipole moments, we find that the dipolar dark matter reproduces the phenomenology of the modified Newtonian dynamics at galactic scales. The dipolar medium action naturally contains a cosmological constant, and we show that if the model is to come from some fundamental underlying physics, the cosmological constant {lambda} should be of the order of a{sub 0}{sup 2}/c{sup 4}, where a{sub 0} denotes the modified Newtonian dynamics constant acceleration scale, in good agreement with observations.

Blanchet, Luc; Le Tiec, Alexandre [GRECO, Institut d'Astrophysique de Paris-UMR 7095 du CNRS, Universite Pierre and Marie Curie, 98 bis boulevard Arago, 75014 Paris (France)

2008-07-15

358

The Dark Energy Survey Data Management System

The Dark Energy Survey (DES) is a project with the goal of building, installing and exploiting a new 74 CCD-camera at the Blanco telescope, in order to study the nature of cosmic acceleration. It will cover 5000 square degrees of the southern hemisphere sky and will record the positions and shapes of 300 million galaxies up to redshift 1.4. The survey will be completed using 525 nights during a 5-year period starting in 2012. About O(1 TB) of raw data will be produced every night, including science and calibration images. The DES data management system has been designed for the processing, calibration and archiving of these data. It is being developed by collaborating DES institutions, led by NCSA. In this contribution, we describe the basic functions of the system, what kind of scientific codes are involved and how the Data Challenge process works, to improve simultaneously the Data Management system algorithms and the Science Working Group analysis codes.

Sevilla, I.; /Madrid, CIEMAT; Armstrong, R.; Jarvis, M.; /Pennsylvania U.; Bertin, E.; /Paris, Inst. Astrophys.; Carlson, A.; Desai, S.; Mohr, J.; /Munich U.; Daues, G.; Gower, M.; Gruendl, R.; Petravick, D.; /Illinois U., Urbana /Illinois U., Urbana /Chicago U. /Fermilab /Brookhaven /Harvard-Smithsonian Ctr. Astrophys.

2011-09-01

359

Cooling the dark energy camera instrument

DECam, camera for the Dark Energy Survey (DES), is undergoing general design and component testing. For an overview see DePoy, et al in these proceedings. For a description of the imager, see Cease, et al in these proceedings. The CCD instrument will be mounted at the prime focus of the CTIO Blanco 4m telescope. The instrument temperature will be 173K with a heat load of 113W. In similar applications, cooling CCD instruments at the prime focus has been accomplished by three general methods. Liquid nitrogen reservoirs have been constructed to operate in any orientation, pulse tube cryocoolers have been used when tilt angles are limited and Joule-Thompson or Stirling cryocoolers have been used with smaller heat loads. Gifford-MacMahon cooling has been used at the Cassegrain but not at the prime focus. For DES, the combined requirements of high heat load, temperature stability, low vibration, operation in any orientation, liquid nitrogen cost and limited space available led to the design of a pumped, closed loop, circulating nitrogen system. At zenith the instrument will be twelve meters above the pump/cryocooler station. This cooling system expected to have a 10,000 hour maintenance interval. This paper will describe the engineering basis including the thermal model, unbalanced forces, cooldown time, the single and two-phase flow model.

Schmitt, R.L.; Cease, H.; /Fermilab; DePoy, D.; /Ohio State U.; Diehl, H.T.; Estrada, J.; Flaugher, B.; /Fermilab; Kuhlmann, S.; /Ohio State U.; Onal, Birce; Stefanik, A.; /Fermilab

2008-06-01

360

NASA Astrophysics Data System (ADS)

The effect of spatial curvature in reconstructing the cosmic expansion history and the property of dark energy is studied in this paper by smoothing the noise of the Union2.1 Type Ia Supernovae (SNIa) data with a Gaussian smoothing function. We find that the spatial curvature induces an apparent effect in reconstructing the Hubble parameter H(z), the deceleration parameter q(z), and especially on the equation of state w(z) of dark energy. Thus, when one probes the dark energy property, an assumption of a flat universe may induce critical bias and it is imperative to take account of the spatial curvature.

Fu, Xiangyun; Wu, Puxun; Yu, Hongwei; Zhou, Bingju

2013-05-01

361

Dark Energy Domination In The Virgocentric Flow

NASA Astrophysics Data System (ADS)

Dark energy (DE) was first observationally detected at large Gpc distances. If it is a vacuum energy formulated as Einstein's cosmological constant, ?, DE should also have dynamical effects at much smaller scales. Previously, we found its effects on much smaller Mpc scales in our Local Group (LG) as well as in other nearby groups. We used new HST observations of member 3D distances from the group centers and Doppler shifts. We find each group's gravity dominates a bound central system of galaxies but DE antigravity results in a radial recession increasing with distance from the group center of the outer members. Here we focus on the much larger (but still cosmologically local) Virgo Cluster and systems around it using new observations of velocities and distances. We propose an analytic model whose key parameter is the zero-gravity radius (ZGR) from the cluster center where gravity and DE antigravity balance. DE brings regularity to the Virgocentric flow. Beyond Virgo's 10 Mpc ZGR, the flow curves to approach a linear global Hubble law at larger distances. The Virgo cluster and its outer flow are similar to the Local Group and its local outflow with a scaling factor of about 10; the ZGR for Virgo is 10 times larger than that of the LG. The similarity of the two systems on the scales of 1 to 30 Mpc suggests that a quasi-stationary bound central component and an expanding outflow applies to a wide range of groups and clusters due to small scale action of DE as well as gravity. Chernin, et al 2009 Astronomy and Astrophysics 507, 1271 http://arxiv.org/abs/1006.0066 http://arxiv.org/abs/1006.0555

Byrd, Gene; Chernin, A. D.; Karachentsev, I. D.; Teerikorpi, P.; Valtonen, M.; Dolgachev, V. P.; Domozhilova, L. M.

2011-04-01

362

Stable large-scale perturbations in interacting dark-energy model

NASA Astrophysics Data System (ADS)

It is found that the evolutions of density perturbations on the super-Hubble scales are unstable in the models with dark-sector interaction Q proportional to the energy density of cold dark matter (CDM) and constant equation of state parameter of dark energy (DE). In this paper, to avoid the instabilities, we suggest a new covariant model for the energy-momentum transfer between DE and CDM. Then we show that the the large-scale instabilities of curvature perturbations can be avoided in a universe filled only by DE and CDM. Furthermore, by including the additional components of radiation and baryons, we calculate the dominant non-adiabatic modes in the radiation era and find that the modes grow in the power law with exponent at the order of unit.

Sun, Cheng-Yi; Yue, Rui-Hong

2013-08-01

363

Turbulence Kinetic Energy Equation for Dilute Suspensions.

National Technical Information Service (NTIS)

A multiphase turbulence closure model is presented which employs one transport equation, namely the turbulence kinetic energy equation. The proposed form of this equation is different from the earlier formulations in some aspects. The power spectrum of th...

T. W. Abou-Arab M. C. Roco

1989-01-01

364

Model selection as a science driver for dark energy surveys

NASA Astrophysics Data System (ADS)

A key science goal of upcoming dark energy surveys is to seek time-evolution of the dark energy. This problem is one of model selection, where the aim is to differentiate between cosmological models with different numbers of parameters. However, the power of these surveys is traditionally assessed by estimating their ability to constrain parameters, which is a different statistical problem. In this paper, we use Bayesian model selection techniques, specifically forecasting of the Bayes factors, to compare the abilities of different proposed surveys in discovering dark energy evolution. We consider six experiments - supernova luminosity measurements by the Supernova Legacy Survey, SNAP, JEDI and ALPACA, and baryon acoustic oscillation measurements by WFMOS and JEDI - and use Bayes factor plots to compare their statistical constraining power. The concept of Bayes factor forecasting has much broader applicability than dark energy surveys.

Mukherjee, Pia; Parkinson, David; Corasaniti, Pier Stefano; Liddle, Andrew R.; Kunz, Martin

2006-07-01

365

Unification of Dark Matter and Dark Energy in a Modified Entropic Force Model

NASA Astrophysics Data System (ADS)

In Verlinde's entropic force scenario of gravity, Newton's laws and Einstein equations can be obtained from the first principles and general assumptions. However, the equipartition law of energy is invalid at very low temperatures. We show clearly that the threshold of the equipartition law of energy is related with horizon of the universe. Thus, a one-dimensional Debye (ODD) model in the direction of radius of the modified entropic force (MEF) may be suitable in description of the accelerated expanding universe. We present a Friedmann cosmic dynamical model in the ODD-MEF framework. We examine carefully constraints on the ODD-MEF model from the Union2 compilation of the Supernova Cosmology Project (SCP) collaboration, the data from the observation of the large-scale structure (LSS) and the cosmic microwave background (CMB), i.e. SNe Ia+LSS+CMB. The combined numerical analysis gives the best-fit value of the model parameters ? ? 10-9 and ?m0 = 0.224, with ?2min = 591.156. The corresponding age of the universe agrees with the result of D. Spergel et al. [J.M. Bardeen, B. Carter, and S.W. Hawking, Commun. Math. Phys. 31 (1973) 161] at 95% confidence level. The numerical result also yields an accelerated expanding universe without invoking any kind of dark energy. Taking ?(? 2??D/H0) as a running parameter associated with the structure scale r, we obtain a possible unified scenario of the asymptotic flatness of the radial velocity dispersion of spiral galaxies, the accelerated expanding universe and the Pioneer 10/11 anomaly in the entropic force framework of Verlinde.

