Fluid equations for fast-moving electroweak bubble walls

TL;DR

This paper improves the fluid equations used to model the dynamics of fast-moving electroweak bubble walls during phase transitions, addressing unphysical behaviors in previous models and enabling better predictions for cosmological phenomena.

Contribution

It introduces a refined set of fluid equations that accurately describe both subsonic and supersonic bubble walls, correcting issues in earlier approaches.

Findings

New fluid equations avoid unphysical perturbation behavior

Applicable to both subsonic and supersonic walls

Enhances predictions for gravitational waves and baryogenesis

Abstract

The cosmological electroweak phase transition can be strongly first order in extended particle physics models. To accurately predict the speed and shape of the bubble walls during such a transition, Boltzmann equations for the CP-even fluid perturbations must be solved. We point out that the equations usually adopted lead to unphysical behavior of the perturbations, for walls traveling close to or above the speed of sound in the plasma. This is an artifact that can be overcome by more carefully truncating the full Boltzmann equation. We present an improved set of fluid equations, suitable for studying the dynamics of both subsonic and supersonic walls, of interest for gravitational wave production and electroweak baryogenesis.