Nanohertz gravitational waves from the baryon-dark matter coincidence

Pulsar timing arrays have found a background of very slow gravitational waves that could come from a sudden shift in the early universe, a first-order phase transition when temperatures were about 100 MeV. The authors argue this exact scale is not random but follows from explaining why there is more matter than antimatter. Their model lets dark matter particles oscillate with neutrons in a resonant way, moving an asymmetry from the dark sector into ordinary matter. This process triggers the phase transition that radiates gravitational waves at the observed frequencies. To avoid spoiling the formation of light elements, the model adds heavy neutral leptons lighter than 100 MeV that also help explain neutrino masses. The setup predicts dark matter particles that interact with each other almost as strongly as experiments allow and suggests neutron stars cannot reach the highest masses sometimes claimed. These ideas can be checked by looking for missing energy at the LHC or unusual ways neutrons decay.