Dark Energy after GW170817: dead ends and the road ahead

TL;DR

The paper uses GW170817's precise measurement of gravitational wave speed to severely constrain dark energy models, ruling out many scalar-tensor theories and identifying only a few viable alternatives that preserve the observed GW speed.

Contribution

It demonstrates that most scalar-tensor dark energy models predicting variable GW speed are incompatible with GW170817, and outlines the remaining viable model classes.

Findings

Most scalar-tensor theories are disfavored by GW speed constraints.

Only specific simplified or transformed models remain compatible with observations.

The results apply broadly to various gravity theories predicting variable GW speed.

Abstract

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.