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The LIGO/VIRGO collaboration has recently announced the detection ofgravitational waves from a neutron star-neutron star merger (GW170817) and thesimultaneous measurement of an optical counterpart (the gamma-ray burst GRB170817A). The close arrival time of the gravitational and electromagnetic waveslimits the difference in speed of photons and gravitons to be less than aboutone part in $10^{15}$. This has three important implications for cosmologicalscalar-tensor gravity theories that are often touted as dark energy candidatesand alternatives to $\Lambda$CDM. First, for the most general scalar-tensortheories---beyond Horndeski models---three of the five parameters appearing inthe effective theory of dark energy can now be severely constrained onastrophysical scales; we present the results of combining the new gravity waveresults with galaxy cluster observations. Second, the combination with the lackof strong equivalence principle violations exhibited by the supermassive blackhole in M87, constrains the quartic galileon model to be cosmologicallyirrelevant. Finally, we derive a new bound on the disformal coupling to photonsthat implies that such couplings are irrelevant for the cosmic evolution of thefield.

Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories