An international team of astronomers witnessed the birth of the first intermediate-mass black hole ever detected. On May 21, 2019, a pair of gravitational wave observatories — The Laser Interferometry Gravitational-wave Observatory (LIGO) in the United States, and Virgo, a three-kilometer-long detector in Italy — detected an unusual signal, dubbed GW190521. This signal, resembling four short wiggles, lasted less than a tenth of a second. Yet, it revealed the formation of the first intermediate-mass black hole ever discovered. “One of the great mysteries in astrophysics is how do supermassive black holes form? They are the million solar-mass elephants in the room. Do…

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