Oscillating modulation to B-mode polarization from varying propagating
  speed of primordial gravitational waves
  In low-energy effective string theory and modified gravity theories, the
propagating speed $c_T$ of primordial gravitational waves may deviate from
unity. We find that the step-like variation of $c_T$ during slow-roll inflation
may result in an oscillating modulation to the B-mode polarization spectrum,
which can hardly be imitated by adjusting other cosmological parameters, and
the intensity of the modulation is determined by the dynamics of $c_T$. Thus
provided that the foreground contribution is under control, high-precision CMB
polarization observations will be able to put tight constraint on the variation
of $c_T$, and so the corresponding theories.
