A strange area around black holes called the “plunging region” has been spotted for the first time. This area, where matter stops circling a black hole and instead falls straight in, was predicted by Albert Einstein’s general theory of relativity, but it has never been observed before. Studying plunging regions could teach us about how black holes form and evolve, as well as reveal new information about the fundamental nature of space-time.

When any matter gets too close to a black hole, it rips apart and forms an orbiting ring around it called an accretion disc. General relativity predicts there should be an inner boundary to the accretion disc past which nothing can orbit the black hole – instead, it should plunge straight in, rapidly accelerating to near the speed of light as it falls.

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“It’s like a river turning into a waterfall, and until now we’ve only been looking at the river,” says at the University of Oxford. “If Einstein was wrong, then it would be stable all the way down – there would only be a river.” Now we’ve gotten our first peek at the waterfall, suggesting Einstein was correct.

Mummery and his colleagues spotted evidence of the plunging region around a black hole in a binary system called MAXI J1820+070, which is about 10,000 light years from Earth. They used data from the Nuclear Spectroscopic Telescope Array (NuSTAR), a space-based X-ray telescope, to build models of the light from the black hole’s accretion disc.

They found the models only fit the data when they included the light emitted by matter in the plunging region in addition to light from the accretion disc. “Before, we sort of thought that anything that crosses this boundary would have no time to really radiate appreciably before it plunges into the black hole”, so researchers wouldn’t see anything, says at Los Alamos National Laboratory in New Mexico, who was not involved with this work. “But it turns out that this plunging region gives you extra light that you wouldn’t have expected.”

This extra light could solve a long-standing problem in X-ray astronomy, in which black holes appear to be spinning faster than theory predicts. The spin of a black hole and the brightness of the area around it are connected, so adding some extra light could bring the spins back in line with predictions. “Black hole spins tell us about all kinds of things, so if we could measure it better, we could answer loads of questions in astrophysics,” says Salvesen.

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That includes questions about the nature of gravity and space-time itself, because plunging regions are some of the most extreme regions of space we can observe. The plunging region is just outside the event horizon, beyond which the gravitational forces are so strong, no matter or even light can escape.

“Technically, if the matter had a rocket it could escape the plunging region, but it’s doomed – its orbit has become unstable and it’s rapidly accelerating toward the speed of light,” says Mummery. “This stuff has about as much chance of coming back as water off the edge of a waterfall.” The researchers are now trying to make more observations of these strange cosmic waterfalls to illuminate the conditions in these extraordinary areas.

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A pair of black holes have been seen wobbling at a rate of three times per second as they merged, in an extreme example of a prediction made by Albert Einstein’s��general theory of relativity that has been seen clearly for the first time.

This wobbling, known as precession, occurs when the orbit or rotation of an object slowly changes with time – a common example is when a spinning top begins to spin at a different angle as it slows down. Gravity-induced orbital precession, a consequence of general relativity’s prediction that heavy objects bend space-time, sees the shape of such an object’s orbit change over time.

This effect had been observed very weakly in neutron stars orbiting one another, but was so subtle that the orbits only wobbled, or precessed, at a rate of a few times a year.

Now, at Cardiff University, UK, and his colleagues have seen a much more extreme effect in a pair of black holes moving at a fifth of the speed of light, caused by one of them spinning at a 90-degree angle to its orbital motion. As they merged, the black holes released a gravitational wave, known as GW200129, that carried the signature of precession at a rate of three times a second.

“It’s 10 billion times faster than what was found in earlier measurements, so it really is the most extreme regime of Einstein’s theory where space and time are warped and distorted in completely crazy ways,” says Hannam.

To identify the precession, the team reanalysed data first collected in 2020 by three gravitational wave detectors, based in the US and Italy. A previous analysis was inconclusive, but using a more advanced model of the gravitational wave signal, Hannam and his team found that the best way to explain the signal was with one of the black holes, spinning at almost the upper limit allowed by general relativity, causing the orbit of the system to precess.

“The astrophysical implications of the detection are quite significant,” says at Cardiff University, who wasn’t involved with the work. The extreme spin, and misalignment with its orbit, isn’t predicted by current ideas of black hole formation, which involve imploding stars, and needs another explanation, he says.

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Einstein’s famous theory of general relativity has passed its most stringent test to date. After 16 years of observing a pair of pulsars – highly compact neutron stars that emit beams of radio waves from their poles – researchers saw relativistic effects that have previously only been predicted by the theory.

General relativity describes the effect of gravity on space-time. “If we have something that’s really massive, it’ll warp the space-time around it to a greater extent than something that’s less massive,” says at the University of East Anglia in the UK.

This means for extremely massive bodies, such as neutron stars, general relativity predicts that light will be bent markedly around them as photons follow the warped path of space-time. What’s more, when neutron stars accelerate, which can happen if two of them are spiralling around one another, they will emit gravitational waves – ripples in space-time – that will cause their orbits to shrink as they lose energy. This is known as orbital decay.

Now, Ferdman and his colleagues have observed both of these theoretical predictions on two pulsars, known as PSR J0737−3039A/B. These orbit each other in just 147 minutes at speeds of up to 1 million kilometres per hour. As they orbit, one of the pulsars is simultaneously rotating around its axis about 44 times every second, while its companion rotates once every 2.8 seconds. Every time each pulsar rotates, we receive a blast of radio beams on Earth.

“Those pulses will arrive extremely regularly unless there’s something in the way or some astrophysical phenomenon that’s causing a delay in those arrivals,” says Ferdman.

Using seven radio telescopes in Australia, the US and Europe, the researchers monitored the double pulsar system between its discovery in 2003 until 2019.

They found that the radio pulses consistently arrived later than expected, and calculated this was because they were being deflected at an angle of 0.04 degrees due to the strong space-time curvature around the two stars. This is the first experimental evidence of such a high curvature, according to Ferdman’s team.

The researchers also found that the pulsars underwent orbital decay due to the emission of gravitational waves. They could detect the energy carried by these waves with a precision 1000 times better than is possible with direct gravitational wave detectors on Earth, such as the LIGO detectors in the US.

“This is the most stringent test to date of Einstein’s theory, and it sets the bar to which future experiments must [operate] in terms of precision in order to put general relativity to the test with any significance,” says Ferdman.

“[General relativity] is the best theory we have of gravity, but we know because of the incompatibility with quantum mechanics and the standard model [of particle physics], it’s not the final word,” he says.

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Einstein’s forgotten twisted universe

Is the universe fine-tuned?

Closing the quantum loopholes

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Also in New Scientist

From the big bang onwards you might think we know a lot about the universe’s first fraction of a second. But that just isn’t true. Why, for example, didn’t antimatter annihilate matter? Why haven’t we observed any dark matter particles yet? How does dark energy accelerate universe expansion? And why do theories of cosmic inflation lead inevitably to the conclusion that there should be an infinite or nearly infinite number of universes in existence?

For Dan Hooper, head of theoretical astrophysics at the Fermi National Accelerator Laboratory, Chicago, solving these questions involves radically rethinking what we think we know about the universe’s very early history.

Interviewed by Richard Webb, Executive Editor, New Scientist at the Royal Institution, London in Feb 2020.