Testing the speed of gravitational waves over cosmological distances with strong gravitational lensing

TL;DR

This paper proposes a method using strongly lensed gravitational wave and electromagnetic signals to measure the relative speed of GWs and light over cosmological distances without assuming intrinsic emission time-lags, enabling precise tests of gravity theories.

Contribution

It introduces a novel approach leveraging future strongly lensed GW events to constrain the GW and light speed ratio independently of emission time assumptions.

Findings

Potential to constrain GW and light speed ratio to 10^{-7} level

Forecast of 50-100 strongly lensed GW events per year with Einstein Telescope

Method robust against intrinsic emission time-lag uncertainties

Abstract

Probing the relative speeds of gravitational waves and light acts as an important test of General Relativity and alternative theories of gravity. Measuring the arrival time of gravitational waves and electromagnetic counterparts can be used to measure the relative speeds, but only if the intrinsic time-lag between emission of the photons and gravitational waves is well understood. Here we suggest a method that does not make such an assumption, using future strongly lensed GW events and EM counterparts; Biesiada et al forecast that 50-100 strongly lensed GW events will be observed each year with the Einstein Telescope. A single strongly lensed GW event would produce robust constraints on the ratio of speeds of GWs and light at the $10^{-7}$ level, if a high energy EM counterpart is observed within the field-of-view of an observing gamma ray burst monitor.