Chang, Zhe; Li, Ming-Hua; Li, Xin

2011-07-01

366

Remarks on dynamical dark energy measured by the conformal age of the universe

We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R. G. Cai, arXiv:0708.0884] where the presentday dark energy density was taken to be {rho}{sub q}{identical_to}3{alpha}{sup 2}m{sub P}{sup 2}/{eta}{sup 2}, where {eta} is the conformal time and {alpha} is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field q, the model may be adjusted to the present values of the dark energy density fraction {omega}{sub q}{approx_equal}0.73 and the equation of state parameter w{sub q}<-0.78, if the numerical constant {alpha} takes a reasonably large value, {alpha} > or approx. 2.6. However, in the presence of a nontrivial gravitational coupling of q-field to matter, say Q-tilde, the model may be adjusted to the values {omega}{sub q}{approx_equal}0.73 and w{sub q}{approx_equal}-1, even if {alpha}{approx}O(1), given that the present value of Q-tilde is large. Unlike for the model in [R. G. Cai, arXiv:0707.4049], the bound {omega}{sub q}<0.1 during big bang nucleosynthesis (BBN) may be satisfied for almost any value of {alpha}. Here we discuss some other limitations of this proposal as a viable dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.

Neupane, Ishwaree P. [Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch 8020 (New Zealand)

2007-12-15

367

Model independent early expansion history and dark energy

NASA Astrophysics Data System (ADS)

We examine model independent constraints on the high redshift and prerecombination expansion history from upcoming cosmic microwave background observations, using a combination of principal component analysis and other techniques. This can be translated to model independent limits on early dark energy and the number of relativistic species Neff. Models such as scaling (Doran-Robbers), dark radiation (?Neff), and barotropic aether fall into distinct regions of eigenspace and can be easily distinguished from each other. Incoming CMB data will map the expansion history from z=0-105, achieving subpercent precision around recombination, and enable determination of the amount of early dark energy and valuable guidance to its nature.

Samsing, Johan; Linder, Eric V.; Smith, Tristan L.

2012-12-01

368

On dark degeneracy and interacting models

NASA Astrophysics Data System (ADS)

Cosmological background observations cannot fix the dark energy equation of state, which is related to a degeneracy in the definition of the dark sector components. Here we show that this degeneracy can be broken at perturbation level by imposing two observational properties on dark matter. First, dark matter is defined as the clustering component we observe in large scale structures. This definition is meaningful only if dark energy is unperturbed, which is achieved if we additionally assume, as a second condition, that dark matter is cold, i.e. non-relativistic. As a consequence, dark energy models with equation-of-state parameter -1 <= ? < 0 are reduced to two observationally distinguishable classes with ? = -1, equally competitive when tested against observations. The first comprises the ?CDM model with constant dark energy density. The second consists of interacting models with an energy flux from dark energy to dark matter.

Carneiro, S.; Borges, H. A.

2014-06-01

369

New Light on Dark Energy (LBNL Science at the Theater)

A panel of Lab scientists ? including Eric Linder, Shirly Ho, and Greg Aldering ? along with Andrew Fraiknoi, the Bay Area's most popular astronomy explainer, gathered at the Berkeley Repertory Theatre on Monday, April 25, 2011, for a discussion about "New Light on Dark Energy." Topics will include hunting down Type 1a supernovae, measuring the universe using baryon oscillation, and whether dark energy is the true driver of the universe.

Linder, Eric; Ho, Shirly; Aldering, Greg; Fraiknoi, Andrew

2011-06-08

370

What do we really know about dark energy?

In this paper, we discuss what we truly know about dark energy. I shall argue that, to date, our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, cosmic microwave background anisotropies and observations of baryon acoustic oscillations simply tell us that the observed distance to a given redshift z is larger than the one expected from a Friedmann-Lemaître universe with matter only and the locally measured Hubble parameter. PMID:22084297

Durrer, Ruth

2011-12-28

371

New Light on Dark Energy (LBNL Science at the Theater)

A panel of Lab scientists — including Eric Linder, Shirly Ho, and Greg Aldering — along with Andrew Fraiknoi, the Bay Area's most popular astronomy explainer, gathered at the Berkeley Repertory Theatre on Monday, April 25, 2011, for a discussion about "New Light on Dark Energy." Topics will include hunting down Type 1a supernovae, measuring the universe using baryon oscillation, and whether dark energy is the true driver of the universe.

Linder, Eric; Ho, Shirly; Aldering, Greg; Fraiknoi, Andrew

2011-04-25

372

Neutrino dark energy and baryon asymmetry from Higgs sector

NASA Astrophysics Data System (ADS)

We propose a new model to explain the neutrino masses, the dark energy and the baryon asymmetry altogether. In this model, neutrinos naturally acquire small Majorana masses via type-II seesaw mechanism, while the pseudo-Nambu-Goldstone bosons associated with the neutrino mass-generation mechanism provide attractive candidates for dark energy. The baryon asymmetry of the universe is produced from the Higgs triplets decay with CP-violation.

Gu, Pei-Hong; He, Hong-Jian; Sarkar, Utpal

2007-09-01

373

Variable modified Chaplygin gas in the holographic dark energy scenario

NASA Astrophysics Data System (ADS)

The holographic principle emerged in the context of black-holes, where it was noted that a local quantum field theory can not fully describe the black holes [1]. Some long standing debates regarding the time evolution of a system, where a black hole forms and then evaporates, played the key role in the development of the holographic principle [2,3,4]. The Chaplygin gas is characterized by an exotic equation of state p=-B/?. where B is a positive constant. Role of Chaplygin gas in the accelerated universe has been studied by several authors. The above mentioned equation of state has been modified to p=-B/?^{?}, where ? lies between 0 and 1. This equation has been further modified to p=-A+B/?^{?}. This is called the modified Chaplygin gas. Debnath [5] introduced a variable modified Chaplygin gas by considering B as a function of scale factor a. In this work, we have considered that the universe is filled with normal matter and variable modified Chaplygin gas. Also we have considered the interaction between normal matter and variable modified Chaplygin gas in FRW universe. Then we have considered a correspondence between the holographic dark energy density and interacting variable modified Chaplygin gas energy density. Then we have reconstructed the potential of the scalar field which describes the variable modified Chaplygin cosmology References: [1] K. Enqvist, S. Hannested and M. S. Sloth, JCAP 2, 004 (2005). [2] L. Thorlocius, hep-th/0404098. [3] G. T. Hooft, gr-qc/9310026. [4] L. Susskind, J. Math. Phys. 36, 6377 (1995). [5] U. Debnath, Astrophys. Space Sci. 312, 295 (2007).

Chattopadhyay, Surajit; Debnath, Ujjal

2012-07-01

374

We solve the higher order nonlinear Schrödinger equation describing the propagation of ultrashort pulses in optical fibers. By means of the coupled amplitude-phase formulation fundamental (solitary wave) dark soliton solutions are found. PMID:11969877

Palacios, S L; Guinea, A; Fernández-Díaz, J M; Crespo, R D

1999-07-01

375

Nonlinear growth in modified gravity theories of dark energy

Theoretical differences in the growth of structure offer the possibility that we might distinguish between modified gravity theories of dark energy and {lambda}CDM. A significant impediment to applying current and prospective large scale galaxy and weak lensing surveys to this problem is that, while the mildly nonlinear regime is important, there is a lack of numerical simulations of nonlinear growth in modified gravity theories. A major question exists as to whether existing analytical fits, created using simulations of standard gravity, can be confidently applied. In this paper we address this, presenting results of N-body simulations of a variety of models where gravity is altered including the Dvali, Gabadadze, and Porrati model. We consider modifications that alter the Poisson equation and also consider the presence of anisotropic shear stress that alters how particles respond to the gravitational potential gradient. We establish how well analytical fits of the matter power spectrum by Peacock and Dodds and Smith et al. are able to predict the nonlinear growth found in the simulations from z=50 up to today, and also consider implications for the weak lensing convergence power spectrum. We find that the analytical fits provide good agreement with the simulations, being within 1{sigma} of the simulation results for cases with and without anisotropic stress and for scale-dependent and independent modifications of the Poisson equation. No strong preference for either analytical fit is found.

