Showing papers on "Deceleration parameter published in 2009"

Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS

Abstract: We present a tomographic cosmological weak lensing analysis of the HST COSMOS Survey. Applying our lensing-optimized data reduction, principal component interpolation for the ACS PSF, and improved modelling of charge-transfer inefficiency, we measure a lensing signal which is consistent with pure gravitational modes and no significant shape systematics. We carefully estimate the statistical uncertainty from simulated COSMOS-like fields obtained from ray-tracing through the Millennium Simulation. We test our pipeline on simulated space-based data, recalibrate non-linear power spectrum corrections using the ray-tracing, employ photometric redshifts to reduce potential contamination by intrinsic galaxy alignments, and marginalize over systematic uncertainties. We find that the lensing signal scales with redshift as expected from General Relativity for a concordance LCDM cosmology, including the full cross-correlations between different redshift bins. For a flat LCDM cosmology, we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023, sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness and using HST Key Project and BBN priors only, we find a negative deceleration parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing independent evidence for the accelerated expansion of the Universe. For a flat wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark energy constraints are still relatively weak solely due to the limited area of COSMOS. However, they provide an important demonstration for the usefulness of tomographic weak lensing measurements from space. (abridged)

Influence of structure formation on the cosmic expansion

Abstract: We investigate the effect that the average backreaction of structure formation has on the dynamics of the cosmological expansion, within the concordance model. Our approach in the Poisson gauge is fully consistent up to second order in a perturbative expansion about a flat Friedmann background, including a cosmological constant. We discuss the key length scales which are inherent in any averaging procedure of this kind. We identify an intrinsic homogeneity scale that arises from the averaging procedure, beyond which a residual offset remains in the expansion rate and deceleration parameter. In the case of the deceleration parameter, this can lead to a quite large increase in the value, and may therefore have important ramifications for dark energy measurements, even if the underlying nature of dark energy is a cosmological constant. We give the intrinsic variance that affects the value of the effective Hubble rate and deceleration parameter. These considerations serve to add extra intrinsic errors to our determination of the cosmological parameters, and, in particular, may render attempts to measure the Hubble constant to percent precision overly optimistic.

Bayesian analysis and constraints on kinematic models from union SNIa

Abstract: The kinematic expansion history of the universe is investigated by using the 307 supernovae type Ia from the Union Compilation set. Three simple model parameterizations for the deceleration parameter (constant, linear and abrupt transition) and two different models that are explicitly parametrized by the cosmic jerk parameter (constant and variable) are considered. Likelihood and Bayesian analyses are employed to find best fit parameters and compare models among themselves and with the flat ΛCDM model. Analytical expressions and estimates for the deceleration and cosmic jerk parameters today (q0 and j0) and for the transition redshift (zt) between a past phase of cosmic deceleration to a current phase of acceleration are given. All models characterize an accelerated expansion for the universe today and largely indicate that it was decelerating in the past, having a transition redshift around 0.5. The cosmic jerk is not strongly constrained by the present supernovae data. For the most realistic kinematic models the 1σ confidence limits imply the following ranges of values: q0 [−0.96, −0.46], j0 [−3.2, −0.3] and zt [0.36, 0.84], which are compatible with the ΛCDM predictions, q0 = −0.57 ± 0.04, j0 = −1 and zt = 0.71 ± 0.08. We find that even very simple kinematic models are equally good to describe the data compared to the concordance ΛCDM model, and that the current observations are not powerful enough to discriminate among all of them.

Cosmological model with interactions in the dark sector

Abstract: A cosmological model for the present Universe is analyzed whose constituents are a non-interacting baryonic matter field and interacting dark matter and dark energy fields. The dark energy and dark matter are coupled through their effective barotropic indexes, which are considered as functions of the ratio of their energy densities. Two asymptotically stable cases are investigated for the ratio of the dark energy densities which have their parameters adjusted by considering best fits to Hubble function data. It is shown that the deceleration parameter, the density parameters, and the luminosity distance have the correct behavior which is expected for a viable present scenario of the Universe.

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 $q_0$, jerk $j_0$, snap $s_0$) a definitively negative deceleration parameter $q_0$ up to the 3$\sigma$ confidence level. We present also forecasts for realistic data sets that are likely to be obtained in the next few years.

