Symmetry principles of gravitational perturbations in thermal environments

TL;DR

This paper explores how symmetry principles constrain the mass shift of gravitational perturbations in a thermal plasma, revealing implications for early universe cosmology and the stability of primordial tensor modes.

Contribution

It identifies the symmetry-based criteria that uniquely determine the plasma-induced mass shift of gravitational perturbations in a cosmological setting.

Findings

Mass shift is constrained by invariance under diffeomorphisms and Weyl rescalings.

Stable damping of primordial tensor modes is consistent with these symmetry principles.

The analysis suggests local thermal equilibrium is an emergent, dynamical concept in quantum field theory.

Abstract

The thermal plasma induces a plasmon-like mass shift for gravitational perturbations, which can modify their dynamics near the horizon scale in the early radiation-dominated universe. However, there are several seemingly reasonable ways to introduce this mass shift, reflecting an ambiguity in how one specifies the initial plasma state on a perturbed FLRW background. Invariance under small diffeomorphisms and Weyl rescalings singles out the (grand) canonical ensemble defined in the decoupling limit of gravitational interactions, while excluding ensembles that violate the Weyl identity, including those perturbed by the metric. Large diffeomorphisms further require the mass shift to vanish in the infrared limit. With this consistent choice, primordial tensor modes exhibit stable damping, in agreement with Weinberg's kinetic theory analysis. This cosmological example indicates a more general picture in which local equilibrium in thermal quantum field theory is not an external input but an emergent, dynamical notion.