Binary black hole

A binary black hole (BBH), or black hole binary, is an <a href="/facts/Astronomical_object/BCHLa0Q2">astronomical object</a> consisting of two <a href="/facts/Black_hole/1r2lmkcF">black holes</a> in close orbit around each other. Like black holes themselves, binary black holes are often divided into binary <a href="/facts/Stellar_black_hole/8cz1uzAe">stellar black holes</a>, formed either as remnants of high-mass <a href="/facts/Binary_star/TYlDe1bD">binary star</a> systems or by <a href="/facts/Stellar_dynamics/TTqAf5M1">dynamic processes</a> and mutual capture; and binary <a href="/facts/Supermassive_black_hole/BcuTx8Q5">supermassive black holes</a>, believed to be a result of <a href="/facts/Galactic_merger/WRKsV7t2">galactic mergers</a>.
For many years, proving the existence of binary black holes 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 <a href="/facts/Gravitational_wave/4XWN3NtG">gravitational waves</a>, with distinctive <a href="/facts/Waveform/yAskMREw">waveforms</a> that can be calculated using <a href="/facts/General_relativity/jVsoIg8X">general relativity</a>. Therefore, during the late 20th and early 21st century, binary black holes became of great interest scientifically as a potential source of such waves and a means by which gravitational waves could be proven to exist. Binary black hole mergers would be one of the strongest known sources of gravitational waves in the universe, and thus offer a good chance of <a href="/facts/Gravitational_wave/4XWN3NtG">directly detecting such waves</a>. 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. In the final fraction of a second the black holes can reach extremely high velocity, and the gravitational wave amplitude reaches its peak.
The existence of stellar-mass binary black holes (and gravitational waves themselves) was finally confirmed when the <a href="/facts/Laser_Interferometer_Gravitational-Wave_Observatory/jX6jyJzM">Laser Interferometer Gravitational-Wave Observatory</a> (LIGO) detected <a href="/facts/GW150914/7yep4aM0">GW150914</a> (detected September 2015, announced February 2016), a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 <a href="/facts/Solar_mass/j6Auhku8">solar masses</a> each, occurring about 1.3 billion <a href="/facts/Light-year/IGc5NdQb">light-years</a> away. In its final 20 ms of spiraling inward and merging, GW150914 released around 3 solar masses as gravitational energy, peaking at a rate of 3.6×1049 <a href="/facts/Watt/VK8oqRrm">watts</a> – more than the combined power of all light radiated by all the stars in the <a href="/facts/Observable_universe/CdTGMdsp">observable universe</a> put together. Supermassive binary black hole candidates have been found, but not yet categorically proven.

Binary black hole open-in-new

Binary black hole