Gravitational wave probes of axion-like particles

TL;DR

This paper explores how future gravitational wave detectors can probe axion-like particles in weakly coupled regimes, expanding the potential for understanding dark matter candidates through gravitational signals.

Contribution

It identifies ALP parameter regions accessible to future GW detectors and discusses effects of dark photon mass and couplings on GW signals.

Findings

Future GW detectors can probe weakly coupled ALPs.

Finite dark photon mass affects GW emission.

ALP scenarios can be tested with provided signal templates.

Abstract

We have recently shown that axions and axion-like particles (ALPs) may emit an observable stochastic gravitational wave (GW) background when they begin to oscillate in the early universe. In this note, we identify the regions of ALP parameter space which may be probed by future GW detectors, including ground- and space-based interferometers and pulsar timing arrays. Interestingly, these experiments have the ability to probe axions from the bottom up, i.e. in the very weakly coupled regime which is otherwise unconstrained. Furthermore, we discuss the effects of finite dark photon mass and kinetic mixing on the mechanism, as well as the (in)sensitivity to couplings of the axion to Standard Model fields. We conclude that realistic axion and ALP scenarios may indeed be probed by GW experiments in the future, and provide signal templates for further studies.