Propagating speed of primordial gravitational waves and inflation
  We show that if the propagating speed of gravitational waves (GWs) gradually
diminishes during inflation, the power spectrum of primordial GWs will be
strongly blue, while that of the primordial scalar perturbation may be
unaffected. We also illustrate that such a scenario is actually a disformal
dual to the superinflation, but it does not have the ghost instability. The
blue tilt obtained is $0<n_T\lesssim 1$, which may significantly boost the
stochastic GWs background at the frequency band of Advanced LIGO/Virgo, as well
as the space-based detectors.
