Constant Equation Of State Parameter Research Articles

Quintessence and Holographic Dark Energy infTGravity

We regardfTtheory as an efficient tool to explain the current cosmic acceleration and associate its evolution with the known dark energy models. The numerical scheme is applied to reconstructfTtheory from dark energy model with constant equation of state parameter and holographic dark energy model. We set the model parametersωϑandcas describing the different evolution eras and show the distinctive behavior of each case realized infTtheory. We also present the future evolution of reconstructedfTand find that it is consistent with the recent observations.

Cosmic acceleration in non-canonical scalar field model: an interacting scenario

In this paper we have studied the dynamics of accelerating scenario within the framework of scalar field models possessing a non-canonical kinetic term. In this toy model, the scalar field is allowed to interact with the dark matter component through a source term. We have assumed a specific form for the coupling term and then have studied the dynamics of the scalar field having a constant equation of state parameter. We have also carried out the dynamical system study of such interacting non-canonical scalar field models for power law potentials for some physically relevant specific values of the model parameters. It has been found that the only for two particular stable fixed points of the system, an accelerating solution is possible and the universe will settle down to a $\Lambda$CDM universe in future and thus there is no future singularity in this model.

Extending the supernova Hubble diagram toz~ 1.5 with theEuclidspace mission

We forecast dark energy constraints that could be obtained from a new large sample of Type Ia supernovae where those at high redshift are acquired with the Euclid space mission. We simulate a three-prong SN survey: a z<0.35 nearby sample (8000 SNe), a 0.2<z<0.95 intermediate sample (8800 SNe), and a 0.75<z<1.55 high-z sample (1700 SNe). The nearby and intermediate surveys are assumed to be conducted from the ground, while the high-z is a joint ground- and space-based survey. This latter survey, the "Dark Energy Supernova Infra-Red Experiment" (DESIRE), is designed to fit within 6 months of Euclid observing time, with a dedicated observing program. We simulate the SN events as they would be observed in rolling-search mode by the various instruments, and derive the quality of expected cosmological constraints. We account for known systematic uncertainties, in particular calibration uncertainties including their contribution through the training of the supernova model used to fit the supernovae light curves. Using conservative assumptions and a 1-D geometric Planck prior, we find that the ensemble of surveys would yield competitive constraints: a constant equation of state parameter can be constrained to sigma(w)=0.022, and a Dark Energy Task Force figure of merit of 203 is found for a two-parameter equation of state. Our simulations thus indicate that Euclid can bring a significant contribution to a purely geometrical cosmology constraint by extending a high-quality SN Hubble diagram to z~1.5. We also present other science topics enabled by the DESIRE Euclid observations

Vacuum energy sequestering and cosmic dynamics

We explicitly compute the dynamics of closed homogeneous and isotropic universes permeated by a single perfect fluid with a constant equation of state parameter $w$ in the context of a recent reformulation of general relativity, proposed in [1], which prevents the vacuum energy from acting as a gravitational source. This is done using an iterative algorithm, taking as an initial guess the background cosmological evolution obtained using standard general relativity in the absence of a cosmological constant. We show that, in general, the impact of the vacuum energy sequestering mechanism on the dynamics of the universe is significant, except for the $w=1/3$ case where the results are identical to those obtained in the context of general relativity with a null cosmological constant. We also show that there are well behaved models in general relativity that do not have a well behaved counterpart in the vacuum energy sequestering paradigm studied in this paper, highlighting the specific case of a quintessence scalar field with a linear potential.

Material models of dark energy

A new class of “dark energy” models is reviewed and developed, in which the relativistic theory of solids is used to construct material models of dark energy. These are models which include the effects of a continuous medium with well defined physical properties at the level of linearized perturbations. The formalism is constructed for a medium with arbitrary symmetry, and then specialised to isotropic media (which will be the case of interest for the majority of cosmological applications). The theory of relativistic isotropic viscoelastic media is developed whilst keeping in mind that we ultimately want to observationally constrain the allowed properties of the material model. This is done by obtaining the viscoelastic equations of state for perturbations (the entropy and anisotropic stress), as well as identifying the consistent corner of the theory which has constant equation of state parameter . A connection to the non‐relativistic theory of solids is obtained by identifying the two quadratic invariants that are needed to construct the energy‐momentum tensor, namely the Rayleigh dissipation function and Lagrangian for perturbations. Finally, the notion is developed that the viscoelastic behavior of the medium can be thought of as a non‐minimally coupled massive gravity theory. This also provides a tool‐kit for constructing consistent generalizations of coupled dark energy theories.

