Measuring the propagation speed of gravitational waves with LISA

The propagation speed of gravitational waves, c(T), has been tightly constrained by the binary neutron star merger GW170817 and its electromagnetic counterpart, under the assumption of a frequency-independent c(T). Drawing upon arguments from Effective Field Theory and quantum gravity, we discuss the possibility that modifications of General Relativity allow for transient deviations of c(T) from the speed of light at frequencies well below the band of current ground-based detectors. We motivate two representative Ansatze for c(T)(f), and study their impact upon the gravitational waveforms of massive black hole binary mergers detectable by the LISA mission. We forecast the constraints on c(T)(f) obtainable from individual systems and a population of sources, from both inspiral and a full inspiral-merger-ringdown waveform. We show that LISA will enable us to place stringent independent bounds on departures from General Relativity in unexplored low-frequency regimes, even in the absence of an electromagnetic counterpart.

Automated summary

The possibility of transient deviations of the propagation speed of gravitational waves from the speed of light at low frequencies, based on modifications of General Relativity, is discussed. The impact of these deviations on the gravitational waveforms of black hole binary mergers detectable by the LISA mission is studied. Constraints on these deviations obtainable from individual systems and a population of sources are forecasted.