Supermassive black hole mergers emit highest energy levels.

The merger of two supermassive black holes is expected to be the most energetic event in the post-Big Bang Universe, emitting more energy than any other event ever seen. The merger of two black holes of billions or even tens-of-billions of solar masses is eminently plausible, particularly in rich galaxy clusters. The first supermassive black holes that are inspiraling should be detectable this decade by advanced pulsar timing arrays. With an ever-improving suite of instruments, detectors, and new techniques, the first hints of a supermassive binary black hole merger might appear later this decade, which would be an incredible development for gravitational wave astronomy.