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

Abstract: Author of Ref. [1], M.R. Setare (JCAP 01, 023, 2007, arXiv:hep-th/0701242), 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.

f(R) gravity theories in the Palatini formalism constrained from strong lensing

Abstract: f(R) gravity, capable of driving the late-time acceleration of the universe, is emerging as a promising alternative to dark energy. Various f(R) gravity models have been intensively tested against probes of the expansion history, including Type Ia supernovae (SNIa), the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO). In this paper, we propose to use the statistical lens sample from Sloan Digital Sky Survey Quasar Lens Search Data Release 3 (SQLS DR3) to constrain f(R) gravity models. This sample can probe the expansion history up to z ∼ 2.2, higher than what probed by current SNIa and BAO data. We adopt a typical parametrization of the form f(R) = R - αH(-R/H 2 0 β with a and β constants. For β = 0 [A cold dark matter (ACDM)], we obtain the best-fitting value of the parameter a = -4.193, for which the 95 per cent confidence interval that is [-4.633, -3.754]. This best-fitting value of a corresponds to the matter density parameter Ω m0 = 0.301, consistent with constraints from other probes. Allowing β to be free, the best-fitting parameters are (a, β) = (-3.777,0.06195). Consequently, we give Ω m0 = 0.285 and the deceleration parameter q 0 = -0.544. At the 95 per cent confidence level, a and β are constrained to [-4.67, -2.89] and [-0.078, 0.202], respectively. Clearly, given the currently limited sample size, we can only constrain β within the accuracy of Δβ ∼ 0.1 and thus cannot distinguish between ACDM andf(R) gravity with high significance, and actually, the former lies in the 68 per cent confidence contour. We expect that the extension of the SQLS DR3 lens sample to the SDSS DR5 and SDSS-II will make constraints on the model more stringent.

Constraints on kinematic model from recent cosmic observations: SN Ia, BAO and observational Hubble data

Abstract: In this paper, linear first order expansion of deceleration parameter q(z) = q0+q1(1−a) (M1), constant jerk j = j0 (M2) and third order expansion of luminosity distance (M3) are confronted with cosmic observations: SCP 307 SN Ia, BAO and observational Hubble data (OHD). Likelihood is implemented to find the best fit model parameters. All these models give the same prediction of the evolution of the universe which is undergoing accelerated expansion currently and experiences a transition from decelerated expansion to accelerated expansion. But, the transition redshift depends on the concrete parameterized form of the model assumed. M1 and M2 give value of transition redshift about zt ~ 0.6. M3 gives a larger one, say zt ~ 1. The χ2/dof implies almost the same goodness of the models. But, for its badness of evolution of deceleration parameter at high redshift z > 1, M3 can not be reliable. M1 and M2 are compatible with ΛCDM model at the 2σ and 1σ confidence levels respectively. M3 is not compatible with ΛCDM model at 2σ confidence level. From M1 and M2 models, one can conclude that the cosmic data favor a cosmological model having j0 < −1.

Present acceleration of the universe, holographic energy and brans–dicke theory

Abstract: The present-day accelerated expansion of the universe is naturally addressed within the Brans–Dicke theory just by using holographic dark energy model with inverse of Hubble scale as IR cutoff and power law temporal behavior of scale factor. It is also concluded that if the universe continues to expand, then one day it might be completely filled with dark energy.

The Hubble effective potential

Abstract: We generalize the effective potential to scalar field configurations which are proportional to the Hubble parameter of a homogeneous and isotropic background geometry. This may be useful in situations for which curvature effects are significant. We evaluate the one loop contribution to the Hubble Effective Potential for a massless scalar with arbitrary conformal and quartic couplings, on a background for which the deceleration parameter is constant. Among other things, we find that inflationary particle production leads to symmetry restoration at late times.

Model- and calibration-independent test of cosmic acceleration

Abstract: We present a calibration-independent test of the accelerated expansion of the universe using supernova type Ia data. The test is also model-independent in the sense that no assumptions about the content of the universe or about the parameterization of the deceleration parameter are made and that it does not assume any dynamical equations of motion. Yet, the test assumes the universe and the distribution of supernovae to be statistically homogeneous and isotropic. A significant reduction of systematic effects, as compared to our previous, calibration-dependent test, is achieved. Accelerated expansion is detected at significant level (4.3σ in the 2007 Gold sample, 7.2σ in the 2008 Union sample) if the universe is spatially flat. This result depends, however, crucially on supernovae with a redshift smaller than 0.1, for which the assumption of statistical isotropy and homogeneity is less well established.

