Axion dark matter: strings and their cores

TL;DR

This paper investigates axion string networks in the early universe, revealing a logarithmic dependence of network density on separation-to-core ratio, and finds that this affects axion production efficiency minimally within simulated parameters.

Contribution

It demonstrates that axion string network density depends logarithmically on separation-to-core ratio and assesses its impact on axion production in cosmological simulations.

Findings

Network density depends logarithmically on separation-to-core ratio.

Logarithmic factor is about 10 times larger in cosmology than in simulations.

Changing network density has little impact on axion production efficiency.

Abstract

Axions constitute a well-motivated dark matter candidate, and if PQ symmetry breaking occurred after inflation, it should be possible to make a clean prediction for the relation between the axion mass and the axion dark matter density. We show that axion (or other global) string networks in 3D have a network density that depends logarithmically on the string separation-to-core ratio. This logarithm would be about 10 times larger in axion cosmology than what we can achieve in numerical simulations. We simulate axion production in the early Universe, finding that, for the separation-to-core ratios we can achieve, the changing density of the network has little impact on the axion production efficiency.