Measuring the propagation speed of gravitational waves with LISA

TL;DR

This paper explores how LISA can measure potential deviations in the speed of gravitational waves at low frequencies, providing new tests of General Relativity beyond current constraints.

Contribution

It introduces models for frequency-dependent gravitational wave speeds and forecasts LISA's ability to constrain these deviations using waveform analysis.

Findings

LISA can set stringent bounds on low-frequency deviations of $c_T$ from light speed.

Transient deviations in $c_T$ could be detected without electromagnetic counterparts.

Constraints are improved over existing bounds in the low-frequency regime.

Abstract

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 Ans\"atze 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.