Gravitational Waves from ALPs

Axion-like particles (ALPs) coupled to dark gauge fields can generate a stochastic gravitational wave (GW) background that may be observable by current or future GW observatories. The GWs are sourced by the rapid production of anisotropic stress in the dark photon field due to a tachyonic instability induced by the ALP motion. I provide a brief review of this so-called audible axion mechanism and then present two variations of it. First, I discuss this mechanism within the framework of kinetic misalignment, where the amplitude of the GW spectrum is set by the axion's kinetic energy instead of its decay constant. Then, I consider the relaxion and elaborate how it’s post-inflationary evolution can generate GWs in a similar way.