Sound speed resonance of the stochastic gravitational wave background
  We propose a novel mechanism to test time variation of the propagation speed
of gravitational waves (GWs) in light of GWs astronomy. As the stochastic GWs
experience the whole history of cosmic expansion, they encode potential
observational evidence of such variation. We report that, one feature of a
varying GWs speed is that the energy spectrum of GWs will present
resonantly-enhanced peaks if the GWs speed oscillates in time at high-energy
scales. Such oscillatory behaviour arises in a wide class of modified gravity
theories. The amplitude of these peaks can be at reach by current and
forthcoming GWs instruments, hence making the underlying theories falsifiable.
This mechanism reveals that probing the variation of GWs speed can be a
promising way to search for new physics beyond general relativity.
