Gravitational Waves and Dark Photon Dark Matter from Axion Rotations

Understand

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.