Bubble wall velocity with out-of-equilibrium corrections

TL;DR

This paper investigates how out-of-equilibrium plasma effects influence bubble wall velocities during a cosmological phase transition, revealing that such effects slow the walls and enhance baryogenesis prospects, with implications for gravitational wave signals.

Contribution

It introduces a numerical method to incorporate out-of-equilibrium corrections in bubble wall dynamics, improving the understanding of phase transition phenomenology beyond equilibrium assumptions.

Findings

Out-of-equilibrium effects slow down bubble walls.

Enhanced baryogenesis efficiency due to increased wall friction.

Potentially observable gravitational wave signatures.

Abstract

We study how out-of-equilibrium effects modify the steady-state propagation of bubble walls during a cosmological first-order electroweak phase transition. Going beyond the local thermal equilibrium approximation, we numerically solve the coupled system of scalar field, hydrodynamic and Boltzmann equations using a spectral algorithm that allows a first-principle treatment of the collision integral. This approach enables a quantitative assessment of non-equilibrium perturbations in the plasma and their backreaction on the wall motion. Focusing on the singlet extension of the Standard Model as a minimal benchmark scenario, we find that out-of-equilibrium corrections substantially enhance the effective friction on the expanding front, leading to slower wall velocities and broader wall profiles compared to the equilibrium case. These modifications have significant implications for cosmological observables. For instance, they enhance the efficiency of electroweak baryogenesis, thus improving the viability of baryon asymmetry generation within realistic parameter regions that can also be probed by future gravitational wave interferometers.