Cosmological constraints on interacting dark energy with redshift-space distortion after Planck data

The interacting dark energy model could propose a effective way to avoid the coincidence problem. In this paper, dark energy is taken as a fluid with a constant equation of state parameter $w_x$. In a general gauge, we could obtain two sets of different perturbation equations when the momentum transfer potential is vanished in the rest frame of dark matter or dark energy. There are many kinds of interacting forms from the phenomenological considerations, here, we choose $Q=3H\xi_x\rho_x$ which owns the stable perturbations in most cases. Then, according to the Markov Chain Monte Carlo method, we constrain the model by currently available cosmic observations which include cosmic microwave background radiation, baryon acoustic oscillation, type Ia supernovae, and $f\sigma_8(z)$ data points from redshift-space distortion. Jointing the geometry tests with the large scale structure information, the results show a tighter constraint on the interacting model than the case without $f\sigma_8(z)$ data. We find the interaction rate in 3$\sigma$ regions: $\xi_x=0.00372_{-0.00372- 0.00372-0.00372}^{+0.000768+0.00655+0.0102}$. It means that the recently cosmic observations favor a small interaction rate between the dark sectors, at the same time, the measurement of redshift-space distortion could rule out a large interaction rate in the 1$\sigma$ region.

Dark energy in thermal equilibrium with the cosmological horizon?

According to a generalization of black hole thermodynamics to a cosmological framework, it is possible to define a temperature for the cosmological horizon. The hypothesis of thermal equilibrium between the dark energy and the horizon has been considered by many authors. We find the restrictions imposed by this hypothesis on the energy transfer rate (${Q}_{i}$) between the cosmological fluids, assuming that the temperature of the horizon has the form $T=b/2\ensuremath{\pi}R$, where $R$ is the radius of the horizon. We more specifically consider two types of dark energy: Chaplygin gas (CG) and dark energy with a constant equation of state parameter ($w\mathrm{DE}$). In each case, we show that for a given radius $R$, there is a unique term ${Q}_{\mathrm{de}}$ that is consistent with thermal equilibrium. We also consider the situation where, in addition to dark energy, other fluids (cold matter, radiation) are in thermal equilibrium with the horizon. We find that the interaction terms required for this will generally violate energy conservation ($\ensuremath{\sum}_{i}{Q}_{i}=0$).

Stable large-scale perturbations in interacting dark-energy model

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.

Cosmological solutions of a nonlocal model with a perfect fluid

A nonlocal gravity model which does not assume theexistence of a new dimensional parameter in the action and includesa function f(□−1R), with □ the d'Alembertian operator,is studied. By specifying an exponential form for the function fand including a matter sector with a constant equation of stateparameter, all available power-law solutions in the Jordan frame areobtained. New power-law solutions in the Einstein frame are alsoprobed. Furthermore, the relationship between power-law solutions inboth frames, established through conformal transformation, issubstantially clarified. The correspondence between power-lawsolutions in these two frames is proven to be a very useful tool inorder to obtain new solutions in the Einstein frame.

The importance of local measurements for cosmology

Abstract We explore how local, cosmology-independent measurements of the Hubble constant and the age of the Universe help to provide a powerful consistency check of the currently favored cosmological model (flat ΛCDM) and model-independent constraints on cosmology. We use cosmic microwave background (CMB) data to define the model-dependent cosmological parameters, and add local measurements to assess consistency and determine whether extensions to the model are justified. At current precision, there is no significant tension between the locally measured Hubble constant and age of the Universe (with errors of 3% and 5%, respectively) and the corresponding parameters derived from the CMB. However, if errors on the local measurements could be decreased by a factor of two, one could decisively conclude if there is tension or not. We also compare the local and CMB data assuming simple extensions of the flat, ΛCDM model (including curvature, dark energy with a constant equation of state parameter not equal to −1, non-zero neutrino masses and a non-standard number of neutrino families) and find no need for these extra parameters; in particular, we constrain the effective number of neutrino species to be less than 4 at 95% confidence. We show that local measurements provide constraints on the curvature and the equation of state of dark energy nearly orthogonal to those of the CMB. We argue that cosmology-independent measurements of local quantities at the percent level would be very useful to explore cosmology in a model-independent way.

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

According to the Harrison-Zel'dovich prescription, the amplitudeof matter density perturbations at horizon crossing is the same atall scales. Based on this prescription, we show how to constructthe matter power spectrum of generic dark energy models fromthe power spectrum of a ΛCDM model without the need ofsolving in full the dynamical equations describing the evolutionof all energy density perturbations. Our approach allows to makemodel predictions of observables that can be expressed in terms ofthe matter power spectrum alone, such as the amplitude of matterfluctuations, peculiar velocities, cosmic microwave backgroundtemperature anisotropies on large angular scalesor the weak lensing convergence spectrum.Then, models that have been tested only at the background levelusing the rate of the expansion of the Universe can now be testedusing data on gravitational clustering and on large scalestructure. This method can save a lot of effort in checking thevalidity of dark energy models. As an example of the accurateness of theapproximation 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.

