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

### Submission summary

 As Contributors: Pedro Schwaller Preprint link: scipost_202104_00020v1 Date accepted: 2021-06-18 Date submitted: 2021-04-19 12:14 Submitted by: Schwaller, Pedro Submitted to: SciPost Physics Academic field: Physics Specialties: High-Energy Physics - Theory High-Energy Physics - Phenomenology Cosmology and Nongalactic Astrophysics Approach: Theoretical

### 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.

Published as SciPost Phys. 11, 001 (2021)

### Submission & Refereeing History

Resubmission scipost_202104_00020v1 on 19 April 2021
Submission 2012.11584v2 on 6 January 2021

## Reports on this Submission

### Report

The reply of the authors to the referee report is satisfactory. The new version of the manuscript meets the acceptance criteria of the Journal and I recommend it for publication.

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