Deceleration parameter

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

Cosmological parameter analyses using transversal BAO data

Abstract: We investigate observational constraints on cosmological parameters combining 15 measurements of the transversal BAO scale (obtained free of any fiducial cosmology) with Planck-CMB data to explore the parametric space of some cosmological models. We investigate how much Planck + transversal BAO data can constraint the minimum $\Lambda$CDM model, and extensions, including neutrinos mass scale $M_{ u}$, and the possibility for a dynamical dark energy (DE) scenario. Assuming the $\Lambda$CDM cosmology, we find $H_0 = 69.23 \pm 0.50$ km s${}^{-1}$ Mpc${}^{-1}$, $M_{ u} < 0.11$ eV and $r_{\rm drag} = 147.59 \pm 0.26$ Mpc (the sound horizon at drag epoch) from Planck + transversal BAO data. When assuming a dynamical DE cosmology, we find that the inclusion of the BAO data can indeed break the degeneracy of the DE free parameters, improving the constraints on the full parameter space significantly. We note that the model is compatible with local measurements of $H_0$ and there is no tension on $H_0$ estimates in this dynamical DE context. Also, we discuss constraints and consequences from a joint analysis with the local $H_0$ measurement from SH0ES. Finally, we perform a model-independent analysis for the deceleration parameter, $q(z)$, using only information from transversal BAO data.

Bianchi Type-V Dark Energy Model in a Scalar-Tensor Theory of Gravitation

Abstract: A spatially homogeneous and anisotropic Bianchi type-V universe with variable equation of state (EoS) parameter and constant deceleration parameter is obtained in a scalar-tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). The physical and kinematical properties of the universe have been discussed.

Bulk viscous matter and recent acceleration of the universe

Abstract: We consider a cosmological model dominated by bulk viscous matter with a total bulk viscosity coefficient proportional to the velocity and acceleration of the expansion of the universe in such a way that $$\zeta =\zeta _{0}\,+\,\zeta _{1}\frac{\dot{a}}{a}\,+\,\zeta _{2}\frac{\ddot{a}}{\dot{a}}.$$ We show that there exist two limiting conditions in the bulk viscous coefficients ( $$\zeta _{0}$$ , $$\zeta _{1}$$ , $$\zeta _{2}$$ ) which correspond to a universe having a Big Bang at the origin, followed by an early decelerated epoch and then making a smooth transition into an accelerating epoch. We have constrained the model using the type Ia Supernovae data, evaluated the best estimated values of all the bulk viscous parameters and the Hubble parameter corresponding to the two limiting conditions. We found that even though the evolution of the cosmological parameters are in general different for the two limiting cases, they show identical behavior for the best estimated values of the parameters from both limiting conditions. A recent acceleration would occur if $$\tilde{\zeta }_{0}+\tilde{\zeta }_{1}>1$$ for the first limiting conditions and if $$\tilde{\zeta }_{0}+\tilde{\zeta }_{1}<1$$ for the second limiting conditions. The age of the universe predicted by this model is found to be less than that predicted from the oldest galactic globular clusters. The total bulk viscosity seems to be negative in the past and becomes positive when $$z\le 0.8$$ . So the model violates the local second law of thermodynamics. However, the model satisfies the generalized second law of thermodynamics at the apparent horizon throughout the evolution of the universe. We also made a statefinder analysis of the model and found that it is distinguishably different from the standard $$\varLambda $$ CDM model at present, but it shows a de Sitter type behavior in the far future of the evolution.

Power spectrum of the SDSS luminous red galaxies: constraints on cosmological parameters

Abstract: In this paper we determine the constraints on cosmological parameters using the CMB data from the WMAP experiment together with the recent power spectrum measurement of the SDSS Luminous Red Galaxies (LRGs). Specifically, we focus on spatially flat, low matter density models with adiabatic Gaussian initial conditions. The spatial flatness is achieved with an additional quintessence component whose equation of state parameter w_eff is taken to be independent of redshift. Throughout most of the paper we do not allow any massive neutrino contribution and also the influence of the gravitational waves on the CMB is taken to be negligible. The analysis is carried out separately for two cases: (i) using the acoustic scale measurements as presented in Hutsi (2006), (ii) using the full SDSS LRG power spectrum and its covariance matrix. We are able to obtain a very tight constraint on the Hubble constant: H_0 = 70.8 ^{+2.1}_{-2.0} km/s/Mpc, which helps in breaking several degeneracies between the parameters and allows us to determine the low redshift expansion law with much higher accuracy than available from the WMAP + HST data alone. The positive deceleration parameter q_0 is found to be ruled out at 5.5 \sigma confidence level. Finally, we extend our analysis by investigating the effects of relaxing the assumption of spatial flatness and also allow for a contribution from massive neutrinos.