Study proves black holes have a ‘plunging region,’ just as Einstein predicted

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Albert Einstein was right: There is an area at the edge of black holes where matter can no longer stay in orbit and instead falls in, as predicted by his theory of gravity.

Using telescopes capable of detecting X-rays, a team of astronomers has for the first time observed this area — called the “plunging region” — in a black hole about 10,000 light-years from Earth. “We’ve been ignoring this region, because we didn’t have the data,” said research scientist Andrew Mummery, lead author of the study published Thursday in the journal Monthly Notices of the Royal Astronomical Society. “But now that we do, we couldn’t explain it any other way.”

It’s not the first time that black holes have helped confirm Einstein’s grand theory, which is also known as general relativity. The first photo of a black hole, captured in 2019, had previously strengthened the revolutionary physicist’s core assumption that gravity is just matter bending the space-time fabric.

Many of Einstein’s other predictions have turned out to be correct over the years, among them gravitational waves and the universal speed limit. “He’s a tough man to bet against at this point,” said Mummery, a Leverhulme-Peierls Fellow in the department of physics at the University of Oxford in the United Kingdom.

“We went out searching for this one specifically — that was always the plan. We’ve argued about whether we’d ever be able to find it for a really long time,” Mummery said. “People said it would be impossible, so confirming it’s there is really exciting.”

‘Like the edge of a waterfall’

The observed black hole is in a system called MAXI J1820 + 070, which is made up of a star smaller than the sun and the black hole itself, estimated at 7 to 8 solar masses. The astronomers used NASA’s space-based NuSTAR and NICER telescopes to collect data and understand how hot gas, called plasma, from the star gets sucked into the black hole.

NuSTAR is short for the Nuclear Spectroscopic Telescope Array, which orbits Earth, and NICER, formally known as the Neutron star Interior Composition Explorer, is located on the International Space Station.

“Around these black holes there are big discs of orbiting material (from nearby stars),” Mummery said. “Most of it is stable, which means it can happily flow. It’s like a river, whereas the plunging region is like the edge of a waterfall — all of your support is gone and you’re just crashing headfirst. Most of what you can see is the river, but there’s this tiny region at the very end, which is basically what we found,” he added, noting that while the “river” had been widely observed, this is the first evidence of the “waterfall.”

Unlike the event horizon, which is closer to the center of the black hole and doesn’t let anything escape, including light and radiation, in the “plunging region” light can still escape, but matter is doomed by the powerful gravitational pull, Mummery explained.

The study’s findings could help astronomers better understand the formation and evolution of black holes. “We can really learn about them by studying this region, because it’s right at the edge, so it gives us the most information,” Mummery said.

One thing that’s missing from the study is an actual image of the black hole, because it is too small and far away. But another team of Oxford researchers is working on something even better than a picture: the first movie of a black hole. To achieve that, the team will first need to build a new observatory, the Africa Millimetre Telescope in Namibia, which Mummery expects to be online within a decade. The telescope, which will join the international Event Horizon Telescope collaboration that captured the groundbreaking 2019 image of the black hole, will enable scientists to observe and film large black holes at the center of the Milky Way galaxy and beyond.

A link to the past

According to Christopher Reynolds, a professor of astronomy at the University of Maryland, College Park, finding actual evidence for the “plunging region” is an important step that will let scientists significantly refine models for how matter behaves around a black hole. “For example, it can be used to measure the rotation rate of the black hole,” said Reynolds, who was not involved in the study.

Dan Wilkins, a research scientist at Stanford University in California, calls it an exciting development, and points out that in 2018 there was an enormously bright outburst of light from one of the black holes within our galaxy, paired with an excess of high-energy X-rays.

“We had hypothesized at the time that this excess was from the hot material in the ‘plunging region,’ but we did not have a full theoretical prediction of what that emission would look like,” said Wilkins, who also was not involved with the new study.

This study actually performs that calculation, he added, using Einstein’s theory of gravity to predict what the X-rays emitted by material in the “plunging region” would look like around a black hole, and compares it with the data from that bright outburst in 2018.

“This will be prime discovery space over the next decade or so,” Wilkins said, “as we look towards the next generation of X-ray telescopes that will give us more detailed measurements of the innermost regions just outside the event horizons of black holes.”

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