# A transient phase in cosmological evolution : A multi-fluid approximation for a quasi-thermodynamics equilibrium

## TL;DR

This paper investigates the thermodynamics of a multi-fluid cosmological model during freeze-out, highlighting a transition phase where neither single-fluid nor multi-fluid approximations are fully adequate, and explores implications for dark-sector interactions.

## Contribution

It introduces a multi-fluid approximation for the transition phase in cosmological evolution, extending beyond traditional single-fluid models during freeze-out.

## Key findings

- Second law of thermodynamics holds throughout the transition
- Species' equations of state are modified during freeze-out
- Multi-fluid approach captures the transition dynamics more accurately

## Abstract

This article presents the study of a multi-species fluid characterised by a freeze-out or break-away of one or more species. Whereas single-fluid approximation suffices for modelling of the pre {\it freeze-out} period, multi-fluid approximation is required for the post freeze-out period. Embedded in this is a transition period where neither one of the two approximations is singly appropriate. We examine the thermodynamics of this fluid and find that the second law holds before, during and after the freeze-out. Our application to cosmological modelling involving interacting dark-section species indicates that the species' equations of state are modified.