Max Planck Institute for Gravitational Physics animation
Black holes, as the basic definition goes, are regions of spacetime so deformed by concentrated mass that, beyond their “event horizon”, nothing — not even light — can escape their gravity. We can detect them in various ways, including spotting the high-energy radiation given off by matter as it swirls into the holes or by looking for how their gravity influences the movement of nearby objects. Physicists can also detect the gravitational waves emitted as black holes merge with each other, warping the very fabric of spacetime, using so-called interferometers like Virgo in Italy and LIGO in the US.
Some of the black holes physicists have seen out in the universe appear to be “speeding”, travelling far faster than is expected based on theoretical predictions.
Scientists had proposed that these fast-moving black holes may have gained their energy as a result of previous merger events.
According to the theory, merged black holes can be given a kick if the gravitational waves released during the collision are emitted predominantly in one direction — which can occur if the two original black holes have very unequal masses or spins.
To conserve momentum, the combined black hole recoils in the other direction. Until now, however, physicists had no evidence to support this theory.
Mergers between black holes can give the massive objects a kick — and eject them out of their galaxy
Black holes are regions of spacetime deformed by concentrated mass
Particularly large kicks are expected to occur when the orbital plane of the merger undergoes precession — a change in the orientation of a rotational axis — which should leave a detectable amplitude modulation in the gravitational wave signal.
In their study, physicist Dr Vijay Varma of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and his colleagues analysed a black hole merger event that has been given the thrilling name of “GW200129”.
This is the first evidence of a black hole merger that has been recorded with a strong and unambiguous signature of precession in its gravitational wave signal.
The researchers compared the signals of GW200129 recorded by the LIGO–Virgo detectors with predictions based on numerical relativity simulations.
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Physicists can detect the gravitational waves emitted as black holes merge with each other
Pictured: the Virgo interferometer in Italy can detect gravitational waves
They found that the black hole produced by the merger event, which has a mass 60 times that of the Sun, was given a kick of around 3,355,404mph.
This is well in excess of the escape velocity of most galaxies — and nearly three times that of our own, the Milky Way.
Given this, the team said, the collision would have likely sent the final black hole whizzing off out of its host galaxy.
Dr Varma said: “Given the kick velocity, we estimate that there is at most a 0.48 percent probability that the remnant black hole of GW200129 would be retained by globular, nuclear star clusters.”
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Merged black holes can be given a kick strong enough to eject them from their host galaxy
The findings may have implications for the existence of so-called “heavy black holes”, which form as a result of multiple, successive black hole mergers.
For heavy black holes to form, however, mergers that end with kicks cannot be too common.
If they happen too frequently, by sending merged black holes out in the vast intergalactic void they would make subsequent collisions too unlikely.
Future studies, the team said, should help physicists to better constrain the rate of so-called second generation mergers that can help build up larger black holes.
Theoretical astrophysicist Professor Saul Teukolsky of Cornell University is the leader of the Simulating eXtreme Spacetimes (SXS) Collaboration, under whose auspices the present study was undertaken.
Prof. Teukolsky said: “This research shows how gravitational-wave signals can be used to learn about astrophysical phenomena in an unexpected way.
“It had been believed that we would have to wait more than a decade for detectors sensitive enough to do this kind of work, but this research shows we can in fact do it now — very exciting!”
The full findings of the study were published in the journal Physical Review Letters.