arXiv subjects

Jerin Mohan N D

Publications and source records attributed to Jerin Mohan N D.

4 recordsLinked to original sources

arXiv2022

Running vacuum cosmology with bulk viscous matter

We study the late acceleration of the universe by incorporating bulk viscous matter with the running vacuum. The vacuum energy density varies as the squares of the Hubble parameter ($ρ_Λ\propto H^2$), and the coefficient of bulk viscosity of matter is proportional to the velocity of expansion ($ξ\propto H$). We obtained an analytical solution to the Friedmann equations and estimated the model parameters using the combined data set SN1a+CMB+BAO+OHD. We have evaluated the universe's age as 14 Gyr, which is slightly higher than the age-predicted by the $Λ$CDM model. However, it is an improved result compared to the age-predicted by a class of bulk viscous matter-dominated models. Interestingly, we have obtained the coefficient of bulk viscosity of the matter component as $1.316\times 10^5$ kg $\textnormal m^{-1}$ $\textnormal s^{-1}$ which is one to two orders of magnitude less than the value predicted by most of the bulk viscous matter-dominated models and it falls in the range of highly viscous materials found on the earth. The Hubble parameter is a decreasing function of the scale factor, and it attains a constant value in the far future that corresponds to an end deSitter phase of evolution. The deceleration parameter shows a transition from matter-dominated decelerated phase to vacuum energy-dominated accelerating phase, and the transition redshift is obtained as $z_T = 0.73$. The statefinder analysis distinguishes our model from the $Λ$CDM model at present, and the $r-s$ trajectory reveals the quintessence behaviour of the vacuum energy. The phase space analysis shows that the universe is evolving towards a mechanically stable state in the far future. The entropy evolution satisfies the generalised second law of thermodynamics, and the entropy is maximised in the far future evolution.

gr-qc↗

arXiv2021

On the feasibility of truncated Israel-Stewart model in the context of late acceleration

A dissipative model of the Universe based on the causal relativistic truncated Israel-Stewart theory is analysed in the context of recent accelerated expansion of the Universe. The bulk viscosity and relaxation time are taken as $ξ=αρ^s$ and $τ=\fracα{εγ(2-γ)}ρ^{s-1}$ respectively. For $s=1/2,$ we found an analytical solution for the Hubble parameter of the model. We have estimated the model parameters by treating $γ=1$ and $γ$ as a free parameter using the latest cosmological data. The model predicts a prior decelerated phase and an end de Sitter phase as in the standard $Λ$CDM model. The dynamical system analysis shows that the prior decelerated epoch is an unstable equilibrium, while the far future de Sitter epoch is stable. The age of the Universe obtained around $13.66$ Gyr, which is close to the recent observations. The second law of thermodynamics is found to be satisfied throughout the evolution in this model. The feasibility of the model has been checked by contrasting with models based on the full Israel-Stewart and the Eckart viscous theories. The truncated viscous model appears more compatible with astronomical observations than the Eckart and full causal viscous models.

gr-qc↗

arXiv2020

Dynamical system analysis and thermal evolution of the causal dissipative model

The dynamical system behaviour and thermal evolution of a homogeneous and isotropic dissipative universe are analyzed. The dissipation is driven by the bulk viscosity $ξ= αρ^s $ and the evolution of bulk viscous pressure is described using the full causal Israel-Stewart theory. We find that for $s=1/2$ the model possesses a prior decelerated epoch which is unstable and a stable future accelerated epoch. From the thermodynamic analysis, we have verified that the local as well as the generalised second law of thermodynamics are satisfied throughout the evolution of the universe. We also show that the convexity condition $S''<0$ is satisfied at the end stage of the universe which implies an upper bound to the evolution of the entropy. For $s\neq1/2,$ the case $s<1/2$ is ruled out since it does not predict the conventional evolutionary stages of the universe. On the other hand, the case $s>1/2$ does imply a prior decelerated and a late de Sitter epochs, but both of them are unstable fixed points. The thermal evolution corresponding to the same case implies that GSL is satisfied at both the epochs but convexity condition is violated by both, so that entropy growth is unbounded. Hence for $s>1/2$ the model does not give a stable evolution of the universe.

gr-qc↗

arXiv2018

Bulk viscous matter and recent acceleration of the universe based on causal viscous theory

Evolution of the bulk viscous matter dominated universe has been analysed using the full causal, Israel-Stewart theory for the evolution of bulk viscous pressure in the context of recent acceleration of the universe. The form of bulk viscosity is taken as $ξ=αρ^{1/2}. $ We obtained analytical solutions for the Hubble parameter and scale factor of the universe. The model parameters have been computed using the Type Ia supernovae observational data. The evolution of the prominent cosmological parameters were obtained. The age of the universe for the best estimated model parameters is found to be less than observational value. The viscous matter behaves like stiff fluid in the early evolutionary phase and then evolves to a negative pressure fluid in the later phase. The equation of state is found to be stabilized with value $ω>-1$ and thus, the model is not showing any of the phantom behaviour during the evolution. The local as well as generalized second law of thermodynamics are satisfied in this model, while it was shown by many that the local second law is breaking in the Eckart formalism approach. The statefinder geometric diagnostic shows that the present model is distinct from the standard $Λ$CDM model of the universe. One of the marked deviation seen in this model compared to a corresponding model using Eckart approach is that the bulk viscosity decreases with expansion of the universe, while in it increases from negative value in the early universe towards positive values values in the Eckart formalism.

gr-qc↗