Remarks on the Axion Domain Wall Problem

TL;DR

This paper reviews axion domain wall issues in cosmology, discusses a proposed explicit symmetry breaking solution, and analyzes the radiation and energetic consequences of domain wall collapse, concluding the solution's viability depends on a special coincidence.

Contribution

It provides a critical review of the explicit symmetry breaking approach to axion domain walls and clarifies the radiation and energetic outcomes during domain wall collapse.

Findings

Little axion radiation occurs until domain collapse

Collapse produces highly energetic axions with rare interactions

The solution's acceptability hinges on a specific coincidence

Abstract

Theories in which the Peccei-Quinn phase transition occurs after inflation tend to suffer from problematic domain walls. One possible solution involves a small, explicit breaking ot the symmetry. But this raises other potential issues. We review some aspects of axion domain walls, focussing especially on this proposed solution. We argue, in disagreement with some recent literature, that there is little axion radiation from the system until the domains actually collapse. The same applies to gravitational waves and electromagnetic radiation. The final stages of the collapse yields small numbers of extremely energetic axions, which interact only rarely with ordinary matter, and are thus relatively harmless. We then note that, if one accepts a remarkable coincidence, this solution can be acceptable. We consider a possible explanation of the required coincidence