Suppressing the QCD Axion Abundance by Hidden Monopoles

TL;DR

This paper explores how hidden monopoles influence QCD axion dynamics, showing that the Witten effect can suppress axion abundance and isocurvature perturbations, potentially resolving cosmological issues and relaxing decay constant bounds.

Contribution

It demonstrates that hidden monopoles can significantly suppress axion abundance and solve the domain wall problem via the Witten effect, offering new insights into axion cosmology.

Findings

Suppression of axion abundance with hidden monopoles.

Relaxation of the upper bound on the axion decay constant.

Resolution of the cosmological domain wall problem.

Abstract

We study the Witten effect of hidden monopoles on the QCD axion dynamics, and show that its abundance as well as isocurvature perturbations can be significantly suppressed if there is a sufficient amount of hidden monopoles. When the hidden monopoles make up a significant fraction of dark matter, the Witten effect suppresses the abundance of axion with the decay constant smaller than $10^{12}$ GeV. The cosmological domain wall problem of the QCD axion can also be avoided, relaxing the upper bound on the decay constant when the Peccei-Quinn symmetry is spontaneously broken after inflation.