Showing papers on "Deceleration parameter published in 2010"

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

Abstract: Locally rotationally symmetric Bianchi type I cosmological models are examined in the presence of dynamically anisotropic dark energy and perfect fluid. We assume that the dark energy (DE) is minimally interacting, has dynamical energy density, anisotropic equation of state parameter (EoS). The conservation of the energy-momentum tensor of the DE is assumed to consist of two separately additive conserved parts. A special law is assumed for the deviation from isotropic EoS, which is consistent with the assumption on the conservation of the energy-momentum tensor of the DE. Exact solutions of Einstein’s field equations are obtained by assuming a special law of variation for the mean Hubble parameter, which yields a constant value of the deceleration parameter. Geometrical and kinematic properties of the models and the behaviour of the anisotropy of the dark energy have been carried out. The models give dynamically anisotropic expansion history for the universe that allows to fine tune the isotropization of the Bianchi metric, hence the CMB anisotropy.

High-Redshift Cosmography

Abstract: We constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires a very minimal set of assumptions (namely to specify a metric) and does not rely on the dynamical equations for gravity. On the data side, we consider the most recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows to further extend the cosmographic fit up to z = 6.6, i.e. up to redshift for which one could start to resolve the low z degeneracy among competing cosmological models. In order to reliably control the cosmographic approach at high redshifts, we adopt the expansion in the improved parameter y = z/(1+z). This series has the great advantage to hold also for z > 1 and hence it is the appropriate tool for handling data including non-nearby distance indicators. We find that Gamma Ray Bursts, probing higher redshifts than Supernovae, have constraining power and do require (and statistically allow) a cosmographic expansion at higher order than Supernovae alone. Exploiting the set of data from Union and GRBs catalogues, we show (for the first time in a purely cosmographic approach parametrized by deceleration q0, jerk j0, snap s0) a definitively negative deceleration parameter q0 up to the 3σ confidence level. We present also forecasts for realistic data sets that are likely to be obtained in the next few years.

Viscous dark fluid universe

Abstract: We investigate the cosmological perturbation dynamics for a universe consisting of pressureless baryonic matter and a viscous fluid, the latter representing a unified model of the dark sector. In the homogeneous and isotropic background the total energy density of this mixture behaves as a generalized Chaplygin gas. The perturbations of this energy density are intrinsically nonadiabatic and source relative entropy perturbations. The resulting baryonic matter power spectrum is shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also Hubble-function and supernovae Ia data, shows that, different from other studies, there exists a maximum in the probability distribution for a negative present value ${q}_{0}\ensuremath{\approx}\ensuremath{-}0.53$ of the deceleration parameter. Moreover, while previous descriptions on the basis of generalized Chaplygin-gas models were incompatible with the matter power-spectrum data since they required a much too large amount of pressureless matter, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis.

Bianchi Type-I Anisotropic Dark Energy Models with Constant Deceleration Parameter

Abstract: New dark energy models in anisotropic Bianchi type-I (B-I) space-time with variable EoS parameter and constant deceleration parameter have been investigated in the present paper. The Einstein's field equations have been solved by applying a variation law for generalized Hubble's parameter in B-I space-time. The variation law for Hubble's parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential form. Using these two forms, Einstein's field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. The equation of state (EoS) parameter $\omega$ is found to be time dependent and its existing range for this model is in good agreement with the recent observations of SNe Ia data, SNe Ia data (with CMBR anisotropy) and galaxy clustering statistics. The cosmological constant $\Lambda$ is found to be a decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations.

An Interacting Two-Fluid Scenario for Dark Energy in FRW Universe

Abstract: We study the evolution of the dark energy parameter within the scope of a spatially flat and isotropic Friedmann-Robertson-Walker (FRW) model filled with barotropic fluid and dark energy. To obtain the deterministic solution we choose the scale factor $a(t) = \sqrt{t e^{t}}$ which yields a time dependent deceleration parameter (DP). In doing so we consider the case minimally coupled with dark energy to the perfect fluid as well as direct interaction with it.

Some Bianchi Type-V Models of Accelerating Universe with Dark Energy

Abstract: The paper deals with a spatially homogeneous and anisotropic universe filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally together with a special law of variation for the average Hubble's parameter in order to solve the Einstein's field equations. The law yields two explicit forms of the scale factor governing the Bianchi-V space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of Bianchi-V universe. It has been found that dark energy dominates the Universe at the present epoch, which is consistent with the observations. The Universe achieves flatness after the dominance of dark energy. The physical behavior of the Universe has been discussed in detail.

