Black holes wobbling three times a second have proved Einstein right

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, Mark Hannam at Cardiff University, UK, and his colleagues have seen a much more extreme effect in a pair of black holes, caused by one of them spinning at a fifth of the speed of light 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, Italy and Japan. A previous analysis revealed no precession, but using a more advanced model to account for sources of noise in the data, 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 Fabio Antonini 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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