Laszlo, Istvan; Bean, Rachel [Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York, 14853 (United States)

2008-01-15

376

Cosmological effects of scalar-photon couplings: dark energy and varying-? Models

NASA Astrophysics Data System (ADS)

We study cosmological models involving scalar fields coupled to radiation and discuss their effect on the redshift evolution of the cosmic microwave background temperature, focusing on links with varying fundamental constants and dynamical dark energy. We quantify how allowing for the coupling of scalar fields to photons, and its important effect on luminosity distances, weakens current and future constraints on cosmological parameters. In particular, for evolving dark energy models, joint constraints on the dark energy equation of state combining BAO radial distance and SN luminosity distance determinations, will be strongly dominated by BAO. Thus, to fully exploit future SN data one must also independently constrain photon number non-conservation arising from the possible coupling of SN photons to the dark energy scalar field. We discuss how observational determinations of the background temperature at different redshifts can, in combination with distance measures data, set tight constraints on interactions between scalar fields and photons, thus breaking this degeneracy. We also discuss prospects for future improvements, particularly in the context of Euclid and the E-ELT and show that Euclid can, even on its own, provide useful dark energy constraints while allowing for photon number non-conservation.

Avgoustidis, A.; Martins, C. J. A. P.; Monteiro, A. M. R. V. L.; Vielzeuf, P. E.; Luzzi, G.

2014-06-01

377

Pilgrim dark energy with apparent and event horizons in non-flat universe

NASA Astrophysics Data System (ADS)

Pilgrim dark energy is an interesting proposal which is based on the conjecture that phantom-like dark energy with strong enough repulsive force can prevent the formation of a black hole. We investigate this conjecture by assuming the apparent and event horizons in non-flat universe and we develop different cosmological parameters. We construct the corresponding equation of state parameter, which indicates that its present values lie in the phantom era of the universe for different ranges of ? (pilgrim dark energy parameter) as well as ? 2 (interacting parameter). It is interesting to mention here that the pilgrim dark energy with event horizon yields a phantom region for all cases of ? 2 with ?<0. We also develop the ? ? - plane and explore the thawing as well as freezing region and ?CDM limit for these models. The statefinders plane is also constructed, which shows the correspondence with different models such as quintessence and phantom dark energy, ?CDM and Chaplygin gas. Finally, we investigate the validity of the generalized second law of thermodynamics with event horizon in a flat as well as non-flat universe.

Sharif, M.; Jawad, Abdul

2013-10-01

378

The Dark Energy Survey data management system

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) collaboration will study cosmic acceleration with a 5000 deg2 griZY survey in the southern sky over 525 nights from 2011-2016. The DES data management (DESDM) system will be used to process and archive these data and the resulting science ready data products. The DESDM system consists of an integrated archive, a processing framework, an ensemble of astronomy codes and a data access framework. We are developing the DESDM system for operation in the high performance computing (HPC) environments at the National Center for Supercomputing Applications (NCSA) and Fermilab. Operating the DESDM system in an HPC environment offers both speed and flexibility. We will employ it for our regular nightly processing needs, and for more compute-intensive tasks such as large scale image coaddition campaigns, extraction of weak lensing shear from the full survey dataset, and massive seasonal reprocessing of the DES data. Data products will be available to the Collaboration and later to the public through a virtual-observatory compatible web portal. Our approach leverages investments in publicly available HPC systems, greatly reducing hardware and maintenance costs to the project, which must deploy and maintain only the storage, database platforms and orchestration and web portal nodes that are specific to DESDM. In Fall 2007, we tested the current DESDM system on both simulated and real survey data. We used Teragrid to process 10 simulated DES nights (3TB of raw data), ingesting and calibrating approximately 250 million objects into the DES Archive database. We also used DESDM to process and calibrate over 50 nights of survey data acquired with the Mosaic2 camera. Comparison to truth tables in the case of the simulated data and internal crosschecks in the case of the real data indicate that astrometric and photometric data quality is excellent.

Mohr, Joseph J.; Adams, Darren; Barkhouse, Wayne; Beldica, Cristina; Bertin, Emmanuel; Cai, Y. Dora; da Costa, Luiz A. Nicolaci; Darnell, J. Anthony; Daues, Gregory E.; Jarvis, Michael; Gower, Michelle; Lin, Huan; Martelli, Leandro; Neilsen, Eric; Ngeow, Chow-Choong; Ogando, Ricardo L. C.; Parga, Alex; Sheldon, Erin; Tucker, Douglas; Kuropatkin, Nikolay; Stoughton, Chris

2008-07-01

379

Early dark energy at high redshifts: status and perspectives

Early dark energy models, for which the contribution to the dark energy density at high redshifts is not negligible, influence the growth of cosmic structures and could leave observable signatures that are different from the standard cosmological constant cold dark matter ({Lambda}CDM) model. In this paper, we present updated constraints on early dark energy using geometrical and dynamical probes. From WMAP five-year data, baryon acoustic oscillations and type Ia supernovae luminosity distances, we obtain an upper limit of the dark energy density at the last scattering surface (lss), {Omega}{sub EDE}(z{sub lss}) < 2.3 Multiplication-Sign 10{sup -2} (95% C.L.). When we include higher redshift observational probes, such as measurements of the linear growth factors, Gamma-Ray Bursts (GRBs) and Lyman-{alpha} forest (Ly{alpha}), this limit improves significantly and becomes {Omega}{sub EDE}(z{sub lss}) < 1.4 Multiplication-Sign 10{sup -3} (95% C.L.). Furthermore, we find that future measurements, based on the Alcock-Paczynski test using the 21cm neutral hydrogen line, on GRBs and on the Ly{alpha} forest, could constrain the behavior of the dark energy component and distinguish at a high confidence level between early dark energy models and pure {Lambda}CDM. In this case, the constraints on the amount of early dark energy at the last scattering surface improve by a factor ten, when compared to present constraints. We also discuss the impact on the parameter {gamma}, the growth rate index, which describes the growth of structures in standard and in modified gravity models.

Xia, Jun-Qing [Scuola Internazionale Superiore di Studi Avanzati, Via Beirut 2-4, I-34014 Trieste (Italy)] [Scuola Internazionale Superiore di Studi Avanzati, Via Beirut 2-4, I-34014 Trieste (Italy); Viel, Matteo, E-mail: xia@sissa.it, E-mail: viel@oats.inaf.it [INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste (Italy)] [INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste (Italy)

2009-04-15

380

NASA Astrophysics Data System (ADS)

We construct dark-dark soliton, general breather (GB), Akhmediev breather (AB), Ma soliton (MS), and rogue wave (RW) solutions of a coupled generalized nonlinear Schrödinger (CGNLS) equation. While dark-dark solitons are captured in the defocusing regime of the CGNLS system, the other solutions, namely, GB, AB, MS, and RW, are identified in the focusing regime. We also analyze the structures of GB, AB, MS, and RW profiles with respect to the four-wave mixing parameter. We show that when we increase the value of the real part of the four-wave mixing parameter, the number of peaks in the breather profile increases and the width of each peak shrinks. Interestingly, the direction of this profile also changes due to this change. As far as the RW profile is concerned the width of the peak becomes very thin when we increase the value of this parameter. Further, we consider the RW solution as the starting point, derive AB, MS, and GB in the reverse direction, and show that the solutions obtained in both directions match each other. In the course of the reverse analysis we also demonstrate how to capture the RW solutions directly from AB and MS.

Priya, N. Vishnu; Senthilvelan, M.; Lakshmanan, M.

2014-06-01

381

Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

We present constraints on the mean dark energy density, Omega_X and dark\\u000aenergy equation of state parameter, w_X, based on Chandra measurements of the\\u000aX-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy\\u000aclusters spanning the redshift range 0.07

S. W. Allen; R. W. Schmidt; H. Ebeling; A. C. Fabian; L. van Speybroeck

2004-01-01

382

New agegraphic dark energy model with the sign-changeable interaction

NASA Astrophysics Data System (ADS)

In this paper, we investigate a cosmological model with the sign-changeable interaction between new agegraphic dark energy (NADE) and dark matter. By analysis it is shown that the equation of state (EoS) of NADE can cross the phantom divide under the condition of the model parameter ?<0. In addition, we plot the trajectories of the interacting NADE model for different values of the parameters n and ? in the statefinder plane. It is found that the statefinder trajectories can be distinguished by both n and ?. Furthermore, we study the interacting NADE model by means of the w- w' analysis.

Xu, Y. D.; Huang, Z. G.

2014-04-01

383

Dark Viscous Fluid Described by a Unified Equation of State in Cosmology

NASA Astrophysics Data System (ADS)

We generalize the ?CDM model by introducing a unified EOS to describe the Universe contents modeled as dark viscous fluid, motivated by the fact that a single constant equation of state (EOS) p = -p0 (p0 > 0) reproduces the ?CDM model exactly. This EOS describes the perfect fluid term, the dissipative effect, and the cosmological constant in a unique framework and the Friedmann equations can be analytically solved. Especially, we find a relation between the EOS parameter and the renormalizable condition of a scalar field. We develop a completely numerical method to perform a ?2 minimization to constrain the parameters in a cosmological model directly from the Friedmann equations, and employ the SNe data with the parameter {A} measured from the SDSS data to constrain our model. The result indicates that the dissipative effect is rather small in the late-time Universe.

