Letter to the Editor

Direct measurements of the speed of gravitational waves using the Decihertz Interferometer Gravitational Wave Observatory

¹ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
² School of Physics and Astronomy, Beijing Normal University, Beijing 100875, China
³ Instituto de Física Téorica UAM-CSIC, Universidad Autónoma de Madrid 28049, Spain
⁴ School of Physics and Optoelectronic, Yangtze University, Jingzhou 434023, China
⁵ National Centre for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland
⁶ Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

^⋆ Corresponding authors: shallyn.liu@foxmail.com; Marek.Biesiada@ncbj.gov.pl; zhuzh@bnu.edu.cn

Received: 29 August 2025
Accepted: 7 October 2025

Abstract

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.