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.
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.
The existence of stellar-mass binary black holes (and gravitational waves themselves) was finally confirmed when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected GW150914 (detected September 2015, announced February 2016), a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 solar masses each, occurring about 1.3 billion light-years 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 watts – more than the combined power of all light radiated by all the stars in the observable universe put together.
[9] Stellar-mass binary black holes have been demonstrated to exist, by the first detection of a black-hole merger event GW150914 by LIGO.
[14] Other galactic nuclei have periodic emissions suggesting large objects orbiting a central black hole, for example, in OJ287.
[16] The quasar PKS 1302-102 appears to have a binary black hole with an orbital period of 1900 days.
[17] When two galaxies collide, the supermassive black holes at their centers are very unlikely to hit head-on and would most likely shoot past each other on hyperbolic trajectories, unless some mechanism brings them together.
The most important mechanism is dynamical friction, which transfers kinetic energy from the black holes to nearby matter.
Gravitational waves can cause significant loss of orbital energy, but not until the separation shrinks to a much smaller value, roughly 0.01–0.001 parsec.
If enough stars pass close by to the orbiting pair, their gravitational ejection can bring the two black holes together in an astronomically plausible time.
The first stages of the inspiral take a very long time, as the gravitational waves emitted are very weak when the black holes are distant from each other.
In this region most of the emitted gravitational waves go towards the event horizon, and the amplitude of those escaping reduces.
[28][29] The first observation of stellar-mass binary black holes merging, GW150914, was performed by the LIGO detector.
[2][3][4] Some simplified algebraic models can be used for the case where the black holes are far apart, during the inspiral stage, and also to solve for the final ringdown.
Effective-one-body (EOB) approximation solves the dynamics of the binary black-hole system by transforming the equations to those of a single object.
The final Kerr black hole is distorted, and the spectrum of frequencies it produces can be calculated.
Description of the entire evolution, including merger, requires solving the full equations of general relativity.
In these calculations it is important to have enough fine detail close into the black holes, and yet have enough volume to determine the gravitation radiation that propagates to infinity.
[37] In the ringdown phase of a Kerr black hole, frame-dragging produces a gravitation wave with the horizon frequency.
[37] The radiation reaction force can be calculated by Padé resummation of gravitational wave flux.
, with f(U) being the gravitational wave flux at retarded time U. f is a surface integral of the news function at null infinity varied by solid angle.
It is calculated by integrating the product of the multipolar metric waveform with the news function complement over retarded time.
For non-spinning black holes a maximum recoil velocity of 175 km/s occurs for masses in the ratio of five to one.
[42] Parameters that may be of interest include the point at which the black holes merge, the mass ratio that produces maximum kick, and how much mass/energy is radiated via gravitational waves.