Astronomers probe black hole origins after 39 new cosmic collisions detected

An extreme merger motherlode gives astronomers a fresh view of the universe's most mysterious objects.

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LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

The number of gravitational wave events, caused by massive collisions between black holes and neutron stars, has quadrupled. In a suite of new papers, researchers from the LIGO and Virgo collaborations cataloged 39 "new" events, adding to the 11 already detected since the LIGO and Virgo gravitational wave detectors were switched on in 2015. 

Gravitational waves are ripples in space-time caused by collisions between black holes and other extreme cosmic phenomena. When massive cosmic bodies merge, they release exceptional amounts of energy, causing a wave to ripple out from their location. Eventually, that wave washes over the Earth, pinging detectors in the US (LIGO) and Italy (Virgo). Gravitational wave detections have revolutionized the way we see the universe, helping scientists to understand some of the most mystifying objects in space.

The new catalog, announced on Wednesday, is known as GWTC-2 and features 50 total events, including black hole mergers, neutron star mergers and, potentially, collisions between a black hole and neutron star. Thirty-nine events were detected between April 1 and Sept. 30, 2019 after the LIGO and Virgo facilities received a number of upgrades, increasing their sensitivity. 

The catalog update includes some of the most extreme cosmic collisions ever detected, including one revealed in September which created a black hole 150 times more massive than the sun -- the biggest yet -- and a particularly unusual merger between a black hole and a "mysterious object" that doesn't seem to fit in with previous discoveries.   

But the merger motherlode has excited gravitational wave astronomers because it gives them a ton of new data with which to probe the very nature of these extreme cosmic collisions.

An animation displaying all 50 gravitational wave events detected since 2015.

Zoheyr Doctor/University of Oregon/LIGO-Virgo Collaboration

"It's kind of like the difference between finding a single Iguanodon bone and finding hundreds of Iguanodon fossils," explains Eric Thrane, an astrophysicist at Monash University in Melbourne, Australia, and chief investigator with OzGrav, an Australian research center studying gravitational waves.

In one new preprint paper, submitted to the Astrophysical Journal Letters, the collaboration studied 47 of the 50 events and analyzed the physical properties of black hole mergers.

"Black holes are fascinating objects because they're very simple," says Thrane. "They only have two numbers describing them: their mass and their spin."

The spin of a black hole can be determined by the gravitational wave signal. This gives scientists a window into how black holes meet and fall into each other in deep space, revealing how they met.

Black holes are created when huge stars collapse in on themselves. Sometimes, two stars have been orbiting each other for eons in what is known as a "binary." Over time, they lose their mass and eventually die, collapsing to form black holes. But they continue orbiting each other until they collide and form a much larger black hole. In this instance, the spin doesn't change -- it points in the same direction. 

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On the other hand, if the black holes have been wandering the cosmos in dense clusters of stars, all alone, then bump into one another, theory suggests this would mess with their spin. "When that happens, you'd expect the spin to be pointed in different directions," says Thrane.

Importantly, with the truckload of new observations, the LIGO and Virgo collaboration are seeing both types of black holes. 

"We're getting at the origin of where the black holes come from where [and] how they get together and merge," says Thrane.

The last observation run by LIGO and Virgo, O3b, took place between Nov. 1 2019 and March 27, 2020 before being stopped due to the coronavirus pandemic. Data from this period is currently being analyzed and will expand the catalog of gravitational wave events, once again furthering our understanding of extreme cosmic collisions.