Gravitational wave forest from string axiverse

Abstract

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