  The dynamical generation of right-handed-neutrino (RHN) masses in the early
Universe naturally entails the formation of cosmic strings that give rise to an
observable signal in gravitational waves (GWs). Here, we show that a
characteristic break in the GW spectrum would provide evidence for a new stage
in the cosmological expansion history and a suppression of the RHN mass scale
compared to the scale of spontaneous symmetry breaking. The detection of such a
spectral feature would thus represent a novel and unique possibility to probe
the physics of RHN mass generation in regions of parameter space that allow for
low-scale leptogenesis in accord with electroweak naturalness.
