Axion string cosmology and its controversies

TL;DR

This paper reviews axion string cosmology, emphasizing its impact on axion mass constraints and critically examining claims of lighter axion masses in the literature.

Contribution

It provides a critical analysis of the standard axion string evolution scenario and discusses controversies regarding the axion mass bounds.

Findings

Axion mass constrained to ~100 μeV under standard assumptions.

Uncertainties in axion mass bounds are significant.

Controversies exist over the possibility of lighter axions.

Abstract

Understanding axion cosmology has important experimental consequences since it constrains the range of allowed values for the axion mass. In the standard thermal scenario, which assumes Peccei-Quinn symmetry restoration after inflation, an axion string network forms at the phase transition $T\sim\fa$ and then radiatively decays into a cosmological background of axions. Under standard assumptions for the evolution of this string network and the radiation from it, axions must have a mass of $\ma\sim 100\mu{\rm eV}$ with large specified uncertainties. We discuss critically the various suggestions in the literature that the axion mass migh be lighter.