A sonic boom in bubble wall friction

TL;DR

This paper investigates the behavior of bubble wall friction during a cosmological phase transition, revealing a singularity at the speed of sound due to hydrodynamic effects, contrasting with previous regularity assumptions.

Contribution

The study introduces an extended fluid Ansatz to solve the Boltzmann equation and demonstrates a fundamental singularity at the speed of sound in bubble wall friction calculations.

Findings

Identifies a singularity in fluctuations at the speed of sound.

Shows a discontinuity in hydrodynamics across the speed of sound.

Contrasts with recent approaches predicting regular behavior.

Abstract

We revisit the computation of bubble wall friction during a cosmological first-order phase transition, using an extended fluid Ansatz to solve the linearized Boltzmann equation. A singularity is found in the fluctuations of background species as the wall approaches the speed of sound. Using hydrodynamics, we argue that a discontinuity across the speed of sound is expected on general grounds, which manifests itself as the singularity in the solution of the linearized system. We discuss this result in comparison with alternative approaches proposed recently, which find a regular behaviour of the friction for all velocities.