Two black holes swirl through the disk of hot gas around a supermassive black hole (top left) as they prepare to merge. This artist's concept depicts the events leading to a merger of two black holes, which was seen by Earth-based telescopes in May 2019. Gravitational-wave detectors picked up the space-time ripples caused by the merger, while optical telescopes saw a possible flare following the merger. [Caltech/R. Hurt (IPAC)]