Dark Energy after GW170817: dead ends and the road ahead
  Multi-messenger gravitational wave (GW) astronomy has commenced with the
detection of the binary neutron star merger GW170817 and its associated
electromagnetic counterparts. The almost coincident observation of both signals
places an exquisite bound on the GW speed $|c_g/c-1|\leq5\cdot10^{-16}$. We use
this result to probe the nature of dark energy (DE), showing that a large class
of scalar-tensor theories and DE models are highly disfavored. As an example we
consider the covariant Galileon, a cosmologically viable, well motivated
gravity theory which predicts a variable GW speed at low redshift. Our results
eliminate any late-universe application of these models, as well as their
Horndeski and most of their beyond Horndeski generalizations. Three
alternatives (and their combinations) emerge as the only possible scalar-tensor
DE models: 1) restricting Horndeski's action to its simplest terms, 2) applying
a conformal transformation which preserves the causal structure and 3)
compensating the different terms that modify the GW speed (to be robust, the
compensation has to be independent on the background on which GWs propagate).
Our conclusions extend to any other gravity theory predicting varying $c_g$
such as Einstein-Aether, Ho\v{r}ava gravity, Generalized Proca, TeVeS and other
MOND-like gravities.
