Comments on Axions, Domain Walls, and Cosmic Strings

TL;DR

This paper critically examines the role of axion strings in dark matter production, arguing that their contribution is likely already included in existing misalignment models, based on theoretical and simulation-based considerations.

Contribution

It provides a detailed critique of the significance of axion strings, suggesting they do not add to dark matter abundance beyond current assumptions.

Findings

Parametric strings are likely accounted for by random misalignment assumptions.

Qualitative arguments suggest minimal additional axion radiation from strings.

Reviews of simulations support the conclusion that strings' role is overestimated.

Abstract

Axions have for some time been considered a plausible candidate for dark matter. They can be produced through misalignment, but it has been argued that when inflation occurs before a Peccei-Quinn transition, appreciable production can result from cosmic strings. This has been the subject of extensive simulations. But there are reasons to be skeptical about the possible role of axion strings. We review and elaborate on these questions, and argue that parametrically strings are already accounted for by the assumption of random misalignment angles. We review and elaborate on these questions, and provide several qualitative arguments that parametrically strings are already accounted for by the assumption of random misalignment angles. The arguments are base on considerations of the collective modes of the string solutions, on computations of axion radiation in particular models, and reviews of simulations.