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 h^2\Omega_GW ~ 10^-8 at interferometer
frequencies. Ongoing and future LIGO observations will hence probe the scale of
B-L breaking.
