Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories

TL;DR

The detection of GW170817 has significantly constrained scalar-tensor theories of gravity, limiting their parameters and relevance for dark energy models, and establishing bounds on disformal couplings to photons.

Contribution

This paper analyzes how gravitational wave observations from GW170817 restrict scalar-tensor gravity theories and their implications for dark energy models and disformal couplings.

Findings

Severely constrains three parameters of beyond Horndeski models.

Rules out the cosmological relevance of the quartic galileon model.

Derives bounds on disformal couplings to photons.

Abstract

The LIGO/VIRGO collaboration has recently announced the detection of gravitational waves from a neutron star-neutron star merger (GW170817) and the simultaneous measurement of an optical counterpart (the gamma-ray burst GRB 170817A). The close arrival time of the gravitational and electromagnetic waves limits the difference in speed of photons and gravitons to be less than about one part in $10^{15}$. This has three important implications for cosmological scalar-tensor gravity theories that are often touted as dark energy candidates and alternatives to $\Lambda$CDM. First, for the most general scalar-tensor theories---beyond Horndeski models---three of the five parameters appearing in the effective theory of dark energy can now be severely constrained on astrophysical scales; we present the results of combining the new gravity wave results with galaxy cluster observations. Second, the combination with the lack of strong equivalence principle violations exhibited by the supermassive black hole in M87, constrains the quartic galileon model to be cosmologically irrelevant. Finally, we derive a new bound on the disformal coupling to photons that implies that such couplings are irrelevant for the cosmic evolution of the field.