Dark Energy after GW170817 and GRB170817A

TL;DR

The paper examines how the near-light speed of gravitational waves from GW170817 constrains dark energy and modified gravity models, leading to simplified theories with stable relations among operators.

Contribution

It derives conditions on scalar-tensor theories ensuring gravitational wave speed matches light speed, simplifying models and demonstrating their stability against quantum corrections.

Findings

Gravitational wave speed constraints restrict dark energy models.

Only one function remains in beyond Horndeski theories under these constraints.

The resulting models are stable under quantum corrections.

Abstract

The observation of GW170817 and its electromagnetic counterpart implies that gravitational waves travel at the speed of light, with deviations smaller than a few $\times 10^{-15}$. We discuss the consequences of this experimental result for models of dark energy and modified gravity characterized by a single scalar degree of freedom. To avoid tuning, the speed of gravitational waves must be unaffected not only for our particular cosmological solution, but also for nearby solutions obtained by slightly changing the matter abundance. For this to happen the coefficients of various operators must satisfy precise relations that we discuss both in the language of the Effective Field Theory of Dark Energy and in the covariant one, for Horndeski, beyond Horndeski and degenerate higher-order theories. The simplification is dramatic: of the three functions describing quartic and quintic beyond Horndeski theories, only one remains and reduces to a standard conformal coupling to the Ricci scalar for Horndeski theories. We show that the deduced relations among operators do not introduce further tuning of the models, since they are stable under quantum corrections.