Cosmology and thermodynamics of FRW universe with bulk viscous stiff fluid

TL;DR

This paper investigates a cosmological model with a stiff fluid and bulk viscosity, revealing different universe evolutions, phase transitions, and thermodynamic behaviors depending on the bulk viscous parameter, and compares it with the standard $\Lambda$CDM model.

Contribution

It introduces a detailed analysis of a viscous stiff fluid cosmology, classifies possible universe evolutions, and examines thermodynamic laws within this framework.

Findings

For $0<ar extzeta<6$, universe begins with Big Bang and transitions to acceleration.

For $ar extzeta>6$, universe lacks Big Bang and density saturates.

Model approaches $\Lambda$CDM at late times and satisfies thermodynamic laws under certain conditions.

Abstract

We consider a cosmological model dominated by stiff fluid with a constant bulk viscosity. We classify all the possible cases of the universe predicted by the model and analyzing the scale factor, density as well as the curvature scalar. We find that when the dimensionless constant bulk viscous parameter is in the range $0 < \bar\zeta <6$ the model began with a Big Bang, and make a transition form the decelerating expansion epoch to an accelerating epoch, then tends to the de Sitter phase as $ t\to \infty$. The transition into the accelerating epoch would be in the recent past, when $4<\bar\zeta<6.$ For $\bar\zeta>6$ the model doesn't have a Big Bang and suffered an increase in the fluid density and scalar curvature as the universe expands, which are eventually saturates as the scale factor $a \to \infty$ in the future. We have analyzed the model with statefinder diagnostics and find that the model is different from $\Lambda$CDM model but approaches $\Lambda$CDM point as $a \to \infty.$ We have also analyzed the status of the generalized second law of thermodynamics with apparent horizon as the boundary of the universe and found that the law is generally satisfied when $0 \leq \bar\zeta <6$ and for $\bar\zeta >6$ the law is satisfied when the scale factor is larger than a minimum value.