New Agegraphic Dark Energy in Brans–Dicke Theory

Abstract: In this paper, we investigate the new agegraphic dark energy model in the framework of Brans–Dicke theory, which is a natural extension of the Einstein's general relativity. In this framework the form of the new agegraphic dark energy density takes as ρq = 3n2 Φ(t)η−2, where η is the conformal age of the universe and Φ(t) is the Brans–Dicke scalar field representing the inverse of the time-variable Newton's constant. We derive the equation of state of the new agegraphic dark energy and the deceleration parameter of the universe in the Brans–Dicke theory. It is very interesting to find that in the Brans–Dicke theory the agegraphic dark energy realizes quintom-like behavior, i.e., its equation of state crosses the phantom divide w = –1 during the evolution. We also compare the situation of the agegraphic dark energy model in the Brans–Dicke theory with that in the Einstein's theory. In addition, we discuss the new agegraphic dark energy model with interaction in the framework of the Brans–Dicke theory.

Bianchi Type V Cosmological Models with Constant Deceleration Parameter in General Relativity

Abstract: We investigate Bianchi type V cosmological models with bulk viscous fluid source. Exact solutions of the Einstein field equations are presented via a suitable power law assumption for the Hubble parameter. We show that the corresponding solutions retain the well established features of the standard cosmology and in addition, are in accordance with recent type Ia supernovae observations. Some observational parameters for the models have also been discussed.

COSMIC CONSTRAINTS ON DECELERATION PARAMETER WITH Sne Ia AND CMB

Abstract: In this paper, a parametrized deceleration parameter q(a) = q0+q1(1-a) is constrained by using the current cosmic observational data from type Ia Supernova (Sne Ia) and Cosmic Microwave Background Radiation (CMB). When the CMB dataset is added as a strong constraint, it is found that the 1σ error is largely reduced. The values of transition redshift zT from decelerated expansion to accelerated expansion and current deceleration parameter q0 are larger than that obtained from the case where Sne Ia dataset is used alone. With comparison to the case of Sne Ia 182 dataset used,15 it is found that the value of transition redshift is smaller than that in Sne 192 dataset case. This is the so-called dataset dependence.

A Redshift-Magnitude Relation for Non-Uniform Pressure Universes

Abstract: A redshift-magnitude relation for the two exact non-uniform pressure spherically symmetric Stephani universes is presented. The Kristian-Sachs method expanding the relativistic quantities in series is used, but only first order terms in redshift $z$ are considered. The numerical results are given both for centrally placed and non-centrally placed observers. In the former case the redshift-magnitude relation does not depend on the direction in the sky and the Friedman limit can be easily performed. It appears that the effect of spatial dependence of pressure is similar to the effect of the deceleration parameter in Friedman models. In the latter case the angular dependence of the relation is important. This may serve as another possible explanation of the non-compatibility of the theoretical curve of the redshift-magnitude relation with observations for large redshift objects in the Friedman universe. On the other hand, comparing the magnitudes of equal redshifts objects in different directions in the sky, one can test the reliability of these models.

Bianchi type-V cosmological models with perfect fluid and heat flow in Saez-Ballester theory

Abstract: In this paper we discuss the variation law for Hubble’s parameter with average scale factor in a spatially homogeneous and anisotropic Bianchi type-V space-time model, which yields constant value of the deceleration parameter. We derive two laws of variation of the average scale factor with cosmic time, one is of power-law type and the other is of exponential form. Exact solutions of Einstein field equations with perfect fluid and heat conduction are obtained for Bianchi type-V space-time in these two types of cosmologies. In the cosmology with the power-law, the solutions correspond to a cosmological model which starts expanding from the singular state with positive deceleration parameter. In the case of exponential cosmology, we present an accelerating non-singular model of the Universe. We find that the constant value of deceleration parameter is reasonable for the present day Universe and gives an appropriate description of evolution of Universe. We have also discussed different types of physical and kinematical behaviour of the models in these two types of cosmologies.

Can cosmic acceleration be caused by exotic massless particles

Abstract: To describe dark energy we introduce a fluid model with no free parameter on the microscopic level. The constituents of this fluid are massless particles which are a dynamical realization of the unextended $D=(3+1)$ Galilei algebra. These particles are exotic as they live in an enlarged phase space. Their only interaction is with gravity. A minimal coupling to the gravitational field, satisfying Einstein's equivalence principle, leads to a dynamically active gravitational mass density of either sign. A two-component model containing matter (baryonic and dark) and dark energy leads, through the cosmological principle, to Friedmann-like equations. Their solutions show a deceleration phase for the early universe and an acceleration phase for the late universe. We predict the Hubble parameter $H(z)/{H}_{0}$ and the deceleration parameter $q(z)$ and compare them with available experimental data. We also discuss a reduced model (one-component dark sector) and the inclusion of radiation. Our model shows no stationary modification of Newton's gravitational potential.

