Testing the Neutrino Seesaw Scale with Gravitational Waves

Neutrino masses are not explained within the Standard Model of elementary particles and require postulating new physics. Their smallness can be accommodated through the seesaw mechanism. Here I consider the possibility of a PeV-scale seesaw mechanism within the framework of a model with gauged U(1) lepton number. The theory predicts a primordial stochastic gravitational wave background, at high frequencies arising from a first order phase transition in the early Universe, and at low frequencies generated by domain wall annihilation. This leads to a double-peaked structure in the gravitational wave spectrum searchable in upcoming experiments, such as gravitational wave interferometers, pulsar timing arrays, and astrometry observations.