Axion-like particles are a recurrent feature of models of early-universe phenomena, spanning inflation, dark matter and solutions to the Hubble tension.
The nonperturbative decay of axions into beyond-the-Standard-Model photons is a generic feature of these models.
I will present the complex nonperturbative and nonlinear dynamics of axion-gauge field couplings studied via numerical simulation. These scenarios result in a significant stochastic background of gravitational waves, which provides various means to rule out and constrain models.
In the two examples I will present, the (over)production of GHz gravitational waves at preheating imposes the tightest constraints on the inflaton's axial coupling to gauge fields, while early dark energy realized via axion-dark photon systems are ruled out by CMB constraints at 10^-16 Hz.