The Stochastic Axiverse

TL;DR

This paper explores how a long period of low-scale inflation can prevent relic axion overproduction, introduces the maximal-misalignment mechanism to produce axion dark matter, and discusses its implications for dark energy and string theory constraints.

Contribution

It proposes the maximal-misalignment mechanism as a novel way to control axion relic abundance and allows axions to act as dark energy with sub-Planckian decay constants.

Findings

Long inflationary periods suppress relic axion production.

Maximal-misalignment shifts axion potential, enabling controlled dark matter production.

Axions with specific masses can behave as dark energy, compatible with swampland criteria.

Abstract

In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications. An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant. In this work, we study the minimal requirements to soften this problem and show that the fundamental requirement is a long period of low-scale inflation. However, in this case, if the inflationary Hubble scale is lower than around $O(100)$ eV, no relic DM axion is produced in the early Universe. Cosmological production of some axions may be activated, via the misalignment mechanism, if their potential minimum changes between inflation and today. As a particular example, we study in detail how the maximal-misalignment mechanism dilutes the effect of dangerous axions and allows the production of axion DM in a controlled way. In this case, the potential of the axion that realises the mechanism shifts by a factor $\Delta\theta=\pi$ between the inflationary epoch and today, and the axion starts to oscillate from the top of its potential. We also show that axions with masses $m_a\sim O(1-100)\, H_0$ realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales. Finally, we briefly study the basic phenomenological implications of the mechanism and comment on the compatibility of this type of maximally-misaligned quintessence with the swampland criteria.