Probing the scale of grand unification with gravitational waves

The spontaneous breaking of U(1)B−L around the scale of grand unification can simultaneously account for hybrid inflation, leptogenesis, and neutralino dark matter, thus resolving three major puzzles of particle physics and cosmology in a single predictive framework. The B−L phase transition also results in a network of cosmic strings. If strong and electroweak interactions are unified in an SO(10) gauge group, containing U(1)B−L as a subgroup, these strings are metastable. In this case, they produce a stochastic background of gravitational waves that evades current pulsar timing bounds, but features a flat spectrum with amplitude h2ΩGW∼10−8 at interferometer frequencies. Ongoing and future LIGO observations will hence probe the scale of B−L breaking.