LRS Bianchi type -V cosmology with heat flow in scalar: tensor theory

Abstract: In this paper we present a spatially homogeneous locally rotationally symmetric (LRS) Bianchi type -V perfect fluid model with heat conduction in scalar tensor theory proposed by Saez and Ballester. The field equations are solved with and without heat conduction by using a law of variation for the mean Hubble parameter, which is related to the average scale factor of metric and yields a constant value for the deceleration parameter. The law of variation for the mean Hubble parameter generates two types of cosmologies one is of power -law form and second the exponential form. Using these two forms singular and non -singular solutions are obtained with and without heat conduction. We observe that a constant value of the deceleration parameter is reasonable a description of the different phases of the universe. We arrive to the conclusion that the universe decelerates for positive value of deceleration parameter where as it accelerates for negative one. The physical constraints on the solutions of the field equations, and, in particular, the thermodynamical laws and energy conditions that govern such solutions are discussed in some detail.The behavior of the observationally important parameters like expansion scalar, anisotropy parameter and shear scalar is considered in detail.

A Plane-Symmetric Magnetized Inhomogeneous Cosmological Models of Perfect Fluid Distribution with Variable Magnetic Permeability in Lyra Geometry

Abstract: A plane-symmetric magnetized inhomogeneous cosmological model of the universe with time dependent gauge function β for perfect fluid distribution with variable magnetic permeability within the framework of Lyra geometry is investigated. The source of the magnetic field is due to an electric current produced along the z-axis. Thus F12 is the only non-vanishing component of electromagnetic field tensor Fij. To get a deterministic solution of Einstein’s modified field equations, the free gravitational field is assumed to be Petrov type-II non-degenerate. For our derived model we obtain the deceleration parameter q=−1 as in the case of de Sitter universe. It has been found that the displacement vector β(t) behaves like cosmological term Λ in the normal gauge treatment and the solution is consistent with the observations. The displacement vector β(t) affects entropy. Some physical and geometric properties of the model are also discussed.

Viscous Fluid Cosmology in Bianchi Type-I Space-Time

Abstract: The paper presents a spatially homogeneous and anisotropic Bianchi type-I cosmological model consisting of a dissipative fluid. The field equations are solved explicitly by using a law of variation for mean Hubble parameter, which is related to average scale factor and yields a constant value for deceleration parameter. We find that the constant value of deceleration parameter describes the different phases of the evolution of universe. A barotropic equation of state (p=γρ) together with a linear relation between shear viscosity and expansion scalar, is assumed. It is found that the viscosity plays a key role in the process of the isotropization of the universe. The presence of viscous term does not change the fundamental nature of initial singularity. The thermodynamical properties of the solutions are studied and the entropy distribution is also given explicitly.

Bianchi type-V universe with wet dark fluid

Abstract: The Bianchi Type-I Universe filled with dark energy from a wet dark fluid has been considered. A new equation of state for the dark energy component of the Universe has been used. It is modeled on the equation of state p = γ(ρ − ρ*) which can describe a liquid, for example water. The exact solutions to the corresponding field equations are obtained in quadrature form. The solution for constant deceleration parameter have been studied in detail for both power-law and exponential forms. The cases γ = 1 and γ = 0 have also been analysed.

LRS Bianchi Type-I Universe in Barber’s Second Self Creation Theory

Abstract: We consider Barber’s second self creation theory with perfect fluid source for an LRS Bianchi type-I metric by using deceleration parameter to be time dependent where the metric potentials are taken as function of x and t. The present models are free from singularity and the results are consistent within the observational limit. Some physical properties of the models are also discussed.

Locally-rotationally-symmetric Bianchi type-V cosmology in general relativity

Abstract: A spatially homogeneous locally-rotationally-symmetric (LRS) Bianchi type-V cosmological model is considered with a perfect fluid in general relativity. We present two types of cosmologies (power-law and exponential forms) by using a law of variation for the mean Hubble parameter that yields a constant value for the deceleration parameter. We discuss the physical properties of the non-flat and flat models in each cosmology. Exact solutions that correspond to singular and non-singular models are presented. In a generic situation, models can be interpolated between different phases of the Universe. We find that a constant value for the deceleration parameter is reasonable for a description of different phases of the Universe. We arrive at the conclusion that the Universe decelerates when the value of the deceleration parameter is positive whereas it accelerates when the value is negative. The dynamical behaviours of the solutions and kinematical parameters like expansion, shear and the anisotropy parameter are discussed in detail in each section. Exact expressions for look-back time, luminosity distance and event horizon vs. redshift are derived and their significances are discussed in some detail. It has been observed that the solutions are compatible with the results of recent observations.

