Gravitational wave forest from string axiverse

TL;DR

This paper explores how string theory axions with shallow potentials undergo resonance instabilities, leading to a gravitational wave forest across multiple frequencies, which could be observed in future GW experiments.

Contribution

It reveals the generation of a gravitational wave forest from string axions due to resonance instabilities, a novel mechanism linking string theory axions to observable GWs.

Findings

Resonance instabilities cause significant non-linear axion dynamics.

String axions produce a multi-frequency gravitational wave spectrum.

Potential detection of the GW forest could probe the string axiverse.

Abstract

Axions predicted in string theory may have a scalar potential which has a much shallower potential region than the conventional cosine potential. We first show that axions which were located at such shallow potential regions generically undergo prominent resonance instabilities: the well-known narrow resonance and/or the flapping resonance, which has not been well investigated. We also study non-linear dynamics of axions caused by these resonance instabilities based on lattice simulation. We find that string axions in various mass ranges generate gravitational waves (GWs) with peaks at various frequencies determined by the mass scales, dubbed the GW forest. This may allow us to explore string axiverse through future multi-frequency GW observations. We also investigate GWs produced by the axion which accounts for present dark matter component.