# Viscous Cosmologies

## TL;DR

This paper investigates the effects of non-zero viscosities, especially shear viscosity, in cosmological models filled with Quark Gluon Plasma, demonstrating conditions for non-singular universe evolution.

## Contribution

It introduces a causal viscous cosmology model incorporating shear viscosity from QGP, extending previous bulk viscosity models, and analyzes conditions for non-singular universe evolution.

## Key findings

- Bulk viscosity leads to a non-singular universe with scale factor zero only asymptotically.
- Shear viscosity can be compatible with FLRW metric under specific conditions.
- The model with shear viscosity also results in a non-singular cosmological evolution.

## Abstract

We probe into universes filled with Quark Gluon Plasma with non-zero viscosities. In particular, we study the evolution of a universe with non-zero shear viscosity motivated by the theoretical result of a non-vanishing shear viscosity in the Quark Gluon Plasma due to quantum-mechanical effects. We first review the consequences of a non-zero bulk viscosity and show explicitly the non-singular nature of the bulk-viscosity-universe by calculating the cosmological scale factor $R(t)$ which goes to zero only asymptotically. We further extend the model of bulk viscosity to include a Cosmological Constant. We contrast the previous results with the cosmology of universes with non-zero shear viscosity. We first clarify under which conditions shear viscosity terms are compatible with the Friedmann-Lama\^itre-Robertson-Walker metric. To this end we use a version of the energy-momentum tensor from the M\"uller-Isreal-Stewart theory which leads to causal Navier-Stoke equations. We then derive the corresponding Friedmann equations and show under which conditions the universe emerges non-singular.