Relativistic interacting fluids in cosmology

TL;DR

This paper extends the action functional for relativistic fluids to include non-adiabatic processes and chemical reactions, with applications in cosmology such as dark sector interactions and early-universe phenomena.

Contribution

It introduces a new variational framework for relativistic fluids that accounts for entropy growth and particle number violation, applicable to various cosmological and astrophysical contexts.

Findings

Unified dark sector model reproduces ΛCDM cosmology

Model predicts new phenomenology beyond standard cosmology

Applicable to early-universe phase transitions and matter creation

Abstract

Motivated by cosmological applications for interacting matters, an extension of the action functional for relativistic fluids is proposed to incorporate the physics of non-adiabatic processes and chemical reactions. The former are characterised by entropy growth, while the latter violate particle number conservation. The relevance of these physics is demonstrated in the contexts of self-interacting fluids, fluids interacting with scalar fields, and hyperhydrodynamical interactions with geometry. The possible cosmological applications range from early-universe phase transitions to astrophysical phenomena, and from matter creation inflationary alternatives to interacting dark sector alternatives to the $\Lambda$CDM model that aim to address its tensions. As an example of the latter, a single fluid model of a unified dark sector is presented. The simple action of the model features one field and one parameter, yet it can both reproduce the $\Lambda$CDM cosmology and predict new phenomenology.