Friction on ALP domain walls and gravitational waves

TL;DR

This paper investigates how friction from particles in the early Universe affects the evolution of axion-like particle domain walls and their gravitational wave signals, especially in relation to pulsar timing observations.

Contribution

It provides new insights into friction effects on ALP domain walls, including fermion contributions, and explores their impact on gravitational wave signals and symmetry quality.

Findings

Friction can significantly alter domain wall dynamics and gravitational wave emission.

ALP domain walls coupled to SM leptons can produce detectable gravitational waves.

The gravitational wave signal correlates with the symmetry breaking quality.

Abstract

We study the early Universe evolution of axion-like particle (ALP) domain walls taking into account the effect of friction from particles in the surrounding plasma, including the case of particles in thermal equilibrium and frozen out species. We characterize the friction force from interactions within the ALP effective theory, providing new results for the fermion contribution as well as identifying simple conditions for friction to be relevant during the domain wall life time. When friction dominates, the domain wall network departs from the standard scaling regime and the corresponding gravitational wave emission is affected. As a relevant example, we show how this can be the case for ALP domain walls emitting at the typical frequencies of Pulsar Timing Array experiments, when the ALP couples to the SM leptons. We then move to a general exploration of the gravitational wave prospects in the ALP parameter space. We finally illustrate how the gravitational wave signal from ALP domain walls is correlated with the quality of the underlying $U(1)$ symmetry.