Friendship in the Axiverse: Late-time direct and astrophysical signatures of early-time nonlinear axion dynamics

TL;DR

This paper explores how nonlinear interactions among axions in the string axiverse can produce unique signatures, enhancing detection prospects and affecting dark matter structure, with implications for upcoming experiments and astrophysical observations.

Contribution

It demonstrates that nonlinearities in the axion potential cause resonant energy transfer between axions, leading to observable signatures not previously considered in axiverse models.

Findings

Resonant energy transfer occurs between axions with nearby masses.

Enhanced direct detection prospects for certain axion pairs.

Boosted small-scale structure if the pair constitutes most of dark matter.

Abstract

A generic low-energy prediction of string theory is the existence of a large collection of axions, commonly known as a string axiverse. Axions also have a natural cosmological production mechanism, vacuum misalignment, making them well-motivated dark matter (DM) candidates. Much work on axion production has considered the case of a single free axion, but in a realistic axiverse, string axions are expected to be distributed densely over many orders of magnitude in mass, and to interact with one another through their joint potential. In this paper, we show that non-linearities in this potential lead to a new type of resonant energy transfer between axions with nearby masses. This resonance generically transfers energy from axions with larger decay constants to those with smaller decay constants, and leads to a multitude of signatures. These include enhanced direct detection prospects for a resonant pair comprising even a small subcomponent of dark matter, and boosted small-scale structure if the pair is the majority of DM. Near-future iterations of experiments such as ADMX and DM Radio will be sensitive to this scenario, as will astrophysical probes of DM substructure.