# Viscous Cosmology for Early- and Late-Time Universe

## TL;DR

This paper reviews how incorporating bulk viscosity into cosmological models impacts early universe inflation, late-time acceleration, and potential singularities, offering a unified perspective on cosmic evolution.

## Contribution

It provides a comprehensive analysis of viscous effects on both early and late universe dynamics, including inflation, acceleration, and singularity avoidance, using modern observational data.

## Key findings

- Viscosity influences inflationary observables.
- Viscous effects can unify inflation and late-time acceleration.
- Viscosity may induce cosmological bounces and alter the quintessence-phantom divide.

## Abstract

From a hydrodynamicist's point of view the inclusion of viscosity concepts in the macroscopic theory of the cosmic fluid would appear most natural, as an ideal fluid is after all an abstraction (excluding special cases such as superconductivity). Making use of modern observational results for the Hubble parameter plus standard Friedmann formalism, we may extrapolate the description of the universe back in time up to the inflationary era, or we may go to the opposite extreme and analyze the probable ultimate fate of the universe. In this review we discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity. Various forms of this viscosity, when expressed in terms of the fluid density or the Hubble parameter, are discussed. Furthermore, we consider homogeneous as well as inhomogeneous equations of state. We investigate viscous cosmology in the early universe, examining the viscosity effects on the various inflationary observables. Additionally, we study viscous cosmology in the late universe, containing current acceleration and the possible future singularities, and we investigate how one may even unify inflationary and late-time acceleration. Finally, we analyze the viscosity-induced crossing through the quintessence-phantom divide, we examine the realization of viscosity-driven cosmological bounces, and we briefly discuss how the Cardy-Verlinde formula is affected by viscosity.