Dark energy models with variable equation of state parameter

Abstract: Two phenomenological variable Λ models, viz.Λ ~ (ȧ/a)2 and Λ ~ ρ, have been studied under the assumption that the equation of state parameter ω is a function of time. The selected Λ models are found to be equivalent both in four and five dimensions. The possibility of signature flip of the deceleration parameter is also shown.

Cosmological Constraints on the Sign-Changeable Interactions

Abstract: Recently, Cai and Su [Phys. Rev. D {\bf 81}, 103514 (2010)] found that the sign of interaction $Q$ in the dark sector changed in the approximate redshift range of $0.45\,\lsim\, z\,\lsim\, 0.9$, by using a model-independent method to deal with the observational data. In fact, this result raises a remarkable problem, since most of the familiar interactions cannot change their signs in the whole cosmic history. Motivated by the work of Cai and Su, we have proposed a new type of interaction in a previous work [H. Wei, Nucl. Phys. B {\bf 845}, 381 (2011)]. The key ingredient is the deceleration parameter $q$ in the interaction $Q$, and hence the interaction $Q$ can change its sign when our universe changes from deceleration ($q>0$) to acceleration ($q<0$). In the present work, we consider the cosmological constraints on this new type of sign-changeable interactions, by using the latest observational data. We find that the cosmological constraints on the model parameters are fairly tight. In particular, the key parameter $\beta$ can be constrained to a narrow range.

Imitating accelerated expansion of the Universe by matter inhomogeneities: corrections of some misunderstandings

Abstract: A number of misunderstandings about modeling the apparent accelerated expansion of the Universe and about the ‘weak singularity’ are clarified: (1) Of the five definitions of the deceleration parameter given by Hirata and Seljak (HS), only q 1 is a correct invariant measure of acceleration/deceleration of expansion. The q 3 and q 4 are unrelated to acceleration in an inhomogeneous model. (2) The averaging over directions involved in the definition of q 4 does not correspond to what is done in observational astronomy. (3) HS’s equation (38) connecting q 4 to the flow invariants gives self-contradictory results when applied at the centre of symmetry of the Lemaitre–Tolman (L–T) model. The intermediate equation (31) that determines q 3' is correct, but approximate, so it cannot be used for determining the sign of the deceleration parameter. Even so, at the centre of symmetry of the L–T model, it puts no limitation on the sign of q 3'(0). (4) The ‘weak singularity’ of Vanderveld et al. is a conical profile of mass density at the centre—a perfectly acceptable configuration. (5) The so-called ‘critical point’ in the equations of the ‘inverse problem’ for a central observer in an L–T model is a manifestation of the apparent horizon (AH)—a common property of the past light cones in zero-lambda L–T models, perfectly manageable if the equations are correctly integrated.

Dark energy models with variable equation of state parameter

Abstract: The dark energy models with variable equation of state parameter $\omega$ is investigated by using law of variation of Hubble's parameter that yields the constant value of deceleration parameter. The equation of state parameter $\omega$ is found to be time dependent and its existing range for this model is consistent with the recent observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy clustering statistics. The physical significance of the dark energy models has also been discussed.

Comment on ``Interacting holographic dark energy model and generalized second law of thermodynamics in a non-flat universe", by M.R. Setare (JCAP 01 (2007) 023)

Abstract: Author of ref. 1, M.R. Setare (JCAP 01 (2007) 023), by redefining the event horizon measured from the sphere of the horizon as the system's IR cut-off for an interacting holographic dark energy model in a non-flat universe, showed that the generalized second law of thermodynamics is satisfied for the special range of the deceleration parameter. His paper includes an erroneous calculation of the entropy of the cold dark matter. Also there are some missing terms and some misprints in the equations of his paper. Here we present that his conclusion is not true and the generalized second law is violated for the present time independently of the deceleration parameter.

Effects of inhomogeneities on apparent cosmological observables: "fake" evolving dark energy

Abstract: Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing cosmological constant $\Lambda$, we investigate how the presence of a local spherically-symmetric inhomogeneity can affect apparent cosmological observables, such as the deceleration parameter or the effective equation of state of dark energy (DE), derived from the luminosity distance under the assumption that the real space-time is exactly homogeneous and isotropic. The presence of a local underdensity is found to produce apparent phantom behavior of DE, while a locally overdense region leads to apparent quintessence behavior. We consider relatively small large scale inhomogeneities which today are not linear and could be seeded by primordial curvature perturbations compatible with CMB bounds. Our study shows how observations in an inhomogeneous $\Lambda$CDM universe with initial conditions compatible with the inflationary beginning, if interpreted under the wrong assumption of homogeneity, can lead to the wrong conclusion about the presence of "fake" evolving dark energy instead of $\Lambda$.

