Particle shells from relativistic bubble walls

TL;DR

This paper investigates the properties of particle shells around relativistic bubble walls during cosmological phase transitions, revealing that these shells typically do not free stream due to various interactions, impacting predictions of gravitational waves and particle production.

Contribution

It systematically characterizes shell properties and identifies processes preventing free streaming, challenging standard assumptions in cosmological phase transition models.

Findings

Shells do not free stream in most studied scenarios.

Standard predictions for bubble wall velocities and gravitational waves need reevaluation.

Shell interactions significantly influence particle production and gravitational wave signals.

Abstract

Relativistic bubble walls from cosmological phase transitions (PT) necessarily accumulate expanding shells of particles. We systematically characterize shell properties, and identify and calculate the processes that prevent them from free streaming: phase-space saturation effects, out-of-equilibrium $2\to2$ and $3\to2$ shell-shell and shell-bath interactions, and shell interactions with bubble walls. We find that shells do not free stream in scenarios widely studied in the literature, where standard predictions will need to be reevaluated, including those of bubble wall velocities, gravitational waves (GW) and particle production. Our results support the use of bulk-flow GW predictions in all regions where shells free stream, irrespectively of whether or not the latent heat is mostly converted in the scalar field gradient.