Ren, Jie; Meng, Xin-He

384

A classical treatment of the problems of dark energy, dark matter, and accelerating expansion

NASA Astrophysics Data System (ADS)

The dynamical equivalence with geodesic motions of the isentropic hydrodynamical flows, in the interior of a bounded gravitating perfect-fluid source, implies that the baryonic mass of a cosmological or astrophysical structure can be larger than its observationally determined mass. This can provide a classical explanation of the problems of the missing mass and of the flat rotation curves of disc galaxies. The generalized mass density producing the above geodesic motions can be either positive, or negative, or even vanish, implying the possibility of a spatially increasing, or decreasing, or even vanishing acceleration, depending on the distance from the center of the source as compared to the inversion distance. The extra ingredient to the generalized mass density, stemming from the source's internal physical characteristics, results in an extra, negative mass, beyond the baryonic mass of the source, which can be larger than the baryonic mass. The above negative extra mass, specifically in the case of a supercluster of galaxies and in accordance with the WMAP observational data, can give a clear and precise physical meaning to the notions of dark energy, dark matter, and baryonic mass of a supercluster, which amount to, approximately, 77%, 18%, and 5%, respectively, of the supercluster's total mass. This interpretation of the WMAP data is in accordance with a hot and extended supercluster of galaxies, as the manifestation of the cosmic-structure building and as a typical picture of the Universe, contributes to the clarification of the composition of the Cosmos, and excludes the existence of third ( and higher)-order clusters of galaxies. Treating the cosmological constant in the general-relativistic field equations as a source, in the form of a perfect fluid, the ?-fluid, the cosmological constant does not prove to be really a constant, provided that the hydrodynamic flows in the ?-fluid are not isentropic. This dictates the possibility of defining classically and for every structure its cosmological function. The cosmological function proves to share the numerical value and the basic properties of the cosmological constant, giving to the latter a clear physical meaning. Thus, the above unified scheme can also provide answers to problems, like repulsive gravity, accelerating/decelerating expansion of the Universe, stop-and-go Universe, Big Rip, and, finally, it predicts the existence of non-cosmological run-away motions at the inversion distance and beyond it.

Spyrou, N. K.

2005-01-01

385

In this paper, the modified nonlinear Schrödinger equation is investigated, which describes the femtosecond optical pulse propagation in a monomodal optical fiber. Based on the Wadati-Konno-Ichikawa system, another type of Lax pair and infinitely many conservation laws are derived. Dark and antidark soliton solutions in the normal dispersion regime are obtained by means of the Hirota method. Parametric regions for the existence of the dark and antidark soliton solutions are given. Asymptotic analysis of the two-soliton solution shows that collisions between two solitons (two antidark solitons, two dark solitons, and dark and antidark solitons) are elastic. In addition, collision between dark and antidark solitons reveals that dark and antidark solitons can co-exist on the same background in the normal dispersion regime. PMID:20481852

Li, Min; Tian, Bo; Liu, Wen-Jun; Zhang, Hai-Qiang; Wang, Pan

2010-04-01

386

Dark energy and matter evolution from lensing tomography

Reconstructed from lensing tomography, the evolution of the dark matter density field in the well-understood linear regime can provide model-independent constraints on the growth function of structure and the evolution of the dark energy density. We examine this potential in the context that high-redshift cosmology has in the future been fixed by cosmic microwave background measurements. We construct sharp tests

Wayne Hu

2002-01-01

387

The signature of dark energy on the local Hubble flow

Using N-body simulations of flat, dark energy-dominated cosmologies, we show that galaxies around simulated binary systems resembling the Local Group (LG) have low peculiar velocities, in good agreement with observational data. We have compared results for LG-like systems selected from large, high-resolution simulations of three cosmologies: a LambdaCDM model, a LambdaWDM model with a 2-keV warm dark matter candidate, and

Andrea V. Macciò; Fabio Governato; Cathy Horellou

2005-01-01

388

Figure of merit for dark energy constraints from current observational data

In order to make useful comparisons of different dark energy experiments, it is important to choose the appropriate figure of merit (FoM) for dark energy constraints. Here we show that for a set of dark energy parameters (f{sub i}), it is most intuitive to define FoM=1/{radical}(detCov(f{sub 1},f{sub 2},f{sub 3},...)), where Cov(f{sub 1},f{sub 2},f{sub 3},...) is the covariance matrix of (f{sub i}). In order for this FoM to represent the dark energy constraints in an optimal manner, the dark energy parameters (f{sub i}) should have clear physical meaning and be minimally correlated. We demonstrate two useful choices of (f{sub i}) using 182 SNe Ia (from the HST/GOODS program, the first year Supernova Legacy Survey, and nearby SN Ia surveys), [R(z{sub *}),l{sub a}(z{sub *}),{omega}{sub b}h{sup 2}] from the five year Wilkinson Microwave Anisotropy Probe observations, and Sloan Digital Sky Survey measurement of the baryon acoustic oscillation scale, assuming the Hubble Space Telescope prior of H{sub 0}=72{+-}8 (km/s) Mpc{sup -1}, and without assuming spatial flatness. We find that for a dark energy equation of state linear in the cosmic scale factor a, the correlation of (w{sub 0},w{sub 0.5}) [w{sub 0}=w{sub X}(z=0), w{sub 0.5}=w{sub X}(z=0.5), with w{sub X}(a)=3w{sub 0.5}-2w{sub 0}+3(w{sub 0}-w{sub 0.5})a] is significantly smaller than that of (w{sub 0},w{sub a}) [with w{sub X}(a)=w{sub 0}+(1-a)w{sub a}]. In order to obtain model-independent constraints on dark energy, we parametrize the dark energy density function X(z)={rho}{sub X}(z)/{rho}{sub X}(0) as a free function with X{sub 0.5}, X{sub 1.0}, and X{sub 1.5} [values of X(z) at z=0.5, 1.0, and 1.5] as free parameters estimated from data. If one assumes a linear dark energy equation of state, current observational data are consistent with a cosmological constant at 68% C.L. If one assumes X(z) to be a free function parametrized by (X{sub 0.5},X{sub 1.0},X{sub 1.5}), current data deviate from a cosmological constant at z=1 at 68% C.L., but are consistent with a cosmological constant at 95% C.L. Future dark energy experiments will allow us to dramatically increase the FoM of constraints on (w{sub 0},w{sub 0.5}), and of (X{sub 0.5},X{sub 1.0},X{sub 1.5}). This will significantly shrink the dark energy parameter space to either enable the discovery of dark energy evolution, or the conclusive evidence for a cosmological constant.

Wang Yun [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019 (United States)

2008-06-15

389

Matter power spectra in dynamical dark energy cosmologies

NASA Astrophysics Data System (ADS)

We used a suite of numerical cosmological simulations in order to investigate the effect of gas cooling and star formation on the large-scale matter distribution. The simulations follow the formation of cosmic structures in five different dark energy models: the fiducial ?cold dark matter (?CDM) cosmology and four models where the dark energy density is allowed to have a non-trivial redshift evolution. Each simulation includes a variety of gas physics, ranging from radiative cooling to UV heating and supernova feedback [although the active galactic nuclei (AGN) feedback is not incorporated]. Moreover, for each cosmology we have a control run with dark matter only, in order to allow a direct assessment of the effect of baryonic processes. We found that the power spectra of gas and stars, as well as the total matter power spectrum, are in qualitative agreement with the results of previous works not including the AGN effects in the framework of the fiducial model, although several quantitative differences exist. We used the physically motivated halo model in order to investigate the backreaction of gas and stars on the dark matter distribution, finding that it is very well reproduced by simply increasing the average dark matter halo concentration by 17 per cent, irrespective of the mass. This is in agreement with the cooling of gas dragging dark matter in the very centre of haloes, as well as adiabatic contraction steepening the relative potential wells. Moving to model universes dominated by dynamical dark energy, it turns out that they introduce a specific signature on the power spectra of the various matter components, which is qualitatively independent of the exact cosmology considered. This generic shape is well captured by the halo model if we blindly consider the cosmology dependences of the halo mass function, bias and concentration. However, the details of the dark matter power spectrum can be precisely captured only at the cost of a few slight modifications to the ingredients entering in the halo model. The backreaction of baryons on to the dark matter distribution works pretty much in the same way as in the reference ?CDM model, in the sense that it is very well described by an increment in the average halo concentration. None the less, this increment is less pronounced than in the fiducial model (only ˜10 per cent), in agreement with a series of other clues pointing towards the fact that star formation is less efficient when dark energy displays a dynamical evolution.

Fedeli, C.; Dolag, K.; Moscardini, L.

