Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles

TL;DR

This paper develops an analytic method to evaluate axion abundance suppression via adiabatic mechanisms, applying it to scenarios involving hidden monopoles and the Witten effect to address cosmological overproduction issues.

Contribution

It introduces a general analytic approach for calculating axion abundance under adiabatic suppression and applies it to models with hidden monopoles and the Witten effect.

Findings

Analytic formula for axion abundance in adiabatic suppression scenarios

Demonstration of suppression effectiveness in models with hidden monopoles

Insights into the role of the Witten effect in axion mass enhancement

Abstract

The string theory predicts many light fields called moduli and axions, which cause a cosmological problem due to the overproduction of their coherent oscillation after inflation. One of the prominent solutions is an adiabatic suppression mechanism, which, however, is non-trivial to achieve in the case of axions because it necessitates a large effective mass term which decreases as a function of time. The purpose of this paper is twofold. First, we provide an analytic method to calculate the cosmological abundance of coherent oscillation in a general situation under the adiabatic suppression mechanism. Secondly, we apply our method to some concrete examples, including the one where a string axion acquires a large effective mass due to the Witten effect in the presence of hidden monopoles.