Gravitational Waves from an Axion-Dark Photon System: A Lattice Study

TL;DR

This study uses lattice simulations to analyze gravitational waves generated by an axion-dark photon system in the early Universe, predicting detectable signals and polarization effects influenced by particle couplings.

Contribution

It provides the first detailed lattice-based prediction of the GW spectrum from axion-dark photon interactions, including non-linear effects and polarization characteristics.

Findings

GW spectrum has more power at high momenta due to $2\rightarrow1$ processes.

The polarization of the GW peak depends on the ALP-dark photon coupling.

ALP relic abundance can be suppressed by two orders of magnitude.

Abstract

In this work, we present a lattice study of an axion - dark photon system in the early Universe and show that the stochastic gravitational wave (GW) background produced by this system may be probed by future GW experiments across a vast range of frequencies. The numerical simulation on the lattice allows us to take into account non-linear backreaction effects and enables us to accurately predict the final relic abundance of the axion or axion-like particle (ALP) as well as its inhomogeneities, and gives a more precise prediction of the GW spectrum. Importantly, we find that the GW spectrum has more power at high momenta due to $2\rightarrow1$ processes. Furthermore, we find the degree of polarization of the peak of the GW spectrum depends on the ALP-dark photon coupling and that the polarization can be washed out or even flipped for large values thereof. In line with recent results in the literature, we find the ALP relic abundance may be suppressed by two orders of magnitude and discuss possible extensions of the model that expand the viable parameter space. Finally, we discuss the possibility to probe ultralight ALP dark matter via spectral distortions of the CMB.