Phase space analysis of bulk viscous matter dominated universe

TL;DR

This paper analyzes a universe model with bulk viscous matter and radiation, exploring how different bulk viscosity dependencies affect cosmic evolution and late acceleration, finding only constant viscosity reproduces realistic cosmic history.

Contribution

It introduces a phase space analysis of bulk viscous matter models with three viscosity dependencies, highlighting the conditions for realistic cosmic evolution.

Findings

Constant bulk viscosity yields a realistic universe evolution.

Velocity and acceleration dependent viscosities do not reproduce standard cosmic phases.

All cases predict late-time acceleration of the universe.

Abstract

We consider a Friedmann model of the universe with bulk viscous matter and radiation as the cosmic components. We study the asymptotic properties in the equivalent phase space by considering the three cases for the bulk viscous coefficient as (i) $\zeta = \zeta_{0}$, a constant (ii) $\zeta = \zeta_{0} + \zeta_{1}\frac{\dot{a}}{a}$, depending on velocity of the expansion of the universe and (iii) $\zeta = \zeta_{0} + \zeta_{1}\frac{\dot{a}}{a} + \zeta_{2}\frac{\ddot{a}}{\dot{a}}$, depending both on velocity and acceleration of the expansion of the universe. It is found that all the three cases predicts the late acceleration of the universe. However, a conventional realistic behaviour of the universe, i.e., a universe having an initial radiation dominated phase and then followed by decelerated matter dominated phase and then finally evolving to accelerated epoch, is shown only when $\zeta = \zeta_{0}$, a constant. For the other two cases, it does not show either a prior conventional radiation dominated phase or a matter dominated phase of the universe.