The Stochastic Axion Scenario

TL;DR

This paper shows that low-scale inflation allows the QCD axion to naturally produce the observed dark matter abundance across a wide mass range, avoiding previous overproduction constraints.

Contribution

It demonstrates that during low-scale inflation, the QCD axion can achieve the correct dark matter abundance without fine-tuning, expanding the viable mass range.

Findings

Axion abundance is set by quantum fluctuations during inflation.

The mechanism produces negligible isocurvature perturbations.

It applies to axion-like particles and other light fields.

Abstract

For the minimal QCD axion model it is generally believed that overproduction of dark matter constrains the axion mass to be above a certain threshold, or at least that the initial misalignment angle must be tuned if the mass is below that threshold. We demonstrate that this is incorrect. During inflation, if the Hubble scale is low, the axion tends toward an equilibrium. This means the minimal QCD axion can naturally give the observed dark matter abundance in the entire lower part of the mass range, down to masses $\sim 10^{-12}$ eV (or $f_a$ up to almost the Planck scale). The axion abundance is generated by quantum fluctuations of the field during inflation. This mechanism generates cold dark matter with negligible isocurvature perturbations. In addition to the QCD axion, this mechanism can also generate a cosmological abundance of axion-like particles and other light fields.