Binary black hole

A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.

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.

The existence of stellar-mass binary black holes (and gravitational waves themselves) were finally confirmed when 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 moments 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 - during its last brief moments.[6][7][8] Supermassive binary black hole candidates have been found but as yet, not categorically proven.[9]

Computer simulation of the black hole binary system GW150914 as seen by a nearby observer, during its final inspiral, merge, and ringdown. The star field behind the black holes is being heavily distorted and appears to rotate and move, due to extreme gravitational lensing, as space-time itself is distorted and dragged around by the rotating black holes.[1]


Supermassive black-hole binaries are believed to form during galaxy mergers. Some likely candidates for binary black holes are galaxies with double cores still far apart. An example double nucleus is NGC 6240.[10] Much closer black-hole binaries are likely in single core galaxies with double emission lines. Examples include SDSS J104807.74+005543.5[11] and EGSD2 J142033.66 525917.5.[10] Other galactic nuclei have periodic emissions suggesting large objects orbiting a central black hole, for example in OJ287.[12]

The quasar PG 1302-102 appears to have a binary black hole with an orbital period of 1900 days.[13]

Stellar mass binary black holes have been demonstrated to exist, by the first detection of a black hole merger event GW150914 by LIGO.[14]

Final parsec problem

When two galaxies collide, the supermassive black holes at their centers do not hit head-on, but would shoot past each other if some mechanism did not bring them together. The most important mechanism is dynamical friction, which brings the black holes to within a few parsecs of each other. At this distance, they form a bound, binary system. The binary system must lose orbital energy somehow, for the black holes to orbit more closely or merge.[15]

Initially, the explanation is easy. The black holes transfer energy to gas and stars between them, ejecting matter at high speed via a gravitational slingshot and thereby losing energy. However, the volume of space subject to this effect shrinks as the orbits do, and when the black holes reach a separation of about one parsec, there is so little matter left between them that it would take billions of years to orbit closely enough to merge - more than the age of the universe. Gravitational waves can be a significant contributor, but not until the separation shrinks to a much smaller value, roughly 0.01–0.001 parsec.

Nonetheless, supermassive black holes appear to have merged, and what appears to be a pair in this intermediate range has been observed, in PKS 1302-102.[16] The question of how this happens is the "final parsec problem".[17]

A number of solutions to the final parsec problem have been proposed. Most involve the interaction of the massive binary with surrounding matter, either stars or gas, which can extract energy from the binary and cause it to shrink. For instance, if enough stars pass close by to the orbiting pair, their gravitational ejection can bring the two black holes together much quicker than would otherwise be the case.[18]



The first stage of the life of a binary black hole is the inspiral which resembles a gradually shrinking orbit. The first stages of the inspiral take a very long time, as the gravitation waves emitted are very weak when the black holes are distant from each other. In addition to the orbit shrinking due to the emission of gravitational waves, extra angular momentum may be lost due to interactions with other matter present, such as other stars.

As the black holes’ orbit shrinks, the speed increases, and gravitational wave emission increases. When the black holes are close the gravitational waves cause the orbit to shrink rapidly.

The last stable orbit or innermost stable circular orbit (ISCO) is the innermost complete orbit before the transition from inspiral to merger.


This is followed by a plunging orbit in which the two black holes meet, followed by the merger. Gravitational wave emission peaks at this time.


Immediately following the merger, the now single black hole will “ring” – oscillating in shape between a distorted, elongated spheroid and a flattened spheroid. This ringing is damped in the next stage, called the ringdown, by the emission of gravitational waves. The distortions from the spherical shape rapidly reduce until the final stable sphere is present, with a possible slight distortion due to remaining spin.


The first observation of stellar mass binary black holes merging was performed by the LIGO detector.[14][19][20] As measured from earth, a pair of black holes with estimated masses around 36 and 29 times that of the Sun spun into each other and merged to form a 62 solar mass black hole (approximate) on 14 September 2015, at 09:50 UTC.[21] Three solar masses were converted to gravitational radiation in the final fraction of a second, with a peak power 3.6×1056 ergs/second (200 solar masses per second),[14] which is 50 times the total output power of all the stars in the observable universe.[22] The merger took place at 1.3 billion light years from Earth.[19] The observed signal is consistent with the predictions of numerical relativity.[2][3][4]

Dynamics modelling

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.

Post-Newtonian approximations can be used for the inspiral. These approximate the general relativity field equations adding extra terms to equations in Newtonian gravity. Orders used in these calculations may be termed 2PN (second order post Newtonian) 2.5PN or 3PN (third order post Newtonian). Effective-one-body (EOB) solves the dynamics of the binary black hole system by transforming the equations to those of a single object. This is especially useful where mass ratios are large, such as a stellar mass black hole merging with a galactic core black hole, but can also be used for equal mass systems.

