Friction on ALP domain walls and gravitational waves

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.

Automated summary

We study the early Universe evolution of axion-like particle (ALP) domain walls by considering the friction effect from particles in the surrounding plasma. The friction force from interactions within the ALP effective theory is characterized, and conditions for relevant friction during the domain wall lifetime are identified. When friction dominates, the domain wall network deviates from the standard scaling regime and affects the corresponding gravitational wave emission. We explore the gravitational wave prospects in the ALP parameter space and illustrate the correlation between the gravitational wave signal from ALP domain walls and the quality of the underlying U(1) symmetry.