Luminal scalar-tensor theories for a not so dark dark energy

In general the speed of gravitational waves (GWs) in scalar-tensor modifications of Einstein’s gravity is different from the speed of light. Nevertheless, it has been measured that their speeds are nearly the same. For the most general scalar-tensor theories classified to date that do propagate a graviton—DHOST, —we show that, remarkably, up to five self-consistent couplings of the scalar of dark energy (DE) to the photon are enough to make their GWs luminal in a wide set of cases. We find at least one luminal BH theory for which the GW decay into DE is suppressed in any cosmological background.