No Slip Gravity

TL;DR

No Slip Gravity is a modified gravity theory within the Horndeski class that features equal gravitational couplings to matter and light, no gravitational wave speed deviation, and suppressed structure growth, aligning better with galaxy observations.

Contribution

The paper introduces No Slip Gravity, a subclass of Horndeski theories with unique properties, and provides observational constraints and a method to compare gravitational wave and electromagnetic distances.

Findings

Gravitational wave speed equals light speed in this theory.

Effective gravitational couplings to matter and light are equal.

The theory predicts suppressed growth of cosmic structures.

Abstract

A subclass of the Horndeski modified gravity theory we call No Slip Gravity has particularly interesting properties: 1) a speed of gravitational wave propagation equal to the speed of light, 2) equality between the effective gravitational coupling strengths to matter and light, $G_{\rm matter}$ and $G_{\rm light}$, hence no slip between the metric potentials, yet difference from Newton's constant, and 3) suppressed growth to give better agreement with galaxy clustering observations. We explore the characteristics and implications of this theory, and project observational constraints. We also give a simple expression for the ratio of the gravitational wave standard siren distance to the photon standard candle distance, in this theory and others, and enable a direct comparison of modified gravity in structure growth and in gravitational waves, an important crosscheck.