Agegraphic Dark Energy Model in Non-Flat Universe: Statefinder Diagnostic and $w-w^{\prime}$ Analysis

Abstract: We study the interacting agegraphic dark energy (ADE) model in non-flat universe by means of statefinder diagnostic and $w-w^{\prime}$ analysis. First, the evolution of EoS parameter ($w_d$) and deceleration parameter ($q$) in terms of scale factor for interacting ADE model in non-flat universe are calculated. Dependence of $w_d$ on the ADE model parameters $n$ and $\alpha$ in different spatial curvatures is investigated. We show that the evolution of $q$ is dependent on the type of spatial curvature, beside of dependence on parameters $n$ and $\alpha$. The accelerated expansion takes place sooner in open universe and later in closed universe compare with flat universe. Then, we plot the evolutionary trajectories of the interacting ADE model for different values of the parameters $n$ and $\alpha$ as well as for different contributions of spatial curvature, in the statefinder parameters plane. In addition to statefinder, we also investigate the ADE model in non-flat universe with $w-w^{\prime}$ analysis.

Through the looking glass: why the 'cosmic horizon' is not a horizon

Abstract: The present standard model of cosmology, Λ cold dark matter (ΛCDM), contains some intriguing coincidences. Not only are the dominant contributions to the energy density approximately of the same order at the present epoch, but we also note that contrary to the emergence of cosmic acceleration as a recent phenomenon, the time-averaged value of the deceleration parameter over the age of the Universe is nearly zero. Curious features like these in ΛCDM give rise to a number of alternate cosmologies being proposed to remove them, including models with an equation of state w=−1/3. In this paper, we examine the validity of some of these alternate models and we also address some persistent misconceptions about the Hubble sphere and the event horizon that lead to erroneous conclusions about cosmology.

Cosmic Behavior, Statefinder Diagnostic and $w-w^{\prime}$ Analysis for Interacting NADE model in Non-flat Universe

Abstract: We give a brief review of interacting NADE model in non-flat universe. we study the effect of spatial curvature $\Omega_{k}$, interaction coefficient $\alpha $ and the main parameter of NADE, $n$, On EoS parameter $w_{d}$ and deceleration parameter $q$. We obtain a minimum value for $n$ in both early and present time, in order to that our DE model crosses the phantom divide. Also in a closed universe, changing the sign of $q$ is strongly dependent on $\alpha$. It has been shown that the quantities $w_{d}$ and $q$ have a different treatment for various spatial curvature. At last, we calculate the statefinder diagnostic and $ w-w^{\prime}$ analysis in non flat universe. In non flat universe, the statefinder trajectory is discriminated by both $n$ and $\alpha$.

AGEGRAPHIC DARK ENERGY MODEL IN THE NON-FLAT UNIVERSE: STATEFINDER DIAGNOSTIC AND w–w′ ANALYSIS

Abstract: We study the interacting agegraphic dark energy (ADE) model in the non-flat universe by means of the statefinder diagnostic and an w–w′ analysis. First, the evolution of EoS parameter (wd) and deceleration parameter (q) are calculated in terms of scale-factor for interacting ADE models in the non-flat universe. The dependency of wd on the ADE model parameters n and α in different spatial curvatures is investigated. We show that the evolution of q is dependent on the type of spatial curvature, besides dependening on parameters n and α. The accelerated expansion takes place sooner in the open universe and latter in the closed universe as compared with the flat universe. Then, we plot the evolutionary trajectories of the interacting ADE models for different values of the parameters n and α, as well as for different contributions of spatial curvature, in the plane of statefinder parameters. In addition to the statefinder, we also investigate the ADE model in the non-flat universe with w–w′ analysis.

Cosmographic constraints on a class of Palatini f ( R ) gravity

Abstract: Modified gravity, known as $f(R)$ gravity, has presently been applied to cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryons and dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form $f(R)=R\ensuremath{-}\ensuremath{\alpha}/{R}^{n}$ within the Palatini approach. Although extensively discussed in recent literature, cosmological tests are inconclusive about the true signal of $n$. This is particularly important to define which kind of corrections (infrared or high-energy) to general relativity this class of theory represents. We shed some light on this question by examining the evolution of the deceleration parameter for these theories. We find that for a large range of $\ensuremath{\alpha}$, these models can only have positive values for $n$, placing thus a broad restriction on this class of gravity.

