Catastrogenesis with unstable ALPs as the origin of the NANOGrav 15 yr gravitational wave signal

TL;DR

This paper proposes that the gravitational wave background detected by NANOGrav may originate from a process called catastrogenesis, involving the annihilation of unstable axion-like particle domain walls in the early universe.

Contribution

It introduces the concept of catastrogenesis as a source of gravitational waves and identifies parameter regions of ALPs compatible with NANOGrav data.

Findings

ALP domain wall annihilation can produce detectable gravitational waves.

Parameter space of ALPs consistent with NANOGrav signal identified.

The model fits within current experimental constraints.

Abstract

In post-inflation axion-like particle (ALP) models, a stable domain wall network forms if the model's potential has multiple minima. This system must annihilate before dominating the Universe's energy density, producing ALPs and gravitational waves (a process we dub "catastrogenesis," or "creation via annihilation"). We examine the possibility that the gravitational wave background recently reported by NANOGrav is due to catastrogenesis. For the case of ALP decay into two photons, we identify the region of ALP mass and coupling, just outside current limits, compatible with the NANOGrav signal.