Generalised equilibrium of cosmological fluids in second-order thermodynamics

TL;DR

This paper explores generalized equilibrium states of cosmological fluids using second-order thermodynamics, revealing reduced particle production rates and compatibility with K-matter in an expanding universe.

Contribution

It introduces a second-order thermodynamic framework for cosmological fluids, extending previous first-order models and analyzing equilibrium conditions with nonconserved particle number.

Findings

Reduced particle production rate compared to first-order theory

Compatibility of generalized equilibrium with -matter

Energy density scales as  with the scale factor

Abstract

Combining the second-order entropy flow vector of the causal Israel-Stewart theory with the conformal Killing-vector property of $u_{i}/T$, where $u_{i}$ is the four-velocity of the medium and T its equilibrium temperature, we investigate generalized equilibrium states for cosmological fluids with nonconserved particle number. We calculate the corresponding equilibrium particle production rate and show that this quantity is reduced compared with the results of the previously studied first-order theory. Generalized equilibrium for massive particles turns out to be compatible with a dependence $\rho \propto a ^{-2}$ of the fluid energy density $\rho$ on the scale factor a of the Robertson-Walker metric and may be regarded as a realization of so-called K-matter.