Constraining early dark energy with gravitational waves before
  recombination
  We show that the nonperturbative decay of ultralight scalars into Abelian
gauge bosons, recently proposed as a possible solution to the Hubble tension,
produces a stochastic background of gravitational waves which is constrained by
the cosmic microwave background. We simulate the full nonlinear dynamics of
resonant dark photon production and the associated gravitational wave
production, finding the signals to exceed constraints for the entire parameter
space we consider. Our findings suggest that gravitational wave production from
the decay of early dark energy may provide a unique probe of these models.
