Interacting viscous mixtures

TL;DR

This paper investigates how bulk viscosity in a relativistic plasma influences the evolution of cosmological perturbations, deriving generalized equations that account for fluid interactions and viscosity fluctuations.

Contribution

It introduces a comprehensive framework for analyzing entropy and curvature fluctuations in viscous fluid mixtures within a relativistic cosmological context.

Findings

Bulk viscosity fluctuations act as source terms for perturbation evolution.

The mixing of entropy and curvature perturbations depends on viscosity's functional form.

Numerical analysis confirms analytical predictions about perturbation behavior.

Abstract

Gravitational and hydrodynamical perturbations are analysed in a relativistic plasma containing a mixture of interacting fluids characterized by a non-negligible bulk viscosity coefficient. The energy-momentum transfer between the cosmological fluids, as well as the fluctuations of the bulk viscosity coefficients, are analyzed simultaneously with the aim of deriving a generalized set of evolution equations for the entropy and curvature fluctuations. For typical length scales larger than the Hubble radius, the fluctuations of the bulk viscosity coefficients and of the decay rate provide source terms for the evolution of both the curvature and the entropy fluctuations. According to the functional dependence of the bulk viscosity coefficient on the energy densities of the fluids composing the system, the mixing of entropy and curvature perturbations is scrutinized both analytically and numerically.