Black Hole’s Mysterious ‘Plunging Region’ Matches Einstein’s Predictions

We’ve Finally Seen Matter Plunge into a Black Hole

For the first time, scientists observed matter’s freefall into a black hole’s “plunging region”

Artist's concept of a stellar black hole siphoning material from a companion star

An artist’s concept of a stellar-mass black hole (right) siphoning material from a companion star (left). Much of the material forms an accretion disk around the black hole before falling inside.

ESO/L. Calçada/M.Kornmesser

Black holes stretch the fabric of spacetime to its extreme—and the closer you get to one, the more warped things get. “You can be really very close to a black hole and happily, circularly orbit,” says Andrew Mummery, a physicist at the University of Oxford. But as you draw nearer, a black hole’s gravitational grip becomes overpowering. You hit a precipice, and instead of peacefully circling, you simply fall.

At this point, classical orbital mechanics breaks down, and “[Isaac] Newton has nothing to say,” Mummery notes. Describing the dynamics of an object falling headlong down a black hole’s maw is a task for Albert Einstein’s general theory of relativity.

Einstein used this theory more than a century ago to predict what happens in what would later become known as black holes. Just outside a black hole’s event horizon—the boundary past which not even light can escape—an orbiting object will abruptly encounter a so-called plunging region and plummet to its doom at nearly the speed of light.

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Theorists consider a black hole’s plunging region to be where the fate of all things falling in becomes sealed. Yet beyond that basic insight, this area has remained a near total mystery. “Basically, the preexisting theoretical models ignored this region,” Mummery says—after all, it’s small and hard to see with current telescopes. But thanks to a chance outburst by a black hole feasting on matter in our galaxy, Mummery and his colleagues have now observed the plunging region for the first time. They reported their results in a paper published last week in the Monthly Notices of the Royal Astronomical Society.

“The first time you see it, it’s just nice to know it’s there at all,” Mummery says. “Now that we know we can see this, there’s a lot of things we can, in principle, learn using it.”

No telescope can see black holes directly because even light can’t escape these astronomical objects’ clutches. Instead physicists typically study light from a black hole’s accretion disk—the superheated gas and dust that circles this cosmic drain. The innermost lip of this disk is the plunging region’s threshold. It gives off relatively little light, so until recently, scientists couldn’t get sufficient data to observe it.

“There are two ways to get better data: you can build a better telescope, or you can get lucky,” Mummery says. As luck would have it, in 2018 astronomers using multiple telescopes discovered a black hole, called MAXI J1820+070, that gave Mummery and his team the opportunity they needed. Located about 10,000 light-years away, this black hole has been feeding on material siphoned from a nearby star, and for a few months, observers watched as it gorged itself on a hefty serving of stellar gas, gaining a good view of its thick, hot accretion disk that glowed brightly in x-rays. Two of NASA’s space-based telescopes, Nuclear Spectroscopic Telescope Array (NuSTAR) and Neutron Star Interior Composition Explorer (NICER), tuned to the black hole and gathered a glut of x-ray data.

But by early 2020 study co-author Andrew Fabian of the University of Cambridge and other scientists had realized that standard black hole models couldn’t account for all the light NuSTAR and NICER had observed. Looking closer at the data and consulting simulations, Fabian and his colleagues found that this extra light matched what they would expect from glowing material spilling into the plunging region. Now the researchers have developed a working model that explains these details of the 2018 outburst and that can be applied to other black holes as well, explains Alejandro Cárdenas-Avendaño, a theoretical astrophysicist at Princeton University, who wasn’t involved in the new study.

This confirms and deepens our understanding of what Einstein had predicted must happen to matter approaching a black hole’s point of no return at the inner edge of an accretion disk. In some ways, you can imagine it like going down a funnel waterslide. Gravity and centripetal force send you spiraling down around the inside of the funnel. You circle faster as the spiral tightens before you finally reach the lip of the funnel and plunge into the pool below.

“Once you’ve gone over the funnel, there’s nothing you can do,” Mummery says.

Located so close to a black hole, a plunging region should offer researchers a new way to study other hard-to-probe properties, such as a black hole’s spin, to learn more about how these objects form. So far most black holes that have been studied with traditional telescopes (and with traditional models that have neglected the plunging region) seem to be spinning very fast. But those that have been investigated using gravitational-wave telescopes—observatories that detect ripples in spacetime itself rather than light—seem to be spinning much slower, Cárdenas-Avendaño explains.

Scientists still don’t know if this tentative tension can be physically reconciled or if it instead points to some deeper flaw in our theories. But data from the plunging region could provide a closer look. “Spin is something that you only feel when you’re really, really close to the black hole,” says Amelia Hankla, a theoretical and computational astrophysicist at the University of Maryland, College Park, who was not involved in the new study. “What’s exciting about the plunging region is that the imprint of spacetime actually rotating is [visible] in the emission.”

The new analysis shows that the black hole in question wasn’t spinning very fast, which surprised the researchers. “That’s just totally different from what other people have been finding with models that are neglecting this region,” Mummery says.

And studying black holes’ spin isn’t just a matter of idle curiosity: “The evolution of the universe depends on how black holes behave, and that behavior depends on how much they rotate. So these are fundamental questions,” Cárdenas-Avendaño says.

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