Deceleration parameter

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

Role of deceleration parameter and interacting dark energy in singularity avoidance

Abstract: A class of non-singular bouncing FRW models are obtained by constraining the deceleration parameter in the presence of an interacting dark energy represented by a time-varying cosmological constant. The models being geometrically closed, initially accelerate for a certain period of time and decelerate thereafter and are also free from the entropy and cosmological constant problems. Taking a constant of integration equal to zero one particular model is discussed in some detail and the variation of different cosmological parameters are shown graphically for specific values of the parameters of the model. For some specific choice of the parameters of the model the ever expanding models of Ozer & Taha and Abdel-Rahman and the decelerating models of Berman and also the Einstein de-Sitter model may be obtained as special cases of this particular model.

The three-loop Yang–Mills condensate dark energy model and its cosmological constraints

Abstract: This work is a comprehensive investigation of the Yang–Mills condensate (YMC) dark energy (DE) model, which is extended to include the three-loop quantum corrections. We study its cosmic evolution and the possibility of crossing the phantom divide w = −1, examine in detail the Hubble parameter H, the deceleration parameter q, the statefinder (r,s) diagnostic and the w−w' diagnostic for the model without and with interaction, and compare our results with other DE models. Also, using the observational data for type Ia supernovae (SNIa), the shift parameter from the cosmic microwave background (CMB), and the baryon acoustic oscillation peak from large scale structures (LSS), we give the cosmological constraints on the three-loop YMC model. It is found that the model can solve the coincidence problem naturally, and its prediction of the aforementioned parameter is much closer to the ΛCDM (CDM: cold dark matter) model one than those from other dynamical DE models; the introduction of the matter–DE interaction will make the YMC model deviate from the ΛCDM model, and will give an equation of state crossing −1. Moreover, it is also found that, for fitting the latest SNIa data alone, the ΛCDM model is slightly better than the three-loop YMC model; but in fitting the combination of SNIa, CMB and LSS data, the three-loop YMC model performs better than the ΛCDM model.

Full causal bulk-viscous cosmological models

Abstract: New exact solutions of the gravitational field equations for a homogeneous flat Friedmann–Robertson–Walker Universe filled with a causal bulk-viscous fluid with a bulk viscosity coefficient proportional to the Hubble function are obtained in the framework of the full causal Israel–Stewart–Hiscock (ISH) theory. The evolution equation of the Universe is reduced to a first kind Abel-type first-order differential equation and new classes of exact solutions of the field equations are generated from some particular solutions of this equation leading to two classes of general solutions of the Einstein field equations corresponding to particular values of the parameters entering in the physical model. The obtained solutions are represented mathematically in an exact parametric form and are interpreted physically as describing cosmological particle production. The evolution of the temperature, entropy, deceleration parameter, and bulk viscosity coefficient is also considered for each model.

Viscous cosmology with matter creation in modified f ( R , T ) gravity

Abstract: The Friedmann–Robertson–Walker Universe containing viscous fluid and matter creation is considered in the modified f(R,T) theory of gravitation, which is an arbitrary function of the Ricci scalar R and the trace T of the energy-momentum tensor. We assume the bulk viscosity and the matter creation as two independent processes as discussed by Progogine et al. (1988, 1989). The effects of bulk viscosity and matter creation are investigated by considering the general form of bulk viscous coefficient ζ=ζ 0+ζ 1 H and particle creation rate Γ(t)=3βH. Assuming a particular class of f(R,T)=R+2f(T), where f(T)=αT, various forms of the scale factor are obtained with constant and time-dependent bulk viscous coefficient using equation of state p=(γ−1)ρ. All possible (deceleration, acceleration and their transitions) evolutions of the Universe are discussed by constraining the models on β, ζ 0 and ζ 1 in case of time-dependent deceleration parameter for positive and negative values of α. A big-rip singularity is observed for γ<0 at a finite value of cosmic time with some restrictions on parameters. It is also noted that the finite time big-rip singularity can be removed for a specific range of α in phantom region. The role of bulk viscosity and matter creation are discussed in detail through the tables and graphs of variation of deceleration parameter and the scale factor.

Bianchi type string cosmological models in f(R,T) gravity

Abstract: In this work we have studied Bianchi-III and - VI 0 cosmological models with string fluid source in f(R,T) gravity (T. Harko et al., Phys. Rev. D 84, 024020 (2011)), where R is the Ricci scalar and T the trace of the stress energy-momentum tensor in the context of late time accelerating expansion of the universe as suggested by the present observations. The exact solutions of the field equations are obtained by using a time-varying deceleration parameter. The universe is anisotropic and free from initial singularity. Our model initially shows acceleration for a certain period of time and then decelerates consequently. Several dynamical and physical behaviors of the model are also discussed in detail.