Two black holes merged in outer space and created something colossal

Astronomers have detected the signal of a colossal black hole in deep space that likely formed when two already-large black holes crashed into each other billions of light-years away.

The result is a colossal cosmic object about 225 times heavier than the sun — by far the most massive ever observed through gravitational waves, or ripples in the fabric of spacetime. Before now, the record holder for a black hole merger detection using this method weighed in at just 140 times the sun.

The discovery, announced on July 14, comes not from NASA but a collaboration of observatories around the world, including two U.S. National Science Foundation–funded observatories in Louisiana and Washington. The newfound black hole has defied expectations for its unusual size, based on known ways that stars collapse. 

"We have theories of how black holes form when stars die, and those theories are fine for black holes that are five times the mass of our sun, or 10 times, or even 50 times," wrote Mark Hannam, a Cardiff University scientist who led the research team, on his Substack, The Fictional Aether. "But once you get to about 60 times the mass of the sun, some funky nuclear/quantum/whatever processes come into play, and the star blasts away lots of its mass, and you can't form a really massive black hole. That carries on until you get to really massive stars." 

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Black holes are some of the most inscrutable phenomena in outer space. About 50 years ago, they were little more than a theory — a kooky mathematical answer to a physics problem. Even astronomers at the top of their field weren't entirely convinced they existed. Today, not only are black holes accepted science, some supermassive ones are getting their pictures taken by a collection of synced-up radio dishes on Earth. 

Unlike a planet or star, black holes don't have surfaces. Instead, they have a boundary called an "event horizon," or a point of no return. If anything swoops too close, it will fall in, never to escape the hole's gravitational clutch.

"Nothing can escape a black hole, not even another black hole," Hannam explained, "so what's left is: a bigger black hole."

The most common kind, called a stellar black hole, is thought to be the result of an enormous star dying in a supernova explosion. The star's material then collapses onto itself, condensing into a relatively tiny area. Physics predicts a gap in the sizes of black holes that can form this way. That gap — between about 60 and 130 times the mass of our sun — should be largely empty. 

One of the Laser Interferometer Gravitational-wave Observatory, or LIGO, detectors, in Hanford, Washington. The second detector is located in Livingston, Louisiana. Credit: LIGO

But this merger, designated GW231123, is breaking the rules, according to the LIGO-Virgo-KAGRA Collaboration, which together has detected about 300 since 2015. It involved two black holes estimated to land in the mass gap. Furthermore, researchers say there's something else puzzling about the event.

"The black holes appear to be spinning very rapidly — near the limit allowed by Einstein's theory of general relativity," said Charlie Hoy, a University of Portsmouth scientist, in a statement. "That makes the signal difficult to model and interpret."

One possible explanation is that at least one of the colliding black holes was not born from a collapsing star, but from another prior black hole merger. This would require extreme environments where merged black holes could stick around long enough to crash again.

The event could point to new ways the universe forms black holes that scientists are only beginning to understand.