Bubble expansion at strong coupling

TL;DR

This paper analytically studies bubble wall expansion in strongly coupled first-order phase transitions, revealing a linear velocity-pressure relation consistent with holographic simulations, and provides a hydrodynamics-based framework for future velocity predictions.

Contribution

It derives the non-relativistic expansion limits for bubbles of various geometries in strongly coupled theories, linking wall velocity to phase pressure difference without microphysical details.

Findings

Reproduces the linear velocity-pressure relation for planar walls.

Provides analytical results for cylindrical and spherical walls.

Suggests future numerical tests for these geometries.

Abstract

The cosmological first-order phase transition (FOPT) can be of strong dynamics but with its bubble wall velocity difficult to be determined due to lack of detailed collision terms. Recent holographic numerical simulations of strongly coupled theories with a FOPT prefer a relatively small wall velocity linearly correlated with the phase pressure difference between false and true vacua for a planar wall. In this Letter, we have analytically revealed the non-relativistic limit of a planar/cylindrical/spherical wall expansion of a bubble strongly interacting with the thermal plasma. The planar-wall result reproduces the linear relation found previously in the holographic numerical simulations. The results for cylindrical and spherical walls can be directly tested in future numerical simulations. Once confirmed, the bubble wall velocity for a strongly coupled FOPT can be expressed purely in terms of the hydrodynamics without invoking the underlying microphysics.