Bubble Friction in Symmetry-Restoring Transitions

TL;DR

This paper investigates the frictional dynamics of bubble walls during symmetry-restoring phase transitions, revealing a negative friction regime that allows for larger terminal velocities, with implications for gravitational waves and beyond-Standard-Model physics.

Contribution

It introduces the calculation of bubble wall friction in symmetry-restoring transitions, showing the possibility of negative friction and larger terminal velocities compared to symmetry-breaking cases.

Findings

Negative friction can occur in symmetry-restoring transitions.

Bubble walls can reach significantly higher Lorentz factors.

Implications for gravitational wave signals and BSM physics.

Abstract

In standard (symmetry-breaking) first-order phase transitions, the frictional pressure on expanding bubble walls can be dominated by transition radiation -- the emission of a gauge boson with phase-dependent masses as particles present in the thermal plasma pass through bubble walls. This process is enhanced in the soft limit, and is known to produce a significant frictional effect that is proportional to the Lorentz factor $\gamma$ of the bubble wall, thereby prohibiting runaway behavior. We calculate the analogous pressure for phase transitions with symmetry restoration. In such transitions, we show that the pressure due to this process can be $\textit{negative}$, producing the opposite effect. However, when the Lorentz factor of the wall gets very large, the result approaches the same scaling as the standard scenarios. Therefore, phase transitions with symmetry restoration can feature an intermediate negative friction regime even in the presence of significant interactions with the plasma, and the bubble wall terminal Lorentz factor can be significantly larger (by more than an order of magnitude) than in the corresponding symmetry-breaking scenarios. This can carry important implications for various phenomenological applications, from gravitational waves to physics beyond-the-Standard-Model.