Constraining H0 in general dark energy models from Sunyaev-Zeldovich/X-ray technique and complementary probes

In accelerating dark energy models, the estimates of the Hubbleconstant, H0, from Sunyaev-Zel'dovich effect (SZE) and X-raysurface brightness of galaxy clusters may depend on the mattercontent (ΩM), the curvature (ΩK) and the equationof state parameter (ω). In this article, by using a sampleof 25 angular diameter distances of galaxy clusters described by theelliptical β model obtained through the SZE/X-ray technique,we constrain H0 in the framework of a general ΛCDM model(arbitrary curvature) and a flat XCDM model with a constant equationof state parameter ω = px/ρx. In order to avoid the use ofpriors in the cosmological parameters, we apply a joint analysisinvolving the baryon acoustic oscillations (BAO) and the CMBShift Parameter signature. By taking into account thestatistical and systematic errors of the SZE/X-ray technique weobtain for nonflat ΛCDM model H0 = 74+5.0−4.0 kms−1 Mpc−1(1σ) whereas for a flat universe withconstant equation of state parameter we find H0 = 72+5.5−4.0km s−1 Mpc−1(1σ). By assuming that galaxyclusters are described by a spherical β model these resultschange to H0 = 62+8.0−7.0 and H0 = 59+9.0−6.0 kms−1 Mpc−1(1σ), respectively. The results fromelliptical description are in good agreement with independentstudies from the Hubble Space Telescope key project andrecent estimates based on the Wilkinson Microwave AnisotropyProbe, thereby suggesting that the combination of these threeindependent phenomena provides an interesting method to constrainthe Hubble constant. As an extra bonus, the adoption of theelliptical description is revealed to be a quite realisticassumption. Finally, by comparing these results with a recentdetermination for a flat ΛCDM model using only the SZE/X-raytechnique and BAO, we see that the geometry has a very weakinfluence on H0 estimates for this combination of data.

A parametrization of the growth index of matter perturbations in various Dark Energy models and observational prospects using a Euclid-like survey

We provide exact solutions to the cosmological matter perturbation equation in a homogeneous FLRW universe with a vacuum energy that can be parametrized by a constant equation of state parameter w and a very accurate approximation for the Ansatz w(a) = w0+wa(1−a). We compute the growth index γ = log f(a)/log Ωm(a), and its redshift dependence, using the exact and approximate solutions in terms of Legendre polynomials and show that it can be parametrized as γ(a) = γ0+γa(1−a) in most cases. We then compare four different types of dark energy (DE) models: wΛCDM, DGP, f(R) and a LTB-large-void model, which have very different behaviors at z≳1. This allows us to study the possibility to differentiate between different DE alternatives using wide and deep surveys like Euclid, which will measure both photometric and spectroscopic redshifts for several hundreds of millions of galaxies up to redshift z ≃ 2. We do a Fisher matrix analysis for the prospects of differentiating among the different DE models in terms of the growth index, taken as a given function of redshift or with a principal component analysis, with a value for each redshift bin for a Euclid-like survey. We use as observables the complete and marginalized power spectrum of galaxies P(k) and the Weak Lensing (WL) power spectrum. We find that, using P(k), one can reach (2%, 5%) errors in (w0,wa), and (4%, 12%) errors in (γ0,γa), while using WL we get errors at least twice as large. These estimates allow us to differentiate easily between DGP, f(R) models and ΛCDM, while it would be more difficult to distinguish the latter from a variable equation of state parameter or LTB models using only the growth index.

Thermodynamics of phantom energy in the presence of a Reissner-Nordström black hole

In this paper, we study the validity of the generalized second law (GSL) in phantom dominated universe in the presence of a Reissner-Nordström (RN) black hole. Our study is independent of the origin of the phantom like behavior of the considered universe. We also discuss the GSL in the neighborhood of transition from quintessence to phantom regime. We show that for a constant equation of state parameter, the GSL may be satisfied provided that the temperature is proportional to de Sitter temperature. It is shown that in models with (only) a transition from quintessence to phantom regime the generalized second law does not hold in the transition epoch. Next we show that if the phantom energy has a chemical potential, then the GSL will hold if the mass of black hole is above from a critical value.

BOUNCING MODELS WITH A COSMOLOGICAL CONSTANT

Most bouncing models contain a contracting phase from a very large and rarefied state, where dark energy might have had an important role. If this is that case, the presence of dark energy can modify the initial conditions and evolution of cosmological perturbations, changing the known results already obtained in the literature concerning their amplitude and spectrum. In this work, we assume the simplest and most appealing candidate for dark energy, the cosmological constant, and study its influence on the evolution of cosmological perturbations during the contracting phase of a bouncing model, containing also a perfect fluid with constant equation of state parameter w. We show that, due to the vacuum state choice we have to make when a cosmological constant is present, the spectrum of the perturbations are substantially altered. We conclude that, in this case, the presence of a stiff matter fluid in the contracting phase is needed in order to have a scale invariant spectrum of perturbations in the expanding phase.

