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Cosmic mash-up reveals a type of black hole scientists have never observed before

An illustration of binary black holes about to collide.
An illustration of binary black holes about to collide.
(Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav))

Black holes are getting stranger — even to astronomers. They’ve now detected the signal from a long-ago violent collision of two black holes that created a new one of a size that had never been seen before.

Black holes are compact regions of space so densely packed that not even light can escape. Until now, astronomers had observed them in only two general sizes.

There are “small” ones called stellar black holes that are formed when a star collapses and are about the size of small cities. And there are supermassive black holes that are millions, maybe billions, of times more massive than our sun and around which entire galaxies revolve.

According to astronomers’ calculations, anything in between didn’t quite make sense, because stars that grew too big before collapse would essentially consume themselves, leaving no black holes behind.

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Star collapses couldn’t create stellar black holes much bigger than 70 times the mass of our sun, scientists thought, according to physicist Nelson Christensen, research director of the French National Centre for Scientific Research.

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Then in May 2019, two detectors picked up a signal that turned out to be the energy from two stellar black holes — each large for a stellar black hole — crashing into each other. One was 66 times the mass of our sun and the other a husky 85 times the mass of the sun.

The end result: The first-ever discovered intermediate black hole, at 142 times the mass of the sun.

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Lost in the collision was an enormous amount of energy in the form of a gravitational wave, a ripple in space that travels at the speed of light. It was that wave that physicists in the United States and Europe captured last year with the LIGO and Virgo detectors.

After deciphering the signal and checking their work, scientists published the results Wednesday in Physical Review Letters and Astrophysical Journal Letters.

Because the detectors allow scientists to pick up the gravitational waves as audio signals, scientists actually heard the collision. Yet for all the violence and drama, the signal lasted only one-tenth of a second.

“It just sounds like a thud,” said Caltech physicist Alan Weinstein, another member of the discovery team. “It really doesn’t sound like much on a speaker.”

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This event, known as GW190521, happened about 7 billion years ago, when the universe was about half its current age. But is being detected only now because it was incredibly far away.

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This short video explains the apparatus that was used to detect gravitational waves passing through the Earth.

Black hole collisions have been observed before, but the black holes involved were smaller to begin with — and even after the merger didn’t grow beyond the size of typical stellar black holes.

Scientists still don’t know how supermassive black holes at the center of galaxies formed, Christensen said, but this new discovery may offer a clue.

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Perhaps, like Lego bricks, smaller blocks combine to make bigger ones and those combine to make even bigger ones, said Harvard astronomer Avi Loeb, who wasn’t part of the study but said the results chart new astronomical territory.

And indeed, the bigger of the two black holes involved in this crash could have been the result of an earlier merger, both Christensen and Weinstein said. That further bolstered the theory.

“It’s conceivable that this pair of black holes formed entirely differently, possibly in a dense system with lots of dead stars whizzing about, which allows one black hole to capture another during a fly by,” said Barnard College astronomer Janna Levin, who wasn’t involved in the research.

On the other hand, scientists can’t quite explain how merged black holes, flying around the universe, would meet so many others to merge again and grow ever bigger. It could instead be that supermassive black holes were formed in the immediate aftermath of the Big Bang.

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“In astrophysics we’re always faced with surprises,” Weinstein said.


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