Probing the course of cosmic expansion with a combination of observational data

Abstract: We study the cosmic expansion history by reconstructing the deceleration parameter q(z) from the SDSS-II type Ia supernova sample (SNIa) with two different light curve fits (MLCS2k2 and SALT-II), the baryon acoustic oscillation (BAO) distance ratio, the cosmic microwave background (CMB) shift parameter, and the lookback time-redshift (LT) from the age of old passive galaxies. Three parametrization forms for the equation of state of dark energy (CPL, JBP, and UIS) are considered. Our results show that, for the CPL and the UIS forms, MLCS2k2 SDSS-II SNIa+BAO+CMB and MLCS2k2 SDSS-II SNIa+BAO+CMB+LT favor a currently slowing-down cosmic acceleration, but this does not occur for all other cases, where an increasing cosmic acceleration is still favored. Thus, the reconstructed evolutionary behaviors of dark energy and the course of the cosmic acceleration are highly dependent both on the light curve fitting method for the SNIa and the parametrization form for the equation of state of dark energy.

Probing the course of cosmic expansion with a combination of observational data

Abstract: We study the cosmic expansion history by reconstructing the deceleration parameter $q(z)$ from the SDSS-II type Ia supernova sample (SNIa) with two different light curve fits (MLCS2k2 and SALT-II), the baryon acoustic oscillation (BAO) distance ratio, the cosmic microwave background (CMB) shift parameter, and the lookback time-redshift (LT) from the age of old passive galaxies. Three parametrization forms for the equation of state of dark energy (CPL, JBP, and UIS) are considered. Our results show that, for the CPL and the UIS forms, MLCS2k2 SDSS-II SNIa+BAO+CMB and MLCS2k2 SDSS-II SNIa+BAO+CMB+LT favor a currently slowing-down cosmic acceleration, but this does not occur for all other cases, where an increasing cosmic acceleration is still favored. Thus, the reconstructed evolutionary behaviors of dark energy and the course of the cosmic acceleration are highly dependent both on the light curve fitting method for the SNIa and the parametrization form for the equation of state of dark energy.

Some Exact Bianchi Type-V Perfect Fluid Cosmological Models with Heat Flow and Decaying Vacuum Energy Density Λ: Expressions for Some Observable Quantities

Abstract: In this paper we have obtained some new exact solutions of Einstein’s field equations in a spatially homogeneous and anisotropic Bianchi type-V space-time with perfect fluid distribution along with heat-conduction and decaying vacuum energy density Λ by applying the variation law for generalized Hubble’s parameter that yields a constant value of deceleration parameter. We find that the constant value of deceleration parameter is reasonable for the present day universe. The variation law for Hubble’s parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential form. Using these two forms, Einstein’s field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. The cosmological constant Λ is found to be a decreasing function of time and positive which is corroborated by results from recent supernovae Ia observations. Expressions for look-back time-redshift, neoclassical tests (proper distance d(z)), luminosity distance red-shift and event horizon are derived and their significance are described in detail. The physical and geometric properties of spatially homogeneous and anisotropic cosmological models are discussed.

Five-dimensional metric f(R) gravity and the accelerated universe

Abstract: The metric f(R) theories of gravity are generalized to five-dimensional spacetimes. By assuming a hypersurface-orthogonal Killing vector field representing the compact fifth dimension, the five-dimensional theories are reduced to their four-dimensional formalism. Then we study the cosmology of a special class of f(R) = �R m models in a spatially flat FRW spacetime. It is shown that the parameter m can be constrained to a certain range by the current observed deceleration parameter, and its lower bound corresponds to the Kaluza-Klein theory. It turns out that both expansion and contraction of the extra dimension may prescribe the smooth transition from the deceleration era to the acceleration era in the recent past as well as an accelerated scenario for the present universe. Hence five-dimensional f(R) gravity can naturally account for the present accelerated expansion of the universe. Moreover, the models predict a transition from acceleration to deceleration in the future, followed by a cosmic recollapse within finite time. This differs from the prediction of the five-dimensional Brans-Dicke theory but is in consistent with a recent prediction based on loop quantum cosmology. PACS numbers: 04.50.-h, 98.80.Es

Crossing the phantom divide with Ricci-like holographic dark energy

Abstract: In this work we study the dark energy problem by adopting an holographic model proposed recently in the literature. In this model there has been postulated an energy density ρ∼R, where R is the Ricci scalar curvature. Under this consideration, we have obtained a cosmological scenario which arises from considering two non-interacting fluids along the lines of a reasonable Ansatz for the cosmic coincidence parameter. We have adjusted the involved parameters in the model according to the observational data, showing that the equation of state for the dark energy exhibits a cross through the −1 barrier. Additionally, we have found a disagreement of these parameters in comparison with a scalar field theory approach.