SPECTRA ANDHUBBLE SPACE TELESCOPELIGHT CURVES OF SIX TYPE Ia SUPERNOVAE AT 0.511 &lt;z&lt; 1.12 AND THE UNION2 COMPILATION

We report on work to increase the number of well-measured Type Ia supernovae (SNe Ia) at high redshifts. Light curves, including high signal-to-noise HST data, and spectra of six SNe Ia that were discovered during 2001 are presented. Additionally, for the two SNe with z>1, we present ground-based J-band photometry from Gemini and the VLT. These are among the most distant SNe Ia for which ground based near-IR observations have been obtained. We add these six SNe Ia together with other data sets that have recently become available in the literature to the Union compilation (Kowalski et al. 2008). We have made a number of refinements to the Union analysis chain, the most important ones being the refitting of all light curves with the SALT2 fitter and an improved handling of systematic errors. We call this new compilation, consisting of 557 supernovae, the Union2 compilation. The flat concordance LambdaCDM model remains an excellent fit to the Union2 data with the best fit constant equation of state parameter w=-0.997^{+0.050}_{-0.054} (stat) ^{+0.077}_{-0.082} (stat+sys\ together) for a flat universe, or w=-1.035^{+0.055}_{-0.059} (stat)^{+0.093}_{-0.097} (stat+sys together) with curvature. We also present improved constraints on w(z). While no significant change in w with redshift is detected, there is still considerable room for evolution in w. The strength of the constraints depend strongly on redshift. In particular, at z > 1, the existence and nature of dark energy are only weakly constrained by the data.

Instability in interacting dark energy and dark matter fluids

We consider a model of dark energy interacting with dark matter. An interaction may be interesting for the solution of the coincidence problem. We model dark energy as a fluid with constant equation of state parameter w and study background and linear perturbations. While non-interacting models with constant w are well behaved, a simple interaction leads to an instability in the dark sector perturbations at early times. The instability is severe, non-adiabatic and independent of how weak the coupling is.

Cluster scaling relations from cosmological hydrodynamic simulations in a dark-energy dominated universe

Clusters are potentially powerful tools for cosmology provided their observed properties such as the Sunyaev-Zel'dovich (SZ) or X-ray signals can be translated into physical quantities like mass and temperature. Scaling relations are the appropriate mean to perform this translation. It is therefore, important to understand their evolution and their modifications with respect to the physics and to the underlying cosmology. In this spirit, we investigate the effect of dark energy on the X-ray and SZ scaling relations. The study is based on the first hydro-simulations of cluster formation for diferent models of dark energy. We present results for four dark energy models which differ from each other by their equations of state parameter, $w$. Namely, we use a cosmological constant model $w=-1$ (as a reference), a perfect fluid with constant equation of state parameter $w=-0.8$ and one with $w = -1.2$ and a scalar field model (or quintessence) with varying $w$. We generate N-body/hydrodynamic simulations that include radiative cooling with the public version of the Hydra code, modified to consider an arbitrary dark energy component. We produce cluster catalogues for the four models and derive the associated X-ray and SZ scaling relations. We find that dark energy has little effect on scaling laws making it safe to use the $\Lambda$CDM scalings for conversion of observed quantities into temperature and masses.

Constraints on Dark Energy Models from Weak Gravity Conjecture

We study the constraints on the dark energy model with constant equation of state parameter w = p/ρ and the holographic dark energy model by using the weak gravity conjecture. The combination of weak gravity conjecture and the observational data gives w < -0.7 at the 3σ confidence level. The holographic dark energy model realized by a scalar field is in swampland.

Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets

We present a new compilation of Type Ia supernovae (SNe Ia), a new data set of low-redshift nearby-Hubble-flow SNe, and new analysis procedures to work with these heterogeneous compilations. This compilation of 414 SNe Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older data sets, as well as the recently extended data set of distant supernovae observed with the Hubble Space Telescope (HST). A single, consistent, and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is ΩΛ = 0.713+ 0.027−0.029(stat)+ 0.036−0.039(sys) , for a flat, ΛCDM universe. Assuming a constant equation of state parameter, w, the combined constraints from SNe, BAO, and CMB give w = − 0.969+ 0.059−0.063(stat)+ 0.063−0.066(sys) . While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a w that varies with redshift. In particular, the current SN data do not yet significantly constrain w at z > 1. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.