For the ringdown, black hole perturbation theory can be used. The final Kerr black hole is distorted, and the spectrum of frequencies it produces can be calculated.

To solve for the entire evolution, including merger, requires solving the full equations of general relativity. This can be done in numerical relativity simulations. Numerical relativity models space-time and simulates its change over time. 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. In order to make this have few enough points to be tractable to calculation in a reasonable time, special coordinate systems can be used such as Boyer-Lindquist coordinates or fish-eye coordinates.

Numerical relativity techniques steadily improved from the initial attempts in the 1960s and 1970s.[23][24] Long-term simulations of orbiting black holes, however, were not possible until three groups independently developed groundbreaking new methods to model the inspiral, merger, and ringdown of binary black holes [2][3][4] in 2005.

In the full calculations of an entire merger, several of the above methods can be used together. It is then important to fit the different pieces of the model that were worked out using different algorithms. The Lazarus Project linked the parts on a spacelike hypersurface at the time of the merger.[25]

Results from the calculations can include the binding energy. In a stable orbit the binding energy is a local minimum relative to parameter perturbation. At the innermost stable circular orbit the local minimum becomes an inflection point.

The gravitational waveform produced is important for observation prediction and confirmation. When inspiralling reaches the strong zone of the gravitational field, the waves scatter within the zone producing what is called the post Newtonian tail (PN tail).[25]

In the ringdown phase of a Kerr black hole, frame-dragging produces a gravitation wave with the horizon frequency. In contrast the Schwarzschild black-hole ringdown looks like the scattered wave from the late inspiral, but with no direct wave.[25]

The radiation reaction force can be calculated by Padé resummation of gravitational wave flux. A technique to establish the radiation is the Cauchy characteristic extraction technique CCE which gives a close estimate of the flux at infinity, without having to calculate at larger and larger finite distances.

The final mass of the resultant black hole depends on the definition of mass in general relativity. The Bondi mass MB is calculated from the Bondi-Sach mass loss formula. . With f(U) the gravitational wave flux at retarded time U. f is a surface integral of the News function at null infinity varied by solid angle. The Arnowitt-Deser-Misner (ADM) energy or ADM mass is the mass as measured at infinite distance and includes all the gravitational radiation emitted. .

Angular momentum is also lost in the gravitational radiation. This is primarily in the z axis of the initial orbit. It is calculated by integrating the product of the multipolar metric waveform with the news function complement over retarded time.[26]


One of the problems to solve is the shape or topology of the event horizon during a black-hole merger.

In numerical models, test geodesics are inserted to see if they encounter an event horizon. As two black holes approach each other, a ‘duckbill’ shape protrudes from each of the two event horizons towards the other one. This protrusion extends longer and narrower until it meets the protrusion from the other black hole. At this point in time the event horizon has a very narrow X-shape at the meeting point. The protrusions are drawn out into a thin thread.[27] The meeting point expands to a roughly cylindrical connection called a bridge.[27]

Simulations as of 2011 had not produced any event horizons with toroidal topology (ring-shaped). Some researchers suggested that it would be possible if, for example, several black holes in the same nearly-circular orbit coalesce.[27]

Black-hole merger recoil

An unexpected result can occur with binary black holes that merge in that the gravitational waves carry momentum and the merging black-hole pair accelerates seemingly violating Newton's third law. The center of gravity can add over 1000 km/s of kick velocity.[28] The greatest kick velocities (approaching 5000 km/s) occur for equal-mass and equal-spin-magnitude black-hole binaries, when the spins directions are optimally oriented to be counter-aligned, parallel to the orbital plane or nearly aligned with the orbital angular momentum.[29] This is enough to escape large galaxies. With more likely orientations a smaller effect takes place, perhaps only a few hundred kilometers per second. This sort of speed will eject merging binary black holes from globular clusters, thus preventing the formation of massive black holes in globular cluster cores. In turn this reduces the chances of subsequent mergers, and thus the chance of detecting gravitational waves. For non spinning black holes a maximum recoil velocity of 175 km/s occurs for masses in the ratio of five to one. When spins are aligned in the orbital plane a recoil of 5000 km/s is possible with two identical black holes.[30] Parameters that may be of interest include the point at which the black holes merge, the mass ratio which produces maximum kick, and how much mass/energy is radiated via gravitational waves. In a head-on collision this fraction is calculated at 0.002 or 0.2%.[31] One of the best candidates of the recoiled supermassive black holes is CXO J101527.2+625911.[32]

