Effective speed of gravitational waves and the mirage of modified luminal gravitational wave propagation

Dr. Antonio Enea Romano (Theoretical Physics Department, CERN, CH-1211 Geneva, Switzerland) derives an effective equation and action for the propagation of gravitational waves (GW), encoding the effects of interaction and self-interaction in a time, frequency and polarization dependent effective speed. In terms of an appropriately defined effective metric, the effective action takes the form a massless Klein-Gordon action. This effective approach predicts that for theories with matter coupled to the Einstein frame metric the ratio between gravitational and electromagnetic (EM) luminosity distance depends on the effective speed, while for Jordan frame matter coupling it depends on the effective Planck mass.