Challenges to Self-Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure

TL;DR

This paper discusses how future gravitational wave speed measurements could challenge the viability of certain scalar-tensor theories explaining cosmic acceleration, showing that such theories may be inconsistent with observed cosmological data.

Contribution

It demonstrates that confirming the speed of gravity equals the speed of light constrains and potentially rules out a broad class of self-accelerating scalar-tensor theories.

Findings

Gravitational wave speed measurement can test modified gravity theories.

Self-accelerated models with standard gravitational-wave speed fit data less well.

Equality of gravitational wave speed and light speed challenges scalar-tensor explanations of cosmic acceleration.

Abstract

With the advent of gravitational-wave astronomy marked by the aLIGO GW150914 and GW151226 observations, a measurement of the cosmological speed of gravity will likely soon be realized. We show that a confirmation of equality to the speed of light as indicated by indirect Galactic observations will have important consequences for a very large class of alternative explanations of the late-time accelerated expansion of our Universe. It will break the dark degeneracy of self-accelerated Horndeski scalar-tensor theories in the large-scale structure that currently limits a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy. Signatures of a self-acceleration must then manifest in the linear, unscreened cosmological structure. We describe the minimal modification required for self-acceleration with standard gravitational-wave speed and show that its maximum likelihood yields a 3-sigma poorer fit to cosmological observations compared to a cosmological constant. Hence, equality between the speeds challenges the concept of cosmic acceleration from a genuine scalar-tensor modification of gravity.