This quantity represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Such observations enable GW170817 to be used as a ‘standard siren’ 14, 15, 16, 17, 18 (meaning that the absolute distance to the source can be determined directly from the gravitational-wave measurements) to measure the Hubble constant. This detection of GW170817 in both gravitational waves and electromagnetic waves represents the first ‘multi-messenger’ astronomical observation. This sky region was subsequently observed by optical astronomy facilities 7, resulting in the identification 8, 9, 10, 11, 12, 13 of an optical transient signal within about ten arcseconds of the galaxy NGC 4993. Less than two seconds after the merger, a γ-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO–Virgo-derived location of the gravitational-wave source 4, 5, 6. On 17 August 2017, the Advanced LIGO 1 and Virgo 2 detectors observed the gravitational-wave event GW170817-a strong signal from the merger of a binary neutron-star system 3.
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