Gravitational wave production from axion dark matter in the early universe

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Axions and axion like particles (ALPs) are motivated both as a solution of the strong CP problem and as candidates for dark matter. Recently we have shown that gravitational waves sourced by axion dynamics in the early universe may be detectable in future gravitational wave experiments. This is possible if the axion couples to a dark photon, which then experiences a tachyonic instability while the axion rolls towards the origin of its potential. The goal of this proposal is to further explore this possibility, by i) determining the region of axion parameter space that can be probed using gravitational waves, ii) performing more precise numerical simulations of the production mechanism, iii) developing methods to distinguish the axion signal from other sources of gravitational waves and iv) applying it to other models which undergo similar dynamics. Our results will provide new ways of searching for new physics and dark matter using gravitational wave observations, and improve our understanding of tachyonic particle production and its impact on the axion dark matter abundance and properties.