2012-01-01

390

Spherically symmeteric dark energy structure in the context of Chaplygin gas model

Spherically symmetric dark energy structures are investigated in the\\u000aframework of a generalized Chaplygin gas (GCG), which has an equation of state\\u000aof the form $P = - A\\/\\\\rho^{\\\\alpha}} $. We also study these in a modified GCG\\u000aequation of state, which includes a matter term, i.e. $P = \\\\sigma^{2} \\\\rho -\\u000aA\\/\\\\rho^{\\\\alpha}$. The results of the latter are then

Abiy G. Tekola

2007-01-01

391

Magnetized anisotropic dark energy cosmological models in scale covariant theory of gravitation

NASA Astrophysics Data System (ADS)

In the present paper, we have investigated Bianchi type II, VIII and IX spacetime in the presence of magnetized anisotropic dark energy in the Scale covariant theory of gravitation. Exact solution of the field equations are obtained by assuming the expansion anisotropy (the ratio of shear scalar to expansion scalar) to be a function of average scale factor. The isotropy of the fluid, space and expansion are examined.

Katore, S. D.; Sancheti, M. M.; Hatkar, S. P.

2014-06-01

392

Improved Dark Energy Constraints from ~100 New CfA Supernova Type Ia Light Curves

We combine the CfA3 supernovae Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013+0.066 -0.068 (0.11

Malcolm Hicken; W. Michael Wood-Vasey; Stephane Blondin; Peter Challis; Saurabh Jha; Patrick L. Kelly; Armin Rest; Robert P. Kirshner

2009-01-01

393

High energy positrons from annihilating dark matter

Results from the PAMELA experiment indicate the presence of an excess of cosmic ray positrons above 10 GeV. In this paper, we consider the possibility that this signal is the result of dark matter annihilations taking place in the halo of the Milky Way. Rather than focusing on a specific particle physics model, we take a phenomenological approach and consider a variety of masses and two-body annihilation modes, including W{sup +}W{sup -}, Z{sup 0}Z{sup 0}, bb, {tau}{sup +}{tau}{sup -}, {mu}{sup +}{mu}{sup -}, and e{sup +}e{sup -}. We also consider a range of diffusion parameters consistent with current cosmic ray data. We find that the significant upturn in the positron fraction above 10 GeV can be explained by dark matter annihilation to leptons, although very large annihilation cross sections and/or boost factors arising from inhomogeneities in the local dark matter distribution are required to produce the observed intensity of the signal. We comment on explanations for the large annihilation rate needed to explain the data and additionally on constraints from gamma rays, synchrotron emission, and cosmic ray antiproton measurements.

Cholis, Ilias; Goodenough, Lisa; Weiner, Neal [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States); Hooper, Dan [Theoretical Astrophysics Group, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Simet, Melanie [Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Theoretical Astrophysics Group, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States)

2009-12-15

394

Higgs Seesaw Mechanism as a Source for Dark Energy

NASA Astrophysics Data System (ADS)

Motivated by the seesaw mechanism for neutrinos which naturally generates small neutrino masses, we explore how a small grand-unified-theory-scale mixing between the standard model Higgs boson and an otherwise massless hidden sector scalar can naturally generate a small mass and vacuum expectation value for the new scalar which produces a false vacuum energy density contribution comparable to that of the observed dark energy dominating the current expansion of the Universe. This provides a simple and natural mechanism for producing the correct scale for dark energy, even if it does not address the long-standing question of why much larger dark energy contributions are not produced from the visible sector. The new scalar produces no discernible signatures in existing terrestrial experiments so that one may have to rely on other cosmological tests of this idea.

Krauss, Lawrence M.; Dent, James B.

2013-08-01

395

Future CMB constraints on early, cold, or stressed dark energy

We investigate future constraints on early dark energy (EDE) achievable by the Planck and CMBPol experiments, including cosmic microwave background (CMB) lensing. For the dark energy, we include the possibility of clustering through a sound speed c{sub s}{sup 2}<1 (cold dark energy) and anisotropic stresses parametrized with a viscosity parameter c{sub vis}{sup 2}. We discuss the degeneracies between cosmological parameters and EDE parameters. In particular we show that the presence of anisotropic stresses in EDE models can substantially undermine the determination of the EDE sound speed parameter c{sub s}{sup 2}. The constraints on EDE primordial energy density are however unaffected. We also calculate the future CMB constraints on neutrino masses and find that they are weakened by a factor of 2 when allowing for the presence of EDE, and highly biased if it is incorrectly ignored.

Calabrese, Erminia; Melchiorri, Alessandro [Physics Department and INFN, Universita' di Roma 'La Sapienza', Ple Aldo Moro 2, 00185, Rome (Italy); Putter, Roland de [IFIC, Universidad de Valencia-CSIC, Valencia (Spain); Institut de Ciencies del Cosmos, Barcelona (Spain); 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, USA. (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)

2011-01-15

396

Fermilab's Dark Energy Strategy: Past, Present, and Future

In the late 1990's, observations of distant supernovae led to the discovery that the expansion of the Universe is speeding up. Observations over the past decade have confirmed this remarkable finding. For the coming decade, as noted by the recent Astro 2010 Decadal Survey report, understanding the physical origin of cosmic acceleration--is it dark energy or a modification of Einstein's General Relativity?--will be a major focus of research on the Cosmic Frontier. Fermilab astrophysics experiments have played and will continue to play major roles in probing the nature of dark energy and the cause of cosmic acceleration. In this talk, I will review what we have learned about dark energy from the Sloan Digital Sky Survey and other observations. I will provide an overview of the Dark Energy Survey, which will use a powerful new instrument on a four-meter telescope in Chile to carry out a five-year survey, starting in 2012, of 300 million galaxies and thousands of supernovae and that will provide a major advance in constraining dark energy. I will close by looking toward the end of the decade, when a number of projects are poised to go further: LSST on the ground, WFIRST in space, large spectroscopic surveys such as BigBOSS and DESpec, and 21-cm radio surveys.

397

Constraints on interacting Dark Energy models from galaxy rotation curves

NASA Astrophysics Data System (ADS)

Interacting Dark Energy models have been introduced as a possible alternative to the standard ?CDM concordance cosmological scenario in order to ease the fine-tuning problems of the cosmological constant. However, the interaction of the Dark Energy field with other massive particles in the universe induces also an effective modification of structure formation processes, leading to a different dynamical behavior of density perturbations with respect to the standard scenario. In particular, high-resolution N-body simulations have recently shown that also the structural properties of highly nonlinear objects, as e.g. their average concentration at a given mass, could be significantly modified in the presence of an interaction between Dark Energy and Dark Matter. While a constant interaction strength leads to less concentrated density profiles, a steep growth in time of the coupling function has been shown to determine a large increase of halo concentrations over a wide range of masses, including the typical halos hosting luminous spiral galaxies. This determines a substantial worsening of the "cusp-core" tension arising in the standard ?CDM model and provides a direct way to constrain the form of the Dark Energy interaction. In the present paper we make use of the outcomes of some high-resolution N-body simulations of a specific class of interacting Dark Energy models to compare the predicted rotation curves of luminous spiral galaxies forming in these cosmologies against real observational data. Our results show how some specific interacting Dark Energy scenarios featuring a steep growth in time of the coupling function — which are virtually indistinguishable from ?CDM in the background — cannot fit the observed rotation curves of luminous spiral galaxies and can therefore be ruled out only on the basis of dynamical properties of small-scale structures. Our study is a pilot investigation of the effects of a Dark Energy interaction at small scales, and demonstrates how the dynamical properties of visible galaxies can in some cases provide direct constraints on the nature of Dark Energy.

Baldi, Marco; Salucci, Paolo

2012-02-01

398

Universe Filled with a Binary Mixture of Perfect Fluid and Dark Energy

NASA Astrophysics Data System (ADS)

We consider a self consistent system of Bianchi type-V gravitational field and a binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be the one obeying the usual equation of state, i.e., p= ??, with ??[0,1] whereas, the dark energy is considered to be either the quintessence like equation of state or Chaplygin gas. The equation of state parameter for dark energy ? is found to be consistent with the recent observations of SNe Ia data (Knop et al., Astrophys. J. 598:102, 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al., Astrophys. J. 606:702, 2004) 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., Astrophys. J. Suppl. 180:225, 2009; Komatsu et al., Astrophys. J. Suppl. Ser. 180:330, 2009). The physical and geometrical aspects of the models are also discussed in detail.

Samanta, G. C.; Dhala, S. N.

