Bubble wall velocity from number density current in (non)equilibrium

TL;DR

This paper introduces a novel, model-independent method to determine the bubble wall velocity during cosmological first-order phase transitions by analyzing number density currents across the shock front and bubble wall.

Contribution

It proposes a new approach based on number density current conservation and violation to accurately calculate bubble wall velocity in non-equilibrium conditions.

Findings

Provides a new framework for calculating bubble wall velocity.

Addresses the challenge of non-equilibrium microscopic modeling.

Enhances understanding of phase transition dynamics in the early Universe.

Abstract

Cosmological first-order phase transitions (FOPTs) serve as comprehensive probes into our early Universe with associated generations of stochastic gravitational waves and superhorizon curvature perturbations or even primordial black holes. In characterizing the FOPT, phenomenological parameters like transition temperatures, strength factors, bubble separations, and energy budgets can be easily extracted from the macroscopic equilibrium features of the underlying particle physics models except for the terminal wall velocity of the bubble expansion, making it the last key parameter to be determined most difficultly due to the non-equilibrium nature of the microscopic transition model. In this paper, we propose a new model-independent approach to calculate the bubble wall velocity by virtue of an extra junction condition from the conservation and violation of the total number density current across the shock front (if any) and bubble wall, respectively.