Do super massive black holes merge?
Binary black hole
For many years, proving the existence of BBHs was made difficult because of the nature of black holes themselves, and the limited means of detection available. However, in the event that a pair of black holes were to merge, an immense amount of energy should be given off as gravitational waves, with distinctive waveforms that can be calculated using general relativity.[2][3][4] Therefore, during the late 20th and early 21st century, BBHs became of great interest scientifically as a potential source of such waves, and a means by which gravitational waves could be proven to exist. BBH mergers would be one of the strongest known sources of gravitational waves in the Universe, and thus offer a good chance of directly detecting such waves. As the orbiting black holes give off these waves, the orbit decays, and the orbital period decreases. This stage is called binary black hole inspiral. The black holes will merge once they are close enough. Once merged, the single hole settles down to a stable form, via a stage called ringdown, where any distortion in the shape is dissipated as more gravitational waves.[5] In the final fraction of a second the black holes can reach extremely high velocity, and the gravitational wave amplitude reaches its peak.
Gravitational-wave astronomy
In addition to a merger of black holes, a binary neutron star merger has been directly detected: a gamma-ray burst (GRB) was detected by the orbiting Fermi gamma-ray burst monitor on 2017 August 17 12:41:06 UTC, triggering an automated notice worldwide. Six minutes later a single detector at Hanford LIGO, a gravitational-wave observatory, registered a gravitational-wave candidate occurring 2 seconds before the gamma-ray burst. This set of observations is consistent with a binary neutron star merger, as evidenced by a multi-messenger transient event which was signalled by gravitational-wave, and electromagnetic (gamma-ray burst, optical, and infrared)-spectrum sightings.