Dark energy loopholes some time after GW170817

TL;DR

This paper re-evaluates constraints on scalar-tensor modified gravity theories after GW170817, focusing on dynamical loopholes where gravitational wave speed anomalies can vanish on-shell, using an improved effective field theory approach.

Contribution

It introduces an advanced EFT framework to analyze a broader class of theories, including Beyond Horndeski and DHOST, considering inhomogeneous perturbations and dynamical loopholes.

Findings

Identifies conditions where gravitational wave speed anomalies can vanish on-shell.

Expands analysis to include a wider class of scalar-tensor theories.

Considers effects of galactic inhomogeneities on gravitational wave constraints.

Abstract

We revisit the constraints on scalar tensor theories of modified gravity following the purge of GW170817. We pay particular attention to dynamical loopholes where the anomalous speed of propagation of the gravitational wave can vanish on-shell, when we impose the dynamical field equations. By working in the effective field theory formalism we are able to improve on previous analyses, scanning a much wider class of theories, including Beyond Horndeski and DHOST. Furthermore, the formalism is well adapted to consider the effect of inhomogeneous perturbations, where we explicitly take into account the fact that the galactic overdensities are pressureless to leading order.