65 pags., 22 figs., 9 tabs.
The propagation speed of gravitational waves, c , has been tightly constrained by the binary neutron star merger GW170817 and its electromagnetic counterpart, under the assumption of a frequency-independent c . 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 from the speed of light at frequencies well below the band of current ground-based detectors. We motivate two representative Ansätze for c (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 (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.
T.B. is supported by ERC Starting Grant SHADE (grant no. StG 949572) and a Royal Society University Research Fellowship (grant no. URF\ R1\180009). G.C. is supported by the I+D grant PID2020-118159GB-C41 of the Spanish Ministry of Science and Innovation. A.C. is supported by a PhD grant from the Chinese Scholarship Council (grant no.202008060014); this paper and the codes developed for it form part of his PhD thesis work. M.F. would like to acknowledge support from the “Atracción de Talento” grant 2019-T1/TIC15784, his work is partially supported by the Spanish Research Agency (Agencia Estatal de Investigación) through the Grant IFT Centro de Excelencia Severo Ochoa No CEX2020-001007-S, funded by MCIN/AEI/10.13039/501100011033. L.L. is supported by a Swiss National Science Foundation Professorship grant (Nos. 170547 & 202671). K.M. is supported by King’s College London through a Postgraduate International Scholarship. M.P. was supported by STFC grants ST/P000762/1 and ST/T000791/1. M.P. acknowledges support by the European Union’s Horizon 2020 Research Council grant 724659 MassiveCosmo ERC- 2016-COG. M.S. is supported in part by the Science and Technology Facility Council (STFC), United Kingdom, under the research grant ST/P000258/1. G.T. is partially funded by the STFC grant ST/T000813/1. D.B. acknowledges partial financial support by ASI Grant No. 2016-24-H.0. I.D.S. is supported by the Grant Agency of the Czech Republic (GAČR), under the grant number 21-16583M.