2013-09-01

399

The Relative Abundance of Isolated Clusters as a Probe of Dark Energy

NASA Astrophysics Data System (ADS)

Those galaxy clusters that do not belong to superclusters are referred to as isolated clusters. Their relative abundance at a given epoch may be a powerful constraint of the dark energy equation of state since it depends strongly on how fast the structures grow on the largest scale in the universe. We note that the mass function of isolated clusters can be separately evaluated through modification of the recently developed Corasaniti-Achitouv (CA) theory according to which the stochastic collapse barrier is quantified by two coefficients: the drifting average coefficient (?) and the diffusion coefficient (DB ). Regarding ? in the CA formalism as an adjustable parameter and assuming that the formation of isolated clusters corresponds to the case of DB = 0, we determine the mass function of isolated clusters by fitting the numerical results from the MICE simulations to the modified CA formula. It is found that the best-fit value of ? changes with redshift and that the CA mass function with DB = 0 agrees very well with the numerical results at various redshifts. Defining the relative abundance of isolated clusters, ? I , as the ratio of the cumulative mass function of isolated clusters to that of non-isolated clusters at a given epoch, we finally show how sensitively ? I changes with the dark energy equation of state. It is also discussed how ? I can help break the degeneracy between the dark energy equation of state and the other key cosmological parameters.

Lee, Jounghun

2012-06-01

400

Ricci dark energy in Chern-Simons modified gravity

NASA Astrophysics Data System (ADS)

In this work, we have considered the Ricci dark energy model, where the energy density of the universe is proportional to the Ricci scalar curvature, in the dynamic Chern-Simons modified gravity. We show that in this context the evolution of the scale factor is similar to that displayed by the modified Chaplygin gas.

Silva, J. G.; Santos, A. F.

2013-08-01

401

The Nature of Dark Energy from Type Ia Supernovae

NASA Astrophysics Data System (ADS)

Type Ia supernovae (SNe Ia) currently provide the most direct evidence for an accelerating Universe and for the existence of an unknown "dark energy" driving this expansion. The 5-year Supernova Legacy Survey (part of the CFHT Legacy Survey) will deliver 1000 SN Ia detections with well-sampled g'r'i'z' light curves. Using this definitive dataset, we will obtain a precise measurement of the cosmological parameters (Omega_Matter, Omega_Lambda); our goal is to determine the cosmological equation of state parameter "w" to a statistical precision of +/-0.07, testing theories for the origin of the universal acceleration. The amount of spectroscopic follow-up performed will define the success of this survey. Approximately 700 of the 1000 SNe Ia detected will be spectroscopically confirmed in a coherent program involving all of the world's major telescopes (Gemini, VLT, Keck). Gemini, with nod-and-shuffle observations, plays a pivotal role within this collaboration (Howell et al 2005). The goal for Gemini this semester is to obtain types and redshifts for 30 SN Ia candidates between redshift 0.6-0.9 (the total for Gemini over 5 years will be 300 SNe Ia candidates), contributing to a large, high quality and homogeneous SN Ia sample with photometry, spectroscopy, light-curve sampling, and colour information superior to any existing - or planned - sample. This continuing QR (quick response) proposal is for GMOS-N time; a similar request for GMOS-S time has been submitted.

Hook, Isobel

2005-08-01

402

The Nature of Dark Energy from Type Ia Supernovae

NASA Astrophysics Data System (ADS)

Type Ia supernovae (SNe Ia) currently provide the most direct evidence for an accelerating Universe and for the existence of an unknown "dark energy" driving this expansion. The 5-year Supernova Legacy Survey (SNLS) will deliver a definitive dataset with well-sampled g'r'i'z' light curves, allowing precise measurement of the cosmological parameters (Omega_Matter, Omega_Lambda). Based on our successful first year results (Astier et al., submitted) we expect the final sample to determine the cosmological equation of state parameter "w" to a statistical precision of +/-0.05, testing theories for the origin of the universal acceleration. The amount of spectroscopic follow-up performed will define the success of this survey. Approximately 700 of the 1000 SNe Ia detected will be spectroscopically confirmed in a coherent program involving all of the world's major telescopes (Gemini, VLT, Keck). Gemini, with nod-and-shuffle observations, plays a pivotal role within this collaboration (Howell et al. 2005). The goal for Gemini this semester is to obtain types and redshifts for 30 SN Ia candidates between redshift 0.6-0.9, contributing to a large, high quality and homogeneous SN Ia sample with photometry, spectroscopy, light-curve sampling, and colour information superior to any existing - or planned - sample. This is a continuing QR (quick response) proposal for GMOS-N.

Hook, Isobel

2006-02-01

403

Accelerating Dark Energy Models in Bianchi Type-V Spacetime

NASA Astrophysics Data System (ADS)

Some new exact solutions of Einstein's field equations in a spatially homogeneous and anisotropic Bianchi type-V spacetime with minimally interaction of perfect fluid and dark energy components have been obtained. To prevail the deterministic solution we choose the scale factor a(t) = ? {tnet}, which yields a time-dependent deceleration parameter (DP), representing a model which generates a transition of the universe from the early decelerating phase to the recent accelerating phase. We find that for n ? 1, the quintessence model is reproducible with present and expected future evolution of the universe. The other models (for n < 1), we observe the phantom scenario. The quintessence as well as phantom models approach to isotropy at late time. For different values of n, we can generate a class of physically viable DE models. The cosmic jerk parameter in our descended model is also found to be in good concordance with the recent data of astrophysical observations under appropriate condition. The physical and geometric properties of spatially homogeneous and anisotropic cosmological models are discussed.

Pradhan, Anirudh; Amirhashchi, Hassan

404

Precision Photometry to Study the Nature of Dark Energy

Over the past decade scientists have collected convincing evidence that the expansion of the universe is accelerating, leading to the conclusion that the content of our universe is dominated by a mysterious 'dark energy'. The fact that present theory cannot account for the dark energy has made the determination of the nature of dark energy central to the field of high energy physics. It is expected that nothing short of a revolution in our understanding of the fundamental laws of physics is required to fully understand the accelerating universe. Discovering the nature of dark energy is a very difficult task, and requires experiments that employ a combination of different observational techniques, such as type-Ia supernovae, gravitational weak lensing surveys, galaxy and galaxy cluster surveys, and baryon acoustic oscillations. A critical component of any approach to understanding the nature of dark energy is precision photometry. This report addresses just that. Most dark energy missions will require photometric calibration over a wide range of intensities using standardized stars and internal reference sources. All of the techniques proposed for these missions rely on a complete understanding of the linearity of the detectors. The technical report focuses on the investigation and characterization of 'reciprocity failure', a newly discovered count-rate dependent nonlinearity in the NICMOS cameras on the Hubble Space Telescope. In order to quantify reciprocity failure for modern astronomical detectors, we built a dedicated reciprocity test setup that produced a known amount of light on a detector, and to measured its response as a function of light intensity and wavelength.

Lorenzon, Wolfgang; Schubnell, Michael

2011-01-30

405

Galaxy Photometry Science Verification Tests for the Dark Energy Survey

NASA Astrophysics Data System (ADS)

The Dark Energy Survey (DES) is an optical imaging survey that will use a new 570-megapixel camera, the Dark Energy Camera (DECam), to image 5000 square degrees of the Southern Galactic Cap in the 5 filters grizY. The primary DES science goal is to constrain dark energy cosmological parameters using the 4 complementary science probes of galaxy clusters, weak lensing, large scale structure, and supernove. Commissioning of DECam has begun Sept. 2012, and DES science verification and the beginning of first season of operations are scheduled for late 2012. Here we report on tests of galaxy photometry undertaken as part of DES science verification. In particular, we will describe the results of measurements of galaxy detection completess and purity, checks of the fidelity of estimated photometric errors, and tests of the homogeneity of photometry across the very large, 2-degree diameter DECam focal plane.

Lin, Huan; Soares-Santos, M.; Diehl, H.; Dark Energy Survey Collaboration

2013-01-01

406

On the determination of curvature and dynamical dark energy

NASA Astrophysics Data System (ADS)

Constraining simultaneously the dark energy (DE) equation of state and the curvature of the universe is difficult due to strong degeneracies. To circumvent this problem when analyzing data it is usual to assume flatness to constrain the DE or, conversely, to assume that the DE is a cosmological constant to constrain the curvature. In this paper, we quantify the impact of such assumptions with an eye to future large surveys. We simulate future data for type Ia supernovae, the cosmic microwave background and baryon acoustic oscillations for a large range of fiducial cosmologies allowing a small spatial curvature. We take into account a possible time evolution of DE through a parameterized equation of state: w(a) = w0+(1-a)wa. We then fit the simulated data with a wrong assumption on the curvature or on the DE parameters. For a fiducial ?CDM cosmology, if flatness is incorrectly assumed in the fit and if the true curvature is within the ranges 0.01

Virey, J.-M.; Talon-Esmieu, D.; Ealet, A.; Taxil, P.; Tilquin, A.

