The fate of large-scale structure in modified gravity after GW170817 and GRB170817A

TL;DR

The paper examines how recent gravitational wave observations constrain modifications to gravity, showing that only certain scalar-tensor theories remain viable and analyzing their implications for cosmic structure formation.

Contribution

It demonstrates that GW170817 constraints limit gravitational slip to conformal scalar-tensor theories and explores the resulting effects on structure growth and scale dependence.

Findings

Dark matter growth rate must be at least as fast as in GR.

Any scale dependence in gravitational slip must match GR at large scales.

Viability of vector-tensor theories for gravitational slip is eliminated.

Abstract

The coincident detection of gravitational waves (GW) and a gamma-ray burst from a merger of neutron stars has placed an extremely stringent bound on the speed of GW. We showed previously that the presence of gravitational slip ($\eta$) in cosmology is intimately tied to modifications of GW propagation. This new constraint implies that the only remaining viable source of gravitational slip is a conformal coupling to gravity in scalar-tensor theories, while viable vector-tensor theories cannot now generate gravitational slip at all. We discuss structure formation in the remaining viable models, demonstrating that (i) the dark-matter growth rate must now be at least as fast as in GR, with the possible exception of the beyond Horndeski model. (ii) If there is any scale-dependence at all in the slip parameter, it is such that it takes the GR value at large scales. We show a consistency relation which must be violated if gravity is modified.