  An axion rotating in field space can produce dark photons in the early
universe via tachyonic instability. This explosive particle production creates
a background of stochastic gravitational waves that may be visible at pulsar
timing arrays or other gravitational wave detectors. This scenario provides a
novel history for dark photon dark matter. The dark photons may be warm at a
level detectable in future 21-cm line surveys. For a consistent cosmology, the
radial direction of the complex field containing the axion must be thermalized.
We explore a concrete thermalization mechanism in detail and also demonstrate
how this setup can be responsible for the generation of the observed baryon
asymmetry.