2008-12-01

407

NASA Astrophysics Data System (ADS)

We present constraints on the mean matter density, ?m, dark energy density, ?DE, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT > 5keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1.1. Using only the fgas data for the six lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measure ?m = 0.28 +/- 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H0, and mean baryon density, ?bh2). Analysing the data for all 42 clusters, employing only weak priors on H0 and ?bh2, we obtain a similar result on ?m and a detection of the effects of dark energy on the distances to the clusters at ~99.99 per cent confidence, with ?DE = 0.86 +/- 0.21 for a non-flat ?CDM model. The detection of dark energy is comparable in significance to recent type Ia supernovae (SNIa) studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the fgas data, despite a weighted mean statistical scatter in the distance measurements of only ~5 per cent. For a flat cosmology with a constant dark energy equation of state, we measure ?m = 0.28 +/- 0.06 and w = -1.14 +/- 0.31. Combining the fgas data with independent constraints from cosmic microwave background and SNIa studies removes the need for priors on ?bh2 and H0 and leads to tighter constraints: ?m = 0.253 +/- 0.021 and w = -0.98 +/- 0.07 for the same constant-w model. Our most general analysis allows the equation of state to evolve with redshift. Marginalizing over possible transition redshifts 0.05 < zt < 1, the combined fgas + CMB + SNIa data set constrains the dark energy equation of state at late and early times to be w0 = -1.05 +/- 0.29 and wet = -0.83 +/- 0.46, respectively, in agreement with the cosmological constant paradigm. Relaxing the assumption of flatness weakens the constraints on the equation of state by only a factor of ~2. Our analysis includes conservative allowances for systematic uncertainties associated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint on ?m and powerful constraints on dark energy from the fgas data bode well for future dark energy studies using the next generation of powerful X-ray observatories, such as Constellation-X.

Allen, S. W.; Rapetti, D. A.; Schmidt, R. W.; Ebeling, H.; Morris, R. G.; Fabian, A. C.

2008-01-01

408

Dark energy model with variable q and ? in LRS Bianchi-II space-time

NASA Astrophysics Data System (ADS)

The present study deals with spatial homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi-II dark energy model in general relativity. The Einstein's field equations have been solved exactly by taking into account the proportionality relation between one of the components of shear scalar (?11) and expansion scalar ( ?), which, for some suitable choices of problem parameters, yields time dependent equation of state (EoS) and deceleration parameter (DP), representing a model which generates a transition of universe from early decelerating phase to present accelerating phase. The physical and geometrical behavior of universe have been discussed in detail.

Saha, Bijan; Yadav, Anil Kumar

2012-10-01

409

Imprint of scalar dark energy on cosmic microwave background polarization

NASA Astrophysics Data System (ADS)

We study the imprint of a coupling of scalar dark energy to a photon on the cosmic microwave background polarization. Both the time-evolving field value and the perturbation of the scalar induce B-mode polarization. For a wide range of scalar dark energy models allowed by current observational data, we conclude that future cosmic microwave background data will find either a cosmic parity violation in a temperature-polarization correlation due to the field value, or perturbation-induced B-mode polarization that is indistinguishable from that generated by primordial gravitational waves.

Lee, Seokcheon; Liu, Guo-Chin; Ng, Kin-Wang

2014-03-01

410

Quintessence and phantom dark energy from ghost D-branes

We present a novel dark-energy candidate, based upon the existence and dynamics of ghost D-branes in a warped compactification of type IIB string theory. Gp-branes cancel the combined BPS sectors of the Dp-branes, while they preserve the same supersymmetries. We show that this scenario can naturally lead to either quintessence or phantomlike behaviors, depending on the form of the involved potentials and brane tension. As a specific example we investigate the static, dark-energy dominated solution subclass.

Saridakis, Emmanuel N. [Department of Physics, University of Athens, GR-15771 Athens (Greece); Ward, John [Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, V8P 1A1 (Canada)

2009-10-15

411

CMB anisotropy induced by tachyonic perturbations of dark energy

We consider the effects of possible tachyonic perturbations of dark energy on the CMB anisotropy. Such perturbations emerge, in particular, in models with phantom dark energy violating Lorentz invariance. Therefore, we discuss tachyonic perturbations with a Lorentz-violating dispersion relation. We show that the corresponding contribution to the CMB anisotropy can have an appreciable amplitude, while the angular spectrum has a distinct maximum. These predictions are compared with observational data. The tachyonic contribution slightly improves the agreement between the theory and observations, but this improvement is statistically insignificant and our analysis gives constraints on the tachyonic perturbation amplitude.

Libanov, M. V.; Rubakov, V. A. [Russian Academy of Sciences, Institute for Nuclear Research (Russian Federation); Sazhina, O. S., E-mail: tedeshka@mail.ru; Sazhin, M. V. [Moscow State University, Sternberg Astronomical Institute (Russian Federation)], E-mail: moimaitre@mail.ru

2009-02-15

412

Reducing Zero-point Systematics in Dark Energy Supernova Experiments

We study the effect of filter zero-point uncertainties on future supernova dark energy missions. Fitting for calibration parameters using simultaneous analysis of all Type Ia supernova standard candles achieves a significant improvement over more traditional fit methods. This conclusion is robust under diverse experimental configurations (number of observed supernovae, maximum survey redshift, inclusion of additional systematics). This approach to supernova fitting considerably eases otherwise stringent mission cali- bration requirements. As an example we simulate a space-based mission based on the proposed JDEM satellite; however the method and conclusions are general and valid for any future supernova dark energy mission, ground or space-based.

Faccioli, Lorenzo; Kim, Alex G; Miquel, Ramon; Bernstein, Gary; Bonissent, Alain; Brown, Matthew; Carithers, William; Christiansen, Jodi; Connolly, Natalia; Deustua, Susana; Gerdes, David; Gladney, Larry; Kushner, Gary; Linder, Eric; McKee, Shawn; Mostek, Nick; Shukla, Hemant; Stebbins, Albert; Stoughton, Chris; Tucker, David

2011-04-01

413

Majorana neutrino superfluidity and stability of neutrino dark energy

We demonstrate that Majorana neutrinos can form Cooper pairs due to long-range attractive forces and show BCS superfluidity in a class of mass varying neutrino dark energy models. We describe the condensates for Majorana neutrinos and estimate the value of the gap, critical temperature, and Pippard coherence length for a simple neutrino dark energy model. In the strong coupling regime bosonic degree of freedom can become important, and Bose-Einstein condensate may govern the dynamics for the mass varying neutrino models. Formation of the condensates can significantly alter the instability scenario in the mass varying neutrino models.

Bhatt, Jitesh R.; Sarkar, Utpal [Physical Research Laboratory, Ahmedabad 380009 (India)

2009-08-15

414

Cosmological coincidence problem in interacting dark energy models

The interacting dark energy model with interaction term Q={lambda}{sub m}H{rho}{sub m}+{lambda}{sub d}H{rho}{sub d} is considered. By studying the model near the transition time, in which the system crosses the {omega}=-1 phantom divide line, the conditions needed to overcome the coincidence problem is investigated. The phantom model, as a candidate for dark energy, is considered, and for two specific examples, the quadratic and exponential phantom potentials, it is shown that it is possible the system crosses the {omega}=-1 line, meanwhile the coincidence problem is alleviated, the two facts that have root in observations.

Sadjadi, H. Mohseni; Alimohammadi, M. [Department of Physics, University of Tehran, North Karegar Avenue, Tehran (Iran, Islamic Republic of)

2006-11-15

415

Limits on decaying dark energy density models from the CMB temperature-redshift relation

NASA Astrophysics Data System (ADS)

The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (Phys Rev D 54:2571, 1996). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the cosmic microwave background (CMB) which depends on the effective equation of state w eff and on the "adiabatic index" ?. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zel'dovich observations and at higher redshift from quasar absorption line spectra, we find w eff = -0.97 ± 0.03, adopting for the adiabatic index ? = 4/3, in good agreement with current estimates and still compatible with w eff = -1, implying that the dark energy content being constant in time.

Jetzer, Philippe; Puy, Denis; Signore, Monique; Tortora, Crescenzo

2011-04-01

416

Strong lensing systems as a probe of dark energy in the universe

Current advances in observational cosmology suggest that our Universe is flat and dominated by dark energy. Out of many particular models of dark energy present in the literature we focus on four: quintessence, quintessence with time varying equation of state, braneworld model and generalized Chaplygin gas model. In this paper we discuss the utility of strong lensing systems for providing additional constraints on dark energy models. In particular, we use an Einstein cross gravitational lensing system HST 14176+5226 to confront its measured characteristics with background cosmologies invoked in the context of dark energy. The image separations in the system depend on angular distances to the lens and to the source, which in turn are determined by background cosmology. This opens a possibility to constrain cosmological model provided that we have good knowledge of the lens model. We demonstrate that recent measurements of velocity dispersion in the lensing galaxy made by Subaru telescope seem to be consistent with independently obtained bounds on parameters of cosmological models considered. The method we describe is based on angular diameter distances and could become a valuable tool of cosmological model inference complementary to Hubble diagram technique based on luminosity distance.

