Cosmological Perturbations of Ultrarelativistic Plasmas

TL;DR

This paper analyzes how ultrarelativistic plasmas influence cosmological scalar perturbations, revealing modifications in phase speed and damping compared to collisionless cases, using advanced perturbative and approximation techniques.

Contribution

It introduces a perturbative approach up to order λ^{3/2} and employs Padé approximants to study plasma effects on cosmological perturbations, including self-interacting neutrinos.

Findings

Altered phase speed of density perturbations

Modified subhorizon damping effects

Effective treatment of self-interacting neutrinos

Abstract

Scalar cosmological perturbations of a weakly self-interacting plasma mixed with a perfect radiation fluid are investigated. Effects of this plasma are considered through order $\lambda^{3/2}$ of perturbative thermal-field-theory in the radiation dominated universe. The breakdown of thermal perturbation theory at vastly subhorizon scales is circumvented by a Pad\'e approximant solution. Compared to collisionless plasmas the phase speed and subhorizon damping of the plasma density perturbations are changed. An example for a self-interacting thermal field is provided by the neutrinos with effective 4-fermion interactions.