  The LIGO/VIRGO collaboration has recently announced the detection of
gravitational waves from a neutron star-neutron star merger (GW170817) and the
simultaneous measurement of an optical counterpart (the gamma-ray burst GRB
170817A). The close arrival time of the gravitational and electromagnetic waves
limits the difference in speed of photons and gravitons to be less than about
one part in $10^{15}$. This has three important implications for cosmological
scalar-tensor gravity theories that are often touted as dark energy candidates
and alternatives to $\Lambda$CDM. First, for the most general scalar-tensor
theories---beyond Horndeski models---three of the five parameters appearing in
the effective theory of dark energy can now be severely constrained on
astrophysical scales; we present the results of combining the new gravity wave
results with galaxy cluster observations. Second, the combination with the lack
of strong equivalence principle violations exhibited by the supermassive black
hole in M87, constrains the quartic galileon model to be cosmologically
irrelevant. Finally, we derive a new bound on the disformal coupling to photons
that implies that such couplings are irrelevant for the cosmic evolution of the
field.
