Superluminal Gravitational Waves
  The quantum gravity effects of vacuum polarization of gravitons propagating
in a curved spacetime cause the quantum vacuum to act as a dispersive medium
with a refractive index. Due to this dispersive medium gravitons acquire
superluminal velocities. The dispersive medium is produced by higher derivative
curvature contributions to the effective gravitational action. It is shown that
in a Friedmann-Lema\^{i}tre-Robertson-Walker spacetime in the early universe
near the Planck time $t_{\rm PL}\gtrsim 10^{-43}\,{\rm sec}$, the speed of
gravitational waves $c_g\gg c_{g0}=c_0$, where $c_{g0}$ and $c_0$ are the
speeds of gravitational waves and light today. The large speed of gravitational
waves stretches their wavelengths to super-horizon sizes, allowing them to be
observed in B-polarization experiments.
