Black hole collision reveals clues to early cosmos

Colliding black holes first spiral in towards each other, throwing off ripples in spacetime as they do.
Colliding black holes first spiral in towards each other, throwing off ripples in spacetime as they do.
R. Hurt / Caltech

Scientists have observed one of the most powerful astronomical events ever seen — the collision of two giant black holes to form an even larger black hole.  

It’s the third time since 2015 that such a collision has been observed via an instrument called LIGO (Laser Interferometer Gravitational-wave Observatory), which consists of a pair of detectors, one in Hanford, Washington, USA, and the other in Livingston, Louisiana, each designed to measure gravitational waves from distant cosmological events.   

Gravitational waves are ripples in the fabric of space, created by movements of massive objects.

An animation representing colliding black holes.
What colliding black holes might look like to an observer.

“Normally we don’t think of space as having any properties at all, so it’s counterintuitive,” says Michael Landry, director of LIGO’s Hanford observatory. Nevertheless, he says, Einstein’s theory of general relativity predicts that space can expand, contract, or vibrate, thereby distorting the medium in which we all live.  

Such waves can be measured, he adds, because the distortions they produce look like changes in the length of any object they pass through. Landry compares it to stretching the canvas of a painting. “If I stretch the medium, the painting gets distorted,” he says. 

In this case, what LIGO saw were the rapidly vibrating distortions produced as the two black holes spiraled toward each other before merging, releasing as much energy in the form of gravitational waves as would be produced if two stars the size of the Sun were converted from mass into energy in about one-third of a second. Once the collision was complete, the new black hole had a mass about 50 times that of the Sun, the scientists report in a paper published today in Physical Review Letters

It’s an important find because it confirms that black holes that size may be fairly common. 

“Before our discoveries we didn’t even know for sure that these black holes existed,” says Laura Cadonati of Georgia Tech University. “We know now they do. They may have played an important role in the early universe.” 

Infographic detailing ligo’s gravitational wave detections.
LIGO’s gravitational wave detections.

The new finding is also important because it was possible to calculate whether the colliding black holes were spinning in the same direction as they were circling each other in their orbit before the collision, or in a different direction.  

“Imagine two tornadoes rotating each other,” says Laura Cadonati of Georgia Tech University. “They could be [spinning] the same as the orbit, or opposed, or at any angle in between.”  {%recommended 1353%}

That’s useful information, she says, because there are two theories for how black-hole pairs might form. One is that they are remnants of stars that were already paired before they collapsed into black holes. In that case, they would most likely be spinning in the direction of their orbit, just like the planets of our own Solar System.

Alternatively, they may have formed separately, then come together in the densely packed star clusters in which their parent stars existed. In this case, their spins wouldn’t necessarily be aligned with their orbit, Cadonati says. 

Careful computer modeling, she adds, shows that the signals detected by LIGO contain the “gravitational fingerprints” of black holes whose spins did not align with their orbit. 

“This favors the theory that these two black holes formed separately then paired up, rather than being formed from the collapse of two already paired stars,” she says.

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