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.
The 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 twisting it is 10 billion times faster than previous observations.
The binary black hole system was found via gravitational waves in early 2020 at the Advanced LIGO and Virgo detectors.
One of the black holes, 40 times larger than our Sun, is probably the fastest spinning black hole ever found via gravitational waves.
And unlike all previous observations, the rapidly spinning black hole warped space and time so much that the binary system’s entire orbit wobbled from side to side.
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’ve always thought that binary black holes can do this,” said Professor Mark Hannam of Cardiff University’s Institute for the Exploration of Gravity.
“We were hoping to find an example from the first gravitational wave detections. We had to wait five years and more than 80 separate detections, but finally we have one!”
A more realistic example of precession is the wobble of a spinning top, which may wobble, or precess, once every few seconds.
By contrast, precession in general relativity is usually such a weak effect that it is imperceptible. In the fastest previously measured example of orbiting neutron stars called binary pulsars, the orbit took more than 75 years to precess.
The black hole binary in this study, colloquially known as GW200129 (named for the date it was observed, January 29, 2020), precesses several times per second, an effect 10 billion times stronger than the one measured above.
Dr Jonathan Thompson, also from Cardiff University, explained: “It’s a very difficult effect to identify. Gravitational waves are extremely weak and to detect them requires the most sensitive measuring device ever. Precession is an effect even weaker buried within the already weak signal, so we had to do some careful analysis to find out.”
Gravitational waves were predicted by Einstein in 1916. Advanced LIGO instruments first directly detected them from the 2015 merger of two black holes, a groundbreaking discovery that led to the 2017 Nobel Prize.
Gravitational-wave astronomy is now one of the most vibrant fields in science, with a network of advanced LIGO, Virgo and KAGRA detectors operating in the US, Europe and Japan.
To date there have been more than 80 detections, all of merging black holes or neutron stars.
“Until now, most of the black holes we’ve found with gravitational waves have been spinning fairly slowly,” said Dr Charlie Hoy, a researcher at Cardiff University during this 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 about as fast as physically possible. Our current models of how binaries form suggest this was an extremely rare, perhaps a rare event.” one in a thousand. Or it could be a sign that our models need to change.”