We show that the relaxion coupled to a dark photon can generate a stochastic gravitational wave background in the early Universe. When the electroweak symmetry is restored after reheating, the relaxion starts rolling again until the back-reaction potential wiggles reappear. Depending on the time of barrier-reappearance, Hubble friction alone is insufficient to trap the relaxion in a large portion of the parameter space. Thus, an additional source of friction is required, which can for instance be provided by coupling to a dark photon. The dark photon experiences a tachyonic instability as the relaxion rolls, which slows down the relaxion by back-reacting to its motion, and creates anisotropies in the dark photon energy-momentum tensor, sourcing gravitational waves. We calculate the spectrum of the resulting stochastic gravitational wave background and evaluate its observability by current and future experiments. We further investigate the case that the coherently oscillating relaxion constitutes dark matter and present the corresponding constraints from gravitational waves.
|Collaboration / Activity||Weizmann Institute of Science|
|First author||Eric Madge|