Bianchi-I Space-time with Variable Gravitational and Cosmological “Constants”

Abstract: We consider Einstein’s field equations with variable gravitational and cosmological “constants” for a spatially homogeneous and anisotropic Bianchi-I space-time. A law of variation for the Hubble parameter, which is related to the average scale factor and yields a constant value of the deceleration parameter, is assumed to solve the field equations. The gravitational constant is allowed to follow a power-law form. We find that a time-increasing gravitational constant is suitable for describing the present evolution of universe. The solutions reveal the dynamics of a universe, which expands forever. The physical interpretation of the solutions is discussed in detail.

Bianchi Type V bulk viscous cosmological models with time dependent Lambda-Term

Abstract: Received 29 May 2009, Accepted 15 August 2009, Published 30 October 2009 Abstract: Spatially homogeneous and anisotropic Bianchi type V space-time with bulk viscous fluid source and time-dependent cosmological term are considered. Cosmological models have been obtained by assuming a variation law for the Hubble parameter which yields a constant value of deceleration parameter. Physical and kinematical parameters of the models are discussed. The models are found to be compatible with the results of cosmological observations. c

Scale-Free model for governing universe dynamics

Abstract: We investigate the effects of scale-free model on cosmology, providing, in this way, a statistical background in the framework of general relativity. In order to discuss properties and time evolution of some relevant universe dynamical parameters (cosmographic parameters), such as $H(t)$ (Hubble parameter), $q(t)$ (deceleration parameter), $j(t)$ (jerk parameter) and $s(t)$ (snap parameter), which are well re-defined in the framework of scale-free model, we analyze a comparison between WMAP data. Hence the basic purpose of the work is to consider this statistical interpretation of mass distribution of universe, in order to have a mass density $\rho$ dynamics, not inferred from Friedmann equations, via scale factor $a(t)$. This model, indeed, has been used also to explain a possible origin and a viable explanation of cosmological constant, which assumes a statistical interpretation without the presence of extended theories of gravity; hence the problem of dark energy could be revisited in the context of a classical probability distribution of mass, which is, in particular, for the scale-free model, $P(k)\sim k^{-\gamma}$, with $2<\gamma<3$. The $\Lambda$CDM model becomes, with these considerations, a consequence of the particular statistics together with the use of general relativity.

Bayesian Analysis and Constraints on Kinematic Models from Union SNIa

Abstract: The kinematic expansion history of the universe is investigated by using the 307 supernovae type Ia from the Union Compilation set. Three simple model parameterizations for the deceleration parameter (constant, linear and abrupt transition) and two different models that are explicitly parametrized by the cosmic jerk parameter (constant and variable) are considered. Likelihood and Bayesian analyses are employed to find best fit parameters and compare models among themselves and with the flat $\Lambda$CDM model. Analytical expressions and estimates for the deceleration and cosmic jerk parameters today ($q_0$ and $j_0$) and for the transition redshift ($z_t$) between a past phase of cosmic deceleration to a current phase of acceleration are given. All models characterize an accelerated expansion for the universe today and largely indicate that it was decelerating in the past, having a transition redshift around 0.5. The cosmic jerk is not strongly constrained by the present supernovae data. For the most realistic kinematic models the $1\sigma$ confidence limits imply the following ranges of values: $q_0\in[-0.96,-0.46]$, $j_0\in[-3.2,-0.3]$ and $z_t\in[0.36,0.84]$, which are compatible with the $\Lambda$CDM predictions, $q_0=-0.57\pm0.04$, $j_0=-1$ and $z_t=0.71\pm0.08$. We find that even very simple kinematic models are equally good to describe the data compared to the concordance $\Lambda$CDM model, and that the current observations are not powerful enough to discriminate among all of them.

Generalized model for λ dark energy

Abstract: Einstein field equations under spherically symmetric space–times are considered here in connection with dark energy investigation. A set of solutions is obtained for a kinematic Λ model, viz. $\Lambda \sim (\dot{a}/a)^2$, without assuming any a priori value for the curvature constant and the equation-of-state parameter ω. Some interesting results, such as the nature of cosmic density Ω and deceleration parameter q, have been obtained with the consideration of two-fluid structure instead of the usual unifluid cosmological model.