Friction forces on phase transition fronts

TL;DR

This paper investigates the behavior of friction forces on phase transition fronts in cosmology, especially at ultra-relativistic speeds, proposing a model that interpolates between different velocity regimes and analyzing conditions for runaway walls.

Contribution

It introduces a phenomenological model for friction forces on phase transition fronts that covers both non-relativistic and ultra-relativistic regimes, including conditions for runaway solutions.

Findings

Stationary solutions exist at ultra-relativistic velocities under certain parameters.

A new phenomenological friction model interpolates between different velocity regimes.

The velocity of phase transition fronts depends on friction, thermodynamics, and supercooling.

Abstract

In cosmological first-order phase transitions, the microscopic interaction of the phase transition fronts with non-equilibrium plasma particles manifests itself macroscopically as friction forces. In general, it is a nontrivial problem to compute these forces, and only two limits have been studied, namely, that of very slow walls and, more recently, ultra-relativistic walls which run away. In this paper we consider ultra-relativistic velocities and show that stationary solutions still exist when the parameters allow the existence of runaway walls. Hence, we discuss the necessary and sufficient conditions for the fronts to actually run away. We also propose a phenomenological model for the friction, which interpolates between the non-relativistic and ultra-relativistic values. Thus, the friction depends on two friction coefficients which can be calculated for specific models. We then study the velocity of phase transition fronts as a function of the friction parameters, the thermodynamic parameters, and the amount of supercooling.