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 | \leq 5 \cdot 10 ^ { -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 , Hořava gravity , Generalized Proca , TeVeS and other MOND-like gravities .