Gravitational waves: biggest black hole merger ever detected revealed

LIGO data adds four more grav-wave events to the record. Alan Duffy reports.

An artist's impression of two black holes about to collide and merge.

An artist's impression of two black holes about to collide and merge.


Billions of light years away, two black holes have collided to create a larger one – the biggest black hole merger yet detected. It has a mass more than 80 times that of the sun.

The resulting energy injected into the fabric of spacetime was also record breaking, with five sun’s worth of mass released in the form gravitational waves as the two holes spiralled in towards each other.

Such titanic amounts of energy meant that the signal was still detectable by the time it reached gravitational wave detectors on Earth. It produced a record-breaking result – the most distant collision detected so far, nine billion light years away.

“This event also had black holes spinning the fastest of all mergers observed so far,” says the Australian National University’s Susan Scott, a chief investigator with the Centre of Excellence for Gravitational Wave Discovery (OzGrav). “It is also by far the most distant merger observed.”

Colm Talbot, from Australia’s Monash University, notes that “each of these black holes formed from huge stars which died in violent explosions called supernovae. By studying these black holes, we act as black hole archaeologists to learn how these cosmic giants die.”

A description of the event, along with three other smaller separate black hole collisions, has been released as part of a collection of detections by the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) in the US and the Advanced Virgo facility in Italy.

aLIGO is in fact two detectors, on either side of the continental US, and are essentially hyper-accurate rulers. All three gravitational wave detectors send laser beams back and forth along long arms in perfect antiphase, meaning they cancel one another out.

However, if one arm length increases slightly relative to the other, say in the stretching of spacetime by passing gravitational waves, then they don’t perfectly cancel and a tiny spot of light remains. This light flickers in time with the passing gravitational wave creating the characteristic “chirp” signal that arises as black holes spiral inwards ever more rapidly towards impact.

All four newly confirmed black hole mergers were found in archived observing runs from 2017, coming to light as a result of routine data-cleaning.

They bring the total number of mergers detected to 10 – plus a single neutron star–neutron star collision – over the past three years.

Simon Stevenson, from OzGrav and Swinburne University, in Australia, says that the additional information of all these binary black holes “means we are learning things about the population, such as how frequently binary black holes merge in the universe”.

Since the detections, aLIGO has been in temporary shutdown for upgrades. When it reopens, it will have a significantly higher level of sensitivity.

Researchers are hoping this will result in even more gravitational wave detections. They also hope it will lead to the discovery of the remaining combination: a black hole–neutron star collision. This would be detectable by both gravitational wave observatories as well as telescopes, permitting exacting tests of Einstein’s Theory of General Relativity and exploration of some of the most energetic events in the visible universe.

Watch: Computer calculations modeling the gravitational waves LIGO has observed to date and the black holes that emitted the waves.

SXS Collaboration
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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
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