LIGO recorded GW150914 on 14 September 2015 and announced the detection in February 2016. The matching signals at two observatories were consistent with two black holes merging, stretching and squeezing spacetime as the waves passed. It ranks fourth because astronomy gained a messenger beyond electromagnetic radiation. The waveform is a measured signal compared with models, not a recording of sound travelling through the vacuum of space.
Source: LIGO directly detects gravitational waves.
Image credit: Caltech/MIT/LIGO Lab · CC0. Resized and JPEG compressed.
