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| Artist’s illustration of two black holes orbiting each other |
Researchers at Cardiff University 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, which is 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 seen in black holes, where
the twisting is 10 billion times faster than in previous observations.
The binary black hole system was found through gravitational waves in early
2020 in the Advanced LIGO and Virgo detectors. One of the black holes, 40
times bigger than our Sun, is likely the fastest spinning black hole to be
found through gravitational waves. And unlike all previous observations, the
rapidly revolving black hole distorted space and time so much that the
binary's entire orbit wobbled 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 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 Gravity Exploration Institute. "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!"
A more down-to-earth example of precession is the wobbling 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 example previously measured from orbiting
neutron stars called binary pulsars, it took over 75 years for the orbit to
precess. The black-hole binary in this study, colloquially known as GW200129
(named after the date it was observed, January 29, 2020), precesses several
times every second—an effect 10 billion times stronger than measured
previously.
Dr. Jonathan Thompson, also of Cardiff University, explained: "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."
Gravitational waves were predicted by Einstein in 1916. They were first
directly detected from the merger of two black holes by the Advanced LIGO
instruments in 2015, a breakthrough discovery that led to the 2017 Nobel
Prize. Gravitational wave astronomy is now one of the most vibrant fields of
science, with a network of the Advanced LIGO, Virgo and KAGRA detectors
operating in the US, Europe and Japan. To date there have been over 80
detections, all of merging black holes or neutron stars.
"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 during this study, and now 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."
The international network of gravitational-wave detectors is currently being
upgraded and will start its next search of the universe in 2023. They are
likely to find hundreds more black holes colliding, and will tell scientists
whether GW200129 was a rare exception, or a sign that our universe is even
stranger than they thought.
Reference:
Mark Hannam, General-relativistic precession in a black-hole binary, Nature
(2022).
DOI: 10.1038/s41586-022-05212-z.