2025A&A...702L..19C

In this Letter, we propose a new model-independent strategy for a direct measurement of the speed of gravitational waves v_g based on the DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO), a future Japanese space gravitational-wave antenna. Our methodology leverages DECIGO's ability to measure the cosmological distance D_L(z) and the cosmic expansion rate H(z) at the same redshift. Each binary neutron star (BNS) inspiral creates a valuable opportunity for a direct measurement of the speed of gravitational waves (GWs) at different redshifts and directions in the sky. In the DECIGO low-frequency band, observation of BNSs during a one-year mission would produce robust measurements of the absolute value of v_g with an accuracy at the 10^–5 level. Such assessments of v_g in the low-frequency domain improve by three orders of magnitude over other direct methods based on ground-based GW detectors (in the high-frequency domain). If General Relativity is not the ultimate theory of gravity, DECIGO will provide the evidence of its failure through a one-year observation of BNS events.