The recent observation, by the Ligo and Virgo interferometers, of gravitational waves emitted by the collision of two neutron stars and their electromagnetic counterpart (in bursts of gamma rays) implies that gravitational waves move at the speed of light, at intervals of less than one part per quadrillion.

Two articles published in Physical Review Letters, one by Filippo Vernizzi (IPhT) and the other by Miguel Zumalaccaregui (currently a member of the Marie Skłodowska-Curie Actions at the Berkeley Center for Cosmological Physics and the IPhT), show the "dramatic" consequences of this result on models of dark energy and modified gravity. In some of these theories, dark energy acts as a refractive medium for the propagation of gravity, thereby excluding a large class of models.

These articles explore the implications for many well-known theories and present the most general scalar-tensor theory to leave the speed of gravity unchanged.

These two articles were inspired by the many stimulating discussions held at the "Dark Energy and Modified-Gravity cosmologies" workshop, organized at the IPhT from September 11 to October 6, 2017 by Philippe Brax, Patrik Valageas and Grégoire Misguich, several days before the announcement of the discovery.