The Viscous Dark Fluid Universe

Abstract: We investigate the cosmological perturbation dynamics for a universe consisting of pressureless baryonic matter and a viscous fluid, the latter representing a unified model of the dark sector. In the homogeneous and isotropic background the \textit{total} energy density of this mixture behaves as a generalized Chaplygin gas. The perturbations of this energy density are intrinsically non-adiabatic and source relative entropy perturbations. The resulting baryonic matter power spectrum is shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also Hubble-function and supernovae Ia data, shows that, different from other studies, there exists a maximum in the probability distribution for a negative present value $q_0 \approx - 0.53$ of the deceleration parameter. Moreover, while previous descriptions on the basis of generalized Chaplygin gas models were incompatible with the matter power spectrum data since they required a much too large amount of pressureless matter, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis.

Power-law solutions and accelerated expansion in scalar-tensor theories

Abstract: We find exact power-law solutions for scalar-tensor theories and clarify the conditions under which they can account for an accelerated expansion of the Universe. These solutions have the property that the signs of both the Hubble rate and the deceleration parameter in the Jordan frame may be different from the signs of their Einstein-frame counterparts. For special parameter combinations we identify these solutions with asymptotic attractors that have been obtained in the literature through dynamical-system analysis. We establish an effective general-relativistic description for which the geometrical equivalent of dark energy is associated with a time dependent equation of state. The present value of the latter is consistent with the observed cosmological ``constant.'' We demonstrate that this type of power-law solution for accelerated expansion cannot be realized in $f(R)$ theories.

Constraints on variable chaplygin gas model from type ia supernovae and baryon acoustic oscillations

Abstract: We apply the type Ia supernovae union dataset and the baryon acoustic oscillations data at z = 0.2 and z = 0.35 to constrain variable Chaplygin gas (VCG) model as the unification of dark matter and dark energy. It is shown that the confidence levels for VCG model parameters are $B_{s}=0.075^{+0.016}_{-0.013}(1\sigma)$ $^{+0.026}_{-0.020}(2\sigma)$, $n=1.30^{+0.46}_{-0.49}(1\sigma)$ $^{+0.74}_{-0.81}(2\sigma)$. And it indicates that the values of transition redshift and current deceleration parameter are: $z_{T}=0.74^{+0.05}_{-0.05}(1\sigma)$, $q_{0}=-0.54^{+0.07}_{-0.07}(1\sigma)$. In addition, we plot the evolution trajectory of the VCG model in the statefinder parameter r–s plane and show the discrimination between this scenario and other dark energy models.

Comment on "A holographic model of dark energy and the thermodynamics of a non-flat accelerated expanding universe", by M.R. Setare and S. Shafei (JCAP 09 (2006) 011)

Abstract: Authors of ref. [1], M.R. Setare and S. Shafei (JCAP 09 (2006) 011), studied the thermodynamics of a holographic dark energy model in a non-flat universe enclosed by the apparent horizon RA and the event horizon measured from the sphere of the horizon named L. In section 3 in ref. [1], Authors showed that for RA the generalized second law of thermodynamics is respected, while for L it is satisfied for the special range of the deceleration parameter. Here we present that their calculations for RA should be revised. Also we show that their conclusion for L is not true and the generalized second law is hold for the present time independently of the deceleration parameter. Also if we take into account the contribution of dark matter in the generalized second law which is absent in ref. [1], then the generalized second law for L is violated for the present time.

Comment on `A holographic model of dark energy and the thermodynamics of a non-flat accelerated expanding universe', by M.R. Setare and S. Shafei (JCAP 09 (2006) 011, arXiv:gr-qc/0606103)

Abstract: Authors of ref. [1], M.R. Setare and S. Shafei (JCAP 09 (2006) 011), studied the thermodynamics of a holographic dark energy model in a non-flat universe enclosed by the apparent horizon $R_A$ and the event horizon measured from the sphere of the horizon named $L$. In section 3 in ref. [1], Authors showed that for $R_A$ the generalized second law of thermodynamics is respected, while for $L$ it is satisfied for the special range of the deceleration parameter. Here we present that their calculations for $R_A$ should be revised. Also we show that their conclusion for $L$ is not true and the generalized second law is hold for the present time independently of the deceleration parameter. Also if we take into account the contribution of dark matter in the generalized second law which is absent in ref. [1], then the generalized second law for $L$ is violated for the present time.