Halo Drive for Space Travel

Binary black holes could transfer energy and momentum to a spacecraft using a "halo drive", exploiting the holographic reflection created by a set of null geodesics looping behind and then around one of the black holes before returning to the spacecraft. The reflection passing through these null geodesics forms one end of a laser cavity, with a mirror on the spacecraft forming the other end of the laser cavity. Even a planet-sized spacecraft can thereby accelerate to speeds exceeding the approaching black hole's relative speed. A network of these binary black holes could permit travel across the galaxy.[33]


  1. ^ Credits: SXS (Simulating eXtreme Spacetimes) project
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External links

3C 75

3C75 (a.k.a. 3C 75) is a binary black hole system in the Abell 400 cluster of galaxies. It has four radio jets (two from each accreting black hole). It is travelling at 1200 kilometers per second through the cluster plasma, causing the jets to be swept back. The binary supermassive black holes are themselves contained in the dumbbell shaped galaxy NGC 1128. 3C 75 may be X-ray source 2A 0252+060 (1H 0253+058, XRS 02522+060).


BBH may refer to:

Brown Brothers Harriman & Co., a private investment bank in the United States

Bartle Bogle Hegarty, British advertising agency

Bruce Barrymore Halpenny, writer and historian

The Darkstalkers character Baby Bonnie Hood

Baltic Beverages Holding, now owned by the Carlsberg Group

Bún bò Huế, a noodle dish from Vietnam

Baseball Heaven, an artificial turf baseball complex on Long Island

Binary black hole, an astrophysical binary star system of two black holes orbiting each other

Byun Baek-hyun, a South Korean singer and actor

Carlos Lousto

Carlos O. Lousto is a Professor in the School of Mathematical Sciences in Rochester Institute of Technology, known for his work on black hole collisions.

First observation of gravitational waves

The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had only been inferred indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent "ringdown" of the single resulting black hole. The signal was named GW150914 (from "Gravitational Wave" and the date of observation 2015-09-14). It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.

This first direct observation was reported around the world as a remarkable accomplishment for many reasons. Efforts to directly prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that Albert Einstein himself doubted that they could ever be detected. The waves given off by the cataclysmic merger of GW150914 reached Earth as a ripple in spacetime that changed the length of a 4-km LIGO arm by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width. The energy released by the binary as it spiralled together and merged was immense, with the energy of 3.0+0.5−0.5 c2 solar masses (5.3+0.9−0.8×1047 joules or 5300+900−800 foes) in total radiated as gravitational waves, reaching a peak emission rate in its final few milliseconds of about 3.6+0.5−0.4×1049 watts – a level greater than the combined power of all light radiated by all the stars in the observable universe.The observation confirms the last remaining directly undetected prediction of general relativity and corroborates its predictions of space-time distortion in the context of large scale cosmic events (known as strong field tests). It was also heralded as inaugurating a new era of gravitational-wave astronomy, which will enable observations of violent astrophysical events that were not previously possible and potentially allow the direct observation of the very earliest history of the universe. On 15 June 2016, two more detections of gravitational waves, made in late 2015, were announced. Eight more observations were made in 2017, including GW170817, the first observed merger of binary neutron stars, which was also observed in electromagnetic radiation.


GW170104 was a gravitational wave signal detected by the LIGO observatory on 4 January 2017. On 1 June 2017, the LIGO and Virgo collaborations announced that they had reliably verified the signal, making it the third such signal announced, after GW150914 and GW151226, and fourth overall.


GW170814 was a gravitational wave signal from two merging black holes, detected by the LIGO and Virgo observatories on 14 August 2017. On 27 September 2017, the LIGO and Virgo collaborations announced the observation of the signal, the fourth confirmed event after GW150914, GW151226 and GW170104. It was the first binary black hole merger detected by LIGO and Virgo together.

Golden binary

In gravitational wave astronomy, a golden binary is a binary black hole collision event whose inspiral and ringdown phases have been measured accurately enough to provide separate measurements of the initial and final black hole masses.


The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These can detect a change in the 4 km mirror spacing of less than a ten-thousandth the charge diameter of a proton.The initial LIGO observatories were funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. They collected data from 2002 to 2010 but no gravitational waves were detected.

The Advanced LIGO Project to enhance the original LIGO detectors began in 2008 and continues to be supported by the NSF, with important contributions from the UK Science and Technology Facilities Council, the Max Planck Society of Germany, and the Australian Research Council. The improved detectors began operation in 2015. The detection of gravitational waves was reported in 2016 by the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration with the international participation of scientists from several universities and research institutions. Scientists involved in the project and the analysis of the data for gravitational-wave astronomy are organized by the LSC, which includes more than 1000 scientists worldwide, as well as 440,000 active Einstein@Home users as of December 2016.LIGO is the largest and most ambitious project ever funded by the NSF.

