Observable gravitational waves from unstable ALP catastrogenesis

Abstract

We focus on gravitational waves (GWs) observable in pulsar timing array and interferometric searches that are produced by the annihilation of cosmic string-wall networks in axion-like particle (ALP) models. We called this process "catastrogenesis." It produces GWs with a characteristic peaked spectrum. These GWs would not be observable if the ALPs were stable. We thus assume that ALPs decay into two photons before Big-Bang nucleosynthesis (BBN). The large ALP-photon coupling required could induce field dissipation and friction on cosmic walls through interactions with the radiation background. We study the conditions under which these effects are negligible and map the parameter regions yielding observable GW signals onto the usual ALP mass-coupling parameter space. We find that, in these regions, ALPs dominate the energy density of the Universe before they decay, leading to a period of intermediate matter domination, which would leave a clear imprint on the GW spectrum.