Axion Interactions with Domain and Bubble Walls

TL;DR

This paper investigates how axion-like particles interact with domain and bubble walls, revealing a frictional force effect that influences early universe dynamics and could impact gravitational wave signals and dark radiation.

Contribution

It introduces a novel mechanism where ALP backgrounds induce friction on relativistic walls, affecting their evolution without requiring a medium like radiation or matter.

Findings

Frictional force on walls proportional to γ² from ALP backgrounds.

Potential modifications to gravitational wave signatures from bubble and domain walls.

Generation of ultra-relativistic axion shells that may persist today.

Abstract

We show that interactions between axion-like particles (ALPs) and co-dimension one defects, such as phase-transition bubble walls and solitonic domain walls, can lead to important changes in the evolution of both walls and ALPs. The leading effect arises from the change in the ALP decay constant across the interface, which naturally follows from shift-symmetric interactions with the corresponding order parameter. Specifically, we show that for thin walls moving relativistically, an ALP background -- such as e.g. axion dark matter -- gives rise to a frictional force on the interface that is proportional to $\gamma^2$, with $\gamma$ the Lorentz factor of the wall, and that this effect is present in both the oscillating and frozen axion regimes. We explore the broader consequences of this effect for bubble and domain walls in the early universe, and show that this source of friction can be present even in the absent of a conventional medium such as radiation or matter. Possible implications include modifications to the dynamics of bubble and domain walls and their corresponding gravitational wave signatures, as well as the generation of a dark radiation component of ALPs in the form of ultra-relativistic `axion shells' with Lorentz factor $\gamma_\text{shell} \simeq 2\gamma^2 \gg 1$ that may remain relativistic until the present day.