Deceleration parameter

About: Deceleration parameter is a research topic. Over the lifetime, 1776 publications have been published within this topic receiving 89440 citations.

A Modified Generalized Chaplygin Gas as the Unified Dark Matter–Dark Energy Revisited

Abstract: A modified generalized Chaplygin gas (MGCG) is considered as the unified dark matter-dark energy revisited. The character of MGCG is endued with the dual role, which behaves as matter at early times and as a quiessence dark energy at late times. The equation of state for MGCG is p = -alpha rho/(1 + alpha) - nu(z)rho(-alpha)/(1 + alpha), where nu(z) = -[rho(0c)(1 + z)(3)]((1 + alpha))(1 - Omega(0B))(alpha) {alpha Omega(0DM) + Omega(0DE)[omega(DE) + alpha(1 + omega(DE))](1 + z)3(omega DE(1+alpha))}. Some cosmological quantities, such as the densities of different components of the universe Omega(i) (i, respectively, denotes baryons, dark matter, and dark energy) and the deceleration parameter q, are obtained. The present deceleration parameter q(0), the transition redshift z(T), and the redshift z(eq), which describes the epoch when the densities in dark matter and dark energy are equal, are also calculated. To distinguish MGCG from others, we then apply the Statefinder diagnostic. Later on, the parameters (alpha and omega(DE)) of MGCG are constrained by combination of the sound speed c(s)(2), the age of the universe t(0), the growth factor m, and the bias parameter b. It yields alpha = -3.07(-4.98)(+5.66) x 10(-2) and omega(DE) = -1.05(-0.11)(+0.06). Through the analysis of the growth of density perturbations for MGCG, it is found that the energy will transfer from dark matter to dark energy which reach equal at z(eq) similar to 0.48 and the density fluctuations start deviating from the linear behavior at z similar to 0.25 caused by the dominance of dark energy.

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

FRW viscous fluid cosmological model with time-dependent inhomogeneous equation of state

Abstract: In this paper, we present two viscous models of non-perfect fluid by avoiding the introduction of exotic dark energy. We consider the first model in terms of deceleration parameter q has a viscosity of the form ζ = ζ0 + (ζ1 − ζ2q)H and the other model in quadratic form of H of the type ζ = ζ0 + ζ1H + ζ2H2. In this framework we find the solutions of field equations by using inhomogeneous equation of state of form p = ωρ + Λ(t) with equation of state parameter ω is constant and ω(ρ) = b0ρα−1.

Accelerated expansion from braneworld models with variable vacuum energy

Abstract: We study the acceleration of the universe as a consequence of the time evolution of the vacuum energy in cosmological models based in braneworld theories in 5D. A variable vacuum energy may appear if the size of the extra dimension changes during the evolution of the universe. In this scenario the acceleration of the universe is related not only to the variation of the cosmological term, but also to the time evolution of G and, possibly, to the variation of other fundamental “constants” as well. This is because the expansion rate of the extra dimensionappears in different contexts, notably in expressions concerning the variation of rest mass and electric charge. We concentrate our attention on spatially-flat, homogeneous and isotropic, brane-universes where the matter density decreases as an inverse power of the scale factor, similar (but at different rate) to the power law in FRW-universes of general relativity. We show that these braneworld cosmologies are consistent with the observed accelerating universe and other observational requirements. In particular, G becomes constant and \({\rm \Lambda}_{(4)} \approx const \times H^2\) asymptotically in time. Another important feature is that the models contain no “adjustable” parameters. All the quantities, even the five-dimensional ones, can be evaluated by means of measurements in 4D. We provide precise constrains on the cosmological parameters and demonstrate that the “effective” equation of state of the universe can, in principle, be determined by measurements of the deceleration parameter alone. We give an explicit expression relating the density parameters \({\rm \Omega}_{\rho}\), \({\rm \Omega}_{\rm \Lambda}\) and the deceleration parameter q. These results constitute concrete predictions that may help in observations for an experimental/observational test of the model.

Bianchi type-V anisotropic cosmological models in certain theories of gravitation with perfect fluid distribution

Abstract: Exact Bianchi type-V cosmological models are presented in Einstein’s theory of gravitation with cosmological constant Λ in case of perfect fluid distribution. Also obtained Bianchi type-V cosmological models in a scalar-tensor theory of gravitation proposed by Saez and Ballester (1986) in case of perfect fluid distribution using and without using negative constant deceleration parameter. Some physical and geometrical properties of the models are also discussed.