http://arxiv.org/abs/1710.05901
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 the GW and the gamma ray burst GRB170817A constrain the speed of GWs at the level of $|c_g/c-1|\leq4.5\cdot10^{-16}$. We use this result to probe the nature of dark energy (DE), showing that scalar-tensor theories with derivative interactions with the curvature are highly disfavored. As an example we consider the case of Galileons, a well motivated gravity theory with viable cosmology, which predicts a variable GW speed at low redshift, and is hence strongly ruled out by GW170817. Our result essentially eliminates any cosmological application of these DE models and, in general, of quartic and quintic Horndeski and most beyond Horndeski theories. We identify the surviving scalar-tensor models and, in particular, present specific beyond Horndeski theories avoiding this constraint. The viable scenarios are either conformally equivalent to theories in which $c_g=c$ or rely on cancellations of the anomalous GW speed that are valid on arbitrary backgrounds. Our conclusions can be extended to any other gravity theory predicting an anomalous GW propagation speed such as Einstein-Aether, Ho\v{r}ava gravity, Generalized Proca, TeVeS and other MOND-like gravities.
J. Ezquiaga and M. Zumalacarregui
Tue, 17 Oct 17
78/163
Comments: 9 pages, 3 figures