Cardiff University researchers have identified a peculiar twisting motion in the orbits of two colliding black holes, an exotic phenomenon predicted by Einstein’s theory of gravity.
Their study, published in Nature and led by Professor Mark Hannam, Dr Charlie Hoy and Dr Jonathan Thompson, reports that this is the first time this effect, known as precession, has been observed in black holes, where the torsion is 10 billion times faster than in previous observations.
The binary black hole system was discovered by gravitational waves in early 2020 in the Advanced LIGO and Virgo detectors. One of the black holes, 40 times larger than our Sun, is probably the fastest spinning black hole thanks to gravitational waves. And contrary to all previous observations, the rapidly rotating black hole distorted space and time so much that the binary’s entire orbit swayed back and forth.
This form of precession is specific to Einstein’s theory of general relativity. These results confirm its existence in the most extreme physical event that we can observe, the collision of two black holes.
“We always thought binary black holes could do this,” said Professor Mark Hannam of Cardiff University’s Gravity Exploration Institute. “We had been hoping to spot an example since the first detections of gravitational waves. We had to wait five years and more than 80 separate detections, but finally we have one!”
A more mundane example of precession is the oscillation of a spinning top, which can oscillate – or precession – once every few seconds. In contrast, precession in general relativity is usually such a weak effect that it is imperceptible. In the fastest example previously measured from orbiting neutron stars called binary pulsars, it took more than 75 years for the orbit to precede. The black hole binary in this study, colloquially known as GW200129 (named after the date it was observed, January 29, 2020), precedes several times per second – an effect 10 billion times stronger than that measured previously.
Dr Jonathan Thompson, also from Cardiff University, explained: “It’s a very tricky effect to identify. Gravitational waves are extremely weak and their detection requires the most sensitive measuring device in history. Precession is an even weaker effect buried in the already weak signal, so we had to do some careful analysis to find that out.”
Gravitational waves were predicted by Einstein in 1916. They were first detected directly from the merger of two black holes by Advanced LIGO instruments in 2015, a groundbreaking discovery that led to the 2017 Nobel Prize. Gravitational wave astronomy is now one of the fastest growing fields in science, with a network of advanced LIGO, Virgo and KAGRA detectors operating in the United States, Europe and Japan. To date, there have been more than 80 detections, all of black hole or neutron star mergers.
“Until now, most of the black holes we’ve found with gravitational waves have been spinning quite slowly,” said Dr Charlie Hoy, a researcher at Cardiff University during the study, and now at the University of Portsmouth. “The largest 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 that this one was extremely rare, possibly -be one in a thousand events. . Or it could be a sign that our patterns need to change.”
The international network of gravitational wave detectors is currently being upgraded and will begin its next search of the universe in 2023. They will likely find hundreds more colliding black holes and tell scientists if GW200129 was an exception rare or a sign that our universe is even stranger than they thought.
The authors were supported in part by funding from the Science and Technology Facilities Council (STFC) and the European Research Council (ERC).
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