Dark energy after GW170817, revisited

TL;DR

This paper reexamines scalar-tensor dark energy theories post-GW170817, identifying a class previously thought unviable that can still be compatible with gravitational wave data under certain conditions, but faces challenges when considering inhomogeneities.

Contribution

It reveals a class of scalar-tensor theories compatible with GW170817 constraints by linking gravitational wave speed anomalies to scalar equations of motion, and discusses limitations due to inhomogeneities.

Findings

A class of theories with anomalous GW speed linked to scalar equations of motion can be compatible with GW data.

The compatibility of these theories is challenged when inhomogeneities are included.

The loophole allowing compatibility is ultimately invalidated by large-scale inhomogeneities.

Abstract

We revisit the status of scalar-tensor theories with applications to dark energy in the aftermath of the gravitational wave signal GW170817 and its optical counterpart GRB170817A. At the level of the cosmological background, we identify a class of theories, previously declared unviable in this context, whose anomalous gravitational wave speed is proportional to the scalar equation of motion. As long as the scalar field is assumed not to couple directly to matter, this raises the possibility of compatibility with the gravitational wave data, for any cosmological sources, thanks to the scalar dynamics. This newly "rescued" class of theories includes examples of generalised quintic galileons from Horndeski theories. Despite the promise of this leading order result, we show that the loophole ultimately fails when we include the effect of large scale inhomogeneities.