The 2015 gravitational-wave discovery was GW150914, the first directly detected signal from merging black holes. Its frequency rose rapidly as the two objects spiraled inward, producing the characteristic “chirp” recorded by the Laser Interferometer Gravitational-Wave Observatory.
GW150914 was detected on September 14, 2015, by the LIGO instruments in Livingston, Louisiana, and Hanford, Washington. The signal lasted slightly more than 0.2 seconds and swept from about 35 Hz to 250 Hz. Each orbit became faster as the black holes lost energy by emitting gravitational radiation, so the wave frequency increased until merger.
The event was not a conventional sound wave and was not discovered through the Doppler effect. The rising frequency was the binary system’s inspiral signature, predicted by general relativity. LIGO measured tiny changes in the lengths of its 4-km interferometer arms. The event’s statistical significance exceeded 5 sigma, and the discovery was announced in 2016, opening the era of gravitational-wave astronomy.