Astronomers have found the first solid evidence that merger events between black holes can provide a “kick” powerful enough to knock a black hole out of its galaxy.
The team, which included Vijay Varma, a physicist at the Max Planck Institute for Gravitational Physics, Albert Einstein Institute, Germany, examined gravitational wave data from the meltdown event known as GW200129 collected by the detectors LIGO and their European counterpart, Virgo. Through this analysis, the scientists discovered that the black hole created in this collision and merger had been sent through space at 3 million mph (4.8 million km/h) – a discovery described by one team member as “both startling and shocking”.
“When two black holes collide, they leave behind a more massive residual black hole. This process can kick back the residual black hole,” said Varma, lead author of a paper detailing the team work, at Space.com. .
Related: 8 Ways We Know Black Holes Really Exist
When black holes revolve around each other, they emit gravitational waves – essentially gravitational radiation – which carries away energy and angular momentum as they ripple through the fabric of space. These emissions cause the orbit to shrink, resulting in black holes colliding and merging.
However, if black holes have unequal masses or spins, this leads to an asymmetry in the emission of gravitational waves, these being mainly emitted in one direction. Because the fundamental laws of physics require momentum to be conserved, this asymmetry results in a large kick, causing the remaining black hole to recoil in the opposite direction.
“Black hole mergers also emit gravitational radiation, similar to astrophysical processes that emit electromagnetic radiation — light,” Varma continued.
These big kicks are expected when the fusion’s orbital plane precedes or “wobbles”. Orbital precession is observable as a small change in amplitude of the gravitational wave signal. “This black hole binary system is also the first signal showing strong signs of orbital precession, whereby the orbital plane wobbles,” co-author Scott Field, a mathematician at the University of Massachusetts Dartmouth, told Space. com.
Varma added that by analyzing gravitational radiation, astronomers and astrophysicists can learn more about black hole mergers. Moreover, since black holes have an influence on the evolution of galaxies, learning more about these processes could reveal how collections of stars like the Milky Way develop.
It’s the first time astronomers have collected strong evidence that such a merger can eject the resulting black hole from its galaxy.
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“Unlike previously observed black hole merger events, this is the first to provide strong evidence of an enormous recoil velocity. Large enough, in fact, that the remaining black hole most likely escaped its surroundings. host,” Field said. “While we knew that general relativity in principle allowed for such extreme possibilities, we did not know if the universe would produce them. The final velocity of the black hole is large enough to most likely exceed the escape velocity of its host environment. “
Field added that this result will also have important implications for binary black hole formation scenarios. This is because supermassive black holes, like Sagittarius A* (Sgr A*) in the heart of the Milky Way, are formed by a series of collisions that scientists call hierarchical mergers. Black holes expelled from a galaxy cannot participate in this process.
Mergers that give the boot to black holes
Discovering mergers that are imbalanced enough to kick black holes powerfully is now possible thanks to technology that enables more precise detections of gravitational waves.
“Black hole mergers emit no light, so gravitational waves are the only way to observe them and learn more. We wouldn’t know anything about this ejected, rogue black hole without gravitational-wave observatories,” said added Field.
Scientists aren’t sure exactly where gravitational wave event GW200129 originated, so Field points out the team can’t be completely sure the black hole was ejected from its galaxy, but that’s the likely result. of its movement at such extreme speeds, according to the researchers.
“If that’s indeed the case, it’s now wandering the universe on its own like a rogue black hole,” Varma said.
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The meltdown that happened here may be a miniature version of an even more dramatic event, he noted. “A similar phenomenon occurs when supermassive black holes merge, which can happen after a galactic merger,” Varma said. “The final supermassive black hole may be displaced from the center of the merged galaxy, or even ejected from it, leaving behind a galaxy without a central black hole.”
Although existing gravitational wave detectors are not powerful enough to observe supermassive black hole mergers, the authors added that future space detectors like the proposed Laser Interferometer Space Antenna (LISA) mission could do it.
“Gravitational-wave astronomy has delivered many high-impact and truly remarkable discoveries over the past five years or so,” Field said. “Before the first detection of gravitational waves, the mantra of our field was that gravitational waves will open a new window on the universe. And this has proven true with each new LIGO observation campaign.”
The research is described in a paper (opens in a new tab) published May 12 in the journal Physical Review Letters.
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