Scientists at Cardiff University have detected a strange winding motion in the orbits of two merging black holes, an unusual occurrence foretold by Einstein’s theory of gravity.
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Dr. Charlie Hoy, Professor Mark Hannam, and Dr. Jonathan Thompson, who lead this study reported in the journal Nature that this is the first time this effect, referred to as precession, has been observed in black holes, where the winding is 10 billion times faster than in earlier observations.
In early 2020, the binary black hole system was discovered via gravitational waves in the Advanced LIGO and Virgo detectors. Of the two black holes, one is 40 times larger than the Sun and is probably the fastest-spinning black hole to be discovered via gravitational waves.
In contrast to all earlier observations, the quickly revolving black hole twisted space and time so much that the binary’s whole orbit shook back and forth.
This kind of precession is particular to Einstein’s theory of general relativity. These findings validate its presence in the most extreme physical event one can witness—two black holes colliding.
We’ve always thought that binary black holes can do this. We have been hoping to spot an example ever since the first gravitational wave detections. We had to wait for five years and over 80 separate detections, but finally, we have one!
Mark Hannam, Study Lead and Professor, Gravity Exploration Institute, Cardiff University
A more realistic illustration of precession is the wobbling of a spinning top, which may wobble once every few seconds. By contrast, precession in basic relativity is typically such a weak effect that it is undetectable.
In the fastest instance formerly measured from orbiting neutron stars known as binary pulsars, it took more than 75 years for the orbit to precess. The black-hole binary in this research, colloquially called GW200129 (labeled after the date it was seen, January 29, 2020), precesses numerous times every second—an effect 10 billion times sturdier than measured earlier.
It’s a very tricky effect to identify. Gravitational waves are extremely weak and to detect them requires the most sensitive measurement apparatus in history. The precession is an even weaker effect buried inside the already weak signal, so we had to do a careful analysis to uncover it.
Dr. Jonathan Thompson, Study Lead and Professor, Cardiff University
Einstein predicted gravitational waves in 1916. They were first instantly spotted from the fusion of two black holes by the Advanced LIGO instruments in 2015, a revolutionary discovery that received the 2017 Nobel Prize.
Gravitational wave astronomy is currently one of the most exciting domains of science, with a network of Virgo, Advanced LIGO, and KAGRA detectors operating in Europe, the US, and Japan. Thus far, there have been more than 80 detections, all of which were merging neutron stars or black holes.
“So far most black holes we’ve found with gravitational waves have been spinning fairly slowly,” said Dr Charlie Hoy, a Researcher at Cardiff University while conducting this study, and currently at the University of Portsmouth.
The larger black hole in this binary, which was about 40 times more massive than the Sun, was spinning almost as fast as physically possible. Our current models of how binaries form suggest this one was extremely rare, maybe a one-in-a-thousand event. Or it could be a sign that our models need to change.
Dr. Charlie Hoy, Study Lead and Researcher, University of Portsmouth
The global network of gravitational-wave detectors is presently being advanced and will begin its next exploration of the universe in 2023. They will probably discover hundreds of black holes colliding and will inform researchers whether GW200129 was an uncommon exception or an indication that the universe is even stranger than once believed.
The researchers were partly funded by the European Research Council (ERC) and the Science and Technology Facilities Council (STFC).
Hannam, M., et al. (2022) General-relativistic precession in a black-hole binary. Nature. doi.org/10.1038/s41586-022-05212-z.