Description
In doubly coupled bigravity the two gravitons are coupled by a non-diagonal mass matrix and show birefringence. One of the two gravitons propagates with a speed which differs from one. This deviation is tightly constrained by both the gravitational Cerenkov effect and the energy loss of binary pulsars. When emitted from astrophysical sources, the wave form displays beats, which could be detected in future nano-Hertz interferometric experiments.