In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry C. Barish "for decisive contributions to the LIGO detector and the observation of gravitational waves."As of December 2018, LIGO has made eleven detections of gravitational waves, of which ten are from binary black hole mergers. The other event was the first detection of a collision of two neutron stars, on 17 August 2017 which simultaneously produced optical signals detectable by conventional telescopes. All eleven events were observed in data from the first and second observing runs of Advanced LIGO.

List of gravitational wave observations

This is a list of observed gravitational wave events. Direct observation of gravitational waves, which commenced with the detection of an event by LIGO in 2015, constitutes part of gravitational wave astronomy. LIGO has played a role in all subsequent detections to date, with Virgo joining in August 2017.

Livingston, Louisiana

Livingston is the parish seat of Livingston Parish, Louisiana, United States. The population was 1,769 at the 2010 census.

Livingston hosts one of the two LIGO gravitational wave detector sites, the other one being located in Hanford, Washington.

Markarian 231

Markarian 231 (UGC 8058) is a Type-1 Seyfert galaxy that was discovered in 1969 as part of a search of galaxies with strong ultraviolet radiation. It contains the nearest known quasar, and in 2015 it was shown that the powerful active galactic nucleus present in the center of the galaxy may in fact be a supermassive binary black hole. It is located about 581 million light years away from Earth.

NGC 300

NGC 300 is a spiral galaxy in the constellation Sculptor. It is one of the closest galaxies to the Local Group, and probably lies between the latter and the Sculptor Group. It is the brightest of the five main spirals in the direction of the Sculptor Group. It is inclined at an angle of 42° when viewed from Earth and shares many characteristics of the Triangulum Galaxy.

NGC 4151

NGC 4151 is an intermediate spiral seyfert galaxy with weak inner ring structure located 19 megaparsecs (62 million light-years) from Earth in the constellation Canes Venatici. The galaxy was first mentioned by William Herschel on March 17, 1787; it was one of the two Seyfert galaxies described in the paper which defined the term. It is one of the nearest galaxies to Earth to contain an actively growing supermassive black hole; it was speculated that the nucleus may host a binary black hole, with about 40 million and about 10 million solar masses respectively, orbiting with a 15.8-year period. This is, however, still a matter of active debate.

Some astronomers nickname it the "Eye of Sauron" from its appearance.

Numerical relativity

Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems. To this end, supercomputers are often employed to study black holes, gravitational waves, neutron stars and many other phenomena governed by Einstein's theory of general relativity. A currently active field of research in numerical relativity is the simulation of relativistic binaries and their associated gravitational waves. Other branches are also active.

OJ 287

OJ 287 is a BL Lac object located 3.5 billion light-years away that has produced quasi-periodic optical outbursts going back approximately 120 years, as first apparent on photographic plates from 1891. It has been seen on photographic plates since at least 1887. It was first detected at radio wavelengths during the course of the Ohio Sky Survey.

Orbiting Binary Black Hole Investigation Satellite

Orbiting Binary Black Hole Investigation Satellite (ORBIS) is a small space telescope still in development by Japan that will study binary black holes in the X-ray region.

The ORBIS concept won the first prize at the 18th Satellite Design Contest in 2010, and of 2015 it was on preliminary design and undergoing thermal simulations by the Tokyo Metropolitan University with support from Japan Aerospace Exploration Agency (JAXA) and the Institute of Space and Astronautical Science (ISAS).The spacecraft will have a mass of about 46 kg and it features a propulsion system using 60 wt% hydrogen peroxide. Launch is aimed for 2020.

Timeline of black hole physics

Timeline of black hole physics

VFTS 352

VFTS 352 is a contact binary star system 160,000 light-years (49,000 pc) away in the Tarantula Nebula, which is part of the Large Magellanic Cloud. It is the most massive and earliest spectral type overcontact system known.The discovery of this O-type binary star system made use of the European Southern Observatory's Very Large Telescope, and the description was published on 13 October 2015. VFTS 352 is composed of two very hot (40,000 °C), bright and massive stars of equal size that orbit each other in little more than a day. The stars are so close that their atmospheres overlap. Extreme stars like the two components of VFTS 352 are thought to be the main producers of elements such as oxygen.The future of VFTS 352 is uncertain, and there are two possible scenarios. If the two stars merge, a very rapidly rotating star will be produced. If it keeps spinning rapidly it might end its life in a long-duration gamma-ray burst. In a second hypothetical scenario, the components would end their lives in supernova explosions, forming a close binary black hole system, hence a potential gravitational wave source through black hole–black hole merger.


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