Gravitational Waves Record Ancient Black Hole Merger Unlike Any Detected Before

The merger occurred when the universe was half its current age, and it may have produced the first known black hole of intermediate size
Two black holes merging

An artist's impression of the two black holes right before they merged. 

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Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

Charles Q. Choi, Contributor

(Inside Science) -- A burst of gravitational waves may have confirmed the existence of a long-sought "missing link" kind of black hole, shedding light on how black holes grow to ever larger sizes, a new study finds.

Gravitational waves are ripples in space-time generated by accelerating masses. Using two gravitational-wave detectors -- LIGO (the Laser Interferometry Gravitational-wave Observatory) in the United States, and Virgo in Italy -- scientists detected a signal less than a tenth of a second long on May 21, 2019.

The outburst, dubbed GW190521, originated roughly 16.3 billion light-years away from Earth when the universe was about half its current age, making this the oldest and most distant gravitational-wave event detected so far. Its most likely cause was a merger between two black holes with masses about 66 and 85 times that of the sun, which would be the most massive merger detected yet through gravitational waves, the researchers said.

The collision released an extraordinary amount of energy, equivalent to the mass of eight of our suns converted to energy in the form of gravitational waves. The final result of the merger was a black hole about 142 times the mass of our sun.

Previous research has discovered "stellar-mass" black holes up to a few dozen times the sun's mass that likely formed when giant stars died and collapsed in on themselves. Researchers have also found supermassive black holes, millions to billions of solar masses in size, that form the hearts of most, if not all, large galaxies. However, proof of so-called intermediate-mass black holes of 100 to 10,000 solar masses has been elusive until now.

"We now finally have concrete evidence of the existence of an intermediate-mass black hole," said study co-author Christopher Berry, an astrophysicist at Northwestern University in Evanston, Illinois. "Having an intermediate-mass black hole to bridge the gap between stellar-mass and supermassive black holes might help shed light on how supermassive black holes form -- through ever-larger mergers of black holes."

The scientists detailed their findings in two studies Sept. 2, one in Physical Review Letters, the other in The Astrophysical Journal Letters.

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Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others.