Thermal suppression of bubble nucleation at first-order phase transitions in the early Universe

TL;DR

This paper investigates how fluid heating during first-order phase transitions in the early Universe suppresses bubble nucleation, leading to larger bubble separations and potentially stronger gravitational wave signals.

Contribution

It quantifies the suppression effect of fluid heating on bubble nucleation and its impact on gravitational wave signals in early Universe phase transitions.

Findings

Fluid heating suppresses bubble nucleation rate.

Suppression increases mean bubble separation.

Gravitational wave signal can be enhanced by up to a factor of 10.

Abstract

One of the key observables in a gravitational wave power spectrum from a first order phase transition in the early Universe is the mean bubble spacing, which depends on the rate of nucleation of bubbles of the stable phase, as well as the bubble wall speed. When the bubbles expand as deflagrations, it is expected that the heating of the fluid in front of the phase boundary suppresses the nucleation rate. We quantify the effect, showing that it increases the mean bubble separation, and acts to enhance the gravitational wave signal by a factor of up to order 10. The effect is largest for small wall speeds and strong transitions.