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

Understand

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.