Gravitational waves from the sound of a first order phase transition

TL;DR

This paper presents pioneering 3D simulations of early universe first-order phase transitions, revealing that sound waves in the cosmic fluid are the dominant source of gravitational waves, a previously underappreciated effect.

Contribution

First comprehensive 3D numerical simulations of phase transition-induced gravitational waves including fluid dynamics and scalar fields, highlighting the significance of sound waves as the main GW source.

Findings

Sound waves persist as GW sources long after bubble collisions.

The dominant GW contribution comes from fluid sound waves, not bubble collisions.

Simulations cover various phase transition strengths and bubble velocities.

Abstract

We report on the first 3-dimensional numerical simulations of first-order phase transitions in the early universe to include the cosmic fluid as well as the scalar field order parameter. We calculate the gravitational wave (GW) spectrum resulting from the nucleation, expansion and collision of bubbles of the low-temperature phase, for phase transition strengths and bubble wall velocities covering many cases of interest. We find that the compression waves in the fluid continue to be a source of GWs long after the bubbles have merged, a new effect not taken properly into account in previous modelling of the GW source. For a wide range of models the main source of the GWs produced by a phase transition is therefore the sound the bubbles make.