Biesiada, Marek [Department of Astrophysics and Cosmology, University of Silesia, Uniwersytecka 4, 40-007 Katowice (Poland)

2006-01-15

417

Exploring parameter constraints on quintessential dark energy: The Albrecht-Skordis model

We consider the effect of future dark energy experiments on 'Albrecht-Skordis' (AS) models of scalar field dark energy using the Monte Carlo Markov chain method. We deal with the issues of parametrization of these models, and have included spatial curvature as a parameter, finding it to be important. We use the Dark Energy Task Force (DETF) simulated data to represent future experiments and report our results in the form of likelihood contours in the chosen parameter space. Simulated data is produced for cases where the background cosmology has a cosmological constant, as well as cases where the dark energy is provided by the AS model. The latter helps us demonstrate the power of DETF Stage 4 data in the context of this specific model. Though the AS model can produce equations of state functions very different from what is possible with the w{sub 0}-w{sub a} parametrization used by the DETF, our results are consistent with those reported by the DETF.

Barnard, Michael; Abrahamse, Augusta; Albrecht, Andreas; Bozek, Brandon; Yashar, Mark [Department of Physics, One Shields Avenue, University of California, Davis, California 95616 (United States)

2008-05-15

418

Mirror world and superstring-inspired hidden sector of the Universe, dark matter and dark energy

NASA Astrophysics Data System (ADS)

We develop a concept of parallel existence of the ordinary (O) and hidden (H) worlds. We compare two cases: (1) when the hidden sector of the Universe is a mirror counterpart of the ordinary world, and (2) when it is a superstring-inspired shadow world described, in contrast to the mirror world, by a symmetry group (or by a chain of groups), which does not coincide with the ordinary world symmetry group. We construct a cosmological model assuming the existence of the superstring-inspired E6 unification, broken at the early stage of the Universe to SO(10)×U(1)Z—in the O-world, and to SU(6)'×SU(2)?'—in the H-world. As a result, we obtain the low-energy symmetry group GSM'×SU(2)?' in the shadow world, instead of the standard model group GSM existing in the O-world. The additional non-Abelian SU(2)?' group with massless gauge fields, ”thetons,” is responsible for dark energy. Considering a quintessence model of cosmology with an inflaton ? and an axion a?, which is a pseudo Nambu-Goldstone boson induced by the SU(2)?'-group anomaly, we explain the origin of dark energy, dark matter and ordinary matter. In the present model we review all cosmological epochs (inflation, reheating, recombination and nucleosynthesis), and give our version of the baryogenesis. The cosmological constant problem is also briefly discussed.

Das, C. R.; Laperashvili, L. V.; Nielsen, H. B.; Tureanu, A.

2011-09-01

419

NASA Astrophysics Data System (ADS)

In order to test if there is energy transfer between dark energy (DE) and dark matter (DM), we investigate cosmological constraints on two forms of nontrivial interaction between the DM sector and the sector responsible for the acceleration of the universe, in light of the newly revised observations including OHD, CMB, BAO and SNe Ia. More precisely, we find the same tendencies for both phenomenological forms of the interaction term Q = 3?H?, i.e. the parameter ? to be a small number, |?| ? 10-2. However, concerning the sign of the interaction parameter, we observe that ? > 0 when the interaction between dark sectors is proportional to the energy density of dust matter, whereas the negative coupling (? < 0) is preferred by observations when the interaction term is proportional to DE density. We further discuss two possible explanations to this incompatibility and apply a quantitative criteria to judge the severity of the coincidence problem. Results suggest that the ?mIDE model with a positive coupling may alleviate the coincidence problem, since its coincidence index C is smaller than that for the ?dIDE model, the interacting quintessence and phantom models by four orders of magnitude.

Cao, Shuo; Liang, Nan

2013-12-01

420

Dynamical behavior of the extended holographic dark energy with the Hubble horizon

The extended holographic dark energy model with the Hubble horizon as the infrared cutoff avoids the problem of the circular reasoning of the holographic dark energy model. Unfortunately, it is hit with the no-go theorem. In this paper, we consider the extended holographic dark energy model with a potential, V({phi}), for the Brans-Dicke scalar field. With the addition of a potential for the Brans-Dicke scalar field, the extended holographic dark energy model using the Hubble horizon as the infrared cutoff is a viable dark energy model, and the model has the dark energy dominated attractor solution.

Liu Jie; Gong Yungui; Chen Ximing [College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China)

2010-04-15

421

The matter power spectrum of dark energy models and the Harrison-Zel'dovich prescription

NASA Astrophysics Data System (ADS)

According to the Harrison-Zel'dovich prescription, the amplitude of matter density perturbations at horizon crossing is the same at all scales. Based on this prescription, we show how to construct the matter power spectrum of generic dark energy models from the power spectrum of a ?CDM model without the need of solving in full the dynamical equations describing the evolution of all energy density perturbations. Our approach allows to make model predictions of observables that can be expressed in terms of the matter power spectrum alone, such as the amplitude of matter fluctuations, peculiar velocities, cosmic microwave background temperature anisotropies on large angular scales or the weak lensing convergence spectrum. Then, models that have been tested only at the background level using the rate of the expansion of the Universe can now be tested using data on gravitational clustering and on large scale structure. This method can save a lot of effort in checking the validity of dark energy models. As an example of the accurateness of the approximation used, we compute the power spectrum of different dark energy models with constant equation of state parameter (wDE = -0.1, -0.5 and -0.8, ruled out by observations but easy to compare to numerical solutions) using our methodology and discuss the constraints imposed by the low multipoles of the cosmic microwave background.

Duran, Ivan; Atrio-Barandela, Fernando; Pavón, Diego

2012-04-01

422

Interacting viscous dark energy in Bianchi type-I Universe

NASA Astrophysics Data System (ADS)

A solution to the coincidence and Big Rip problems on the bases of an anisotropic space-time is proposed. To do so, we study the interaction between viscous dark energy and dark matter in the scope of the Bianchi type-I Universe. We parameterize the viscosity and the interaction between the two fluids by constants ? 0 and ? respectively. A detailed investigation on the cosmological implications of this parametrization has been made. We have also performed a geometrical diagnostic by using the statefinder pairs { s, r} and { q, r} in order to differentiate between different dark energy models. Moreover, we fit the coupling parameter ? as well as the Hubble's parameter H 0 of our model by minimizing the ? 2 through the age differential method, involving a direct measurement of H.

Amirhashchi, Hassan

2014-06-01

423

Dynamics of Kantowski-Sachs universe with magnetized anisotropic Dark Energy

NASA Astrophysics Data System (ADS)

Kantowski-Sachs cosmological model in the presence of magnetized anisotropic dark energy is investigated. The energy-momentum tensor consists of anisotropic fluid with anisotropic EoS p= ?? and a uniform magnetic field of energy density ? 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 does not approach isotropy through the evolution of the universe.

Katore, S. D.; Adhav, K. S.; Sancheti, M. M.

2012-01-01

424

Dark energy from logarithmically modified gravity and deformed Coleman-Weinberg potential

NASA Astrophysics Data System (ADS)

Recent astrophysical measurements strongly suggest the existence of a missing energy component dubbed dark energy that is responsible for the current accelerated expansion of the universe. A new class of modified gravity theory is introduced which yields a universe accelerating in time and dominated by dark energy. The new modified gravity model constructed here concurrently includes a Gauss-Bonnet invariant term, barotropic fluid with a time-dependent equation of state parameter, a Coleman-Weinberg (CW) potential-like expression V(varphi) = ?varphim ln varphin and a new Einstein-Hilbert term f(R, varphi) = E(varphi)R which depends on both the scalar curvature and the scalar field varphi through a generic logarithmic function E(varphi) = ln varphi. Here m and n take different values from the standard CW potential and ? is a real parameter. It was shown that the presence of these terms provides many useful features which are discussed in some detail.

Rami El-Nabulsi, Ahmad

2011-07-01

425

NASA Astrophysics Data System (ADS)

It was found that the model with interaction between cold dark matter (CDM) and dark energy (DE) proportional to the energy density of CDM ? m and constant equation of state of DE w d suffered from instabilities of the density perturbations on the super-Hubble scales. Here we suggest a new covariant model for the energy-momentum transfer between CDM and DE. Then using the covariant model, we analyze the evolution of density perturbations on the super-Hubble scale. We find that the instabilities can be avoided in the model with constant w d and interaction proportional to ? m . Furthermore, we analyze the dominant non-adiabatic mode in the radiation era and find that the mode grows regularly.

Sun, Cheng-Yi; Song, Yu; Yue, Rui-Hong

2013-02-01

426

a New Alternative Model to Dark Energy

NASA Astrophysics Data System (ADS)

The recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. This may be the evidence of the breakdown of the standard Friedmann equation. Instead of a linear function of the matter density, we consider a general function of the matter density to modify the Freidmann equation. We propose a new model which explains the recent acceleration and the past deceleration. Furthermore, the new model also gives a decelerated universe in the future. The new model gives ?m0=0.46 and zT=0.44.

Gong, Yungui; Chen, Xi-Ming; Duan, Chang-Kui