Gravitational Rainbows: LIGO and Dark Energy at its Cutoff

TL;DR

This paper explores how dark energy models near their cutoff scale can alter gravitational wave speeds, potentially reconciling observational constraints with theoretical predictions.

Contribution

It demonstrates that near the cutoff scale, operators in dark energy models can restore gravitational wave speed to light speed, challenging previous assumptions.

Findings

Gravitational wave speed deviations occur near the cutoff scale.

Operators at the cutoff can restore the wave speed to light speed.

Future missions like LISA and PTAs can test these effects.

Abstract

The recent direct detection of a neutron star merger with optical counterpart has been used to severely constrain models of dark energy that typically predict a modification of the speed of gravitational waves. We point out that the energy scales observed at LIGO, and the particular frequency of the neutron star event, lie very close to the strong coupling scale or cutoff associated with many dark energy models. While it is true that at very low energies one expects gravitational waves to travel at a speed different than light in these models, the same is no longer necessarily true as one reaches energy scales close to the cutoff. We show explicitly how this occurs in a simple model with a known partial UV completion. Within the context of Horndeski, we show how the operators that naturally lie at the cutoff scale can affect the speed of propagation of gravitational waves and bring it back to unity at those scales. We discuss how further missions including LISA and PTAs could play an essential role in testing such models.