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

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.

9 pages, 2 figures; additional bound in Figure 1, minor corrections and clarifications in the text, and added references. Matches published version

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• Bounds on Ultralight Dark Matter from NANOGrav
• Enhancement of gravitational waves at Q-ball decay including non-linear density perturbations
• Diraxiogenesis
• Stochastic gravitational wave background: birth from string-wall death
• Irreducible cosmological backgrounds of a real scalar with a broken symmetry
• Interplay between the muon $g-2$ anomaly and the PTA nHZ gravitational waves from domain walls in next-to minimal supersymmetric standard model