Thermodynamics of viscous Matter and Radiation in the Early Universe

TL;DR

This paper models the early Universe as a thermodynamic system of matter and radiation, deriving cosmological parameters and exploring the impact of viscosity, with results consistent with standard cosmological models and implications for universe evolution.

Contribution

It introduces a classical thermodynamic approach to early Universe cosmology, incorporating viscosity effects and quantum considerations to derive key cosmological quantities.

Findings

Cosmological parameters can be derived from thermodynamics of matter and radiation.

Finite bulk viscosity significantly alters the evolution of the Universe.

The model aligns with Friedmann-Lemaitre-Robertson-Walker and Einstein equations.

Abstract

Assuming that the background geometry is filled with free gas consisting of matter and radiation and no phase transitions being occurred in the early Universe, we discuss the thermodynamics of this {\it closed} system using classical approaches. We find that essential cosmological quantities, such as Hubble parameter $H$, scale factor $a$ and curvature parameter $k$, can be derived from this simple model, which on one hand fulfills and entirely obeys the laws of thermodynamics. On the other hand, the results are compatible with the Friedmann-Lemaitre-Robertson-Walker model and the Einstein field equations. The inclusion of finite bulk viscosity coefficient derives to important changes in all these cosmological quantities. Accordingly, our picture about the evolution of the Universe and its astrophysical consequences seems to be a subject of a radical revision. We find that $k$ strongly depends on the thermodynamics of background matter. The time scale, at which negative curvature might take place, depends on the relation between the matter content and the total energy. Using quantum and statistical approaches, we assume that the size of the Universe is given by the volume occupied one particle and one photon. Different types of their interactions are taken into account. Expressions for $H$ and $a$ are introduced. Therefore, the expansion of the Universe turns to be accessible.