Axion dark matter from topological defects

Abstract

The cosmological scenario where the Peccei-Quinn symmetry is broken after inflation is investigated. In this scenario, topological defects such as strings and domain walls produce a large number of axions, which contribute to the cold dark matter of the universe. The previous estimations of the cold dark matter abundance are updated and refined based on the field-theoretic simulations with improved grid sizes. The possible uncertainties originated in the numerical calculations are also discussed. It is found that axions can be responsible for the cold dark matter in the mass range $m_a=(0.8-1.3)\times 10^{-4}\mathrm{eV}$ for the models with the domain wall number $N_{\rm DW}=1$, and $m_a\approx\mathcal{O}(10^{-4}-10^{-2})\mathrm{eV}$ with a mild tuning of parameters for the models with $N_{\rm DW}>1$. Such higher mass ranges can be probed in future experimental studies.