Dark Matter in Axion Landscape

TL;DR

This paper explores how multiple shift-symmetry breaking terms in an axion landscape can produce a very light, stable axion that acts as dark matter, with suppressed isocurvature perturbations and relaxed fine-tuning.

Contribution

It introduces a scenario where multiple terms create a flat-bottomed potential, enabling a heavier axion to be dark matter without fine-tuning and compatible with high-scale inflation.

Findings

Axion abundance is suppressed compared to single cosine potential.

Isocurvature perturbations are significantly reduced.

Allows for heavier axions as dark matter without fine-tuning.

Abstract

If there are a plethora of axions in nature, they may have a complicated potential and create an axion landscape. We study a possibility that one of the axions is so light that it is cosmologically stable, explaining the observed dark matter density. In particular we focus on a case in which two (or more) shift-symmetry breaking terms conspire to make the axion sufficiently light at the potential minimum. In this case the axion has a flat-bottomed potential. In contrast to the case in which a single cosine term dominates the potential, the axion abundance as well as its isocurvature perturbations are significantly suppressed. This allows an axion with a rather large mass to serve as dark matter without fine-tuning of the initial misalignment, and further makes higher-scale inflation to be consistent with the scenario.