Supercooled Audible Axions

Abstract: In the audible axion mechanism, axion-like particles source primordial gravitational waves via their coupling to a dark Abelian gauge field. The original setup, however, relies on a large axion decay constant and coupling to produce sizable signals. In this article, we show that delaying the onset of axion oscillations opens up the testable parameter space and reduces the required coupling to $\alpha \gtrsim 1$. Furthermore, we investigate the emission of gravitational waves via the axion coupling to the Standard Model photon in the presence of Schwinger pair production, generating a strong signal in the $\mu$Hz or ultra-high frequency range. Cosmological constraints and gravitational wave projections are provided for both scenarios.