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

TL;DR

This paper analyzes the evolution of a bulk viscous matter-dominated universe using causal Israel-Stewart theory, deriving analytical solutions and comparing with observational data, revealing distinct thermodynamic and cosmological behaviors from non-causal models.

Contribution

It introduces a causal viscous cosmological model with analytical solutions, demonstrating differences from Eckart formalism and fitting observational data.

Findings

The universe's age is less than observed.

Viscous matter transitions from stiff fluid to negative pressure.

Model satisfies thermodynamic laws and differs from ΛCDM.

Abstract

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 $\xi=\alpha\rho^{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 $\omega >-1$ 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 $\Lambda$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.