(ORDO NEWS) — Recently, scientists observed two black holes that merged into one, and in the process received a “kick” that threw the newly formed black hole at high speed.
This black hole streaked past at about 5 million kilometers per hour, give or take a few million, the researchers report in a paper published in the journal Physical Review Letters. This is amazingly fast: the speed of light is only 200 times faster.
Fluctuations in space-time, called gravitational waves, sent the black hole into its rushing exit. When any two paired black holes swirl inward and merge, they emit these pulsations that stretch and compress space. If these gravitational waves are thrown into space in a predominantly one direction, the black hole will recoil in response.
It’s like a gun being thrown back by a bullet, says astrophysicist Vijay Varma of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
The gravitational wave observatories LIGO and Virgo, located in the US and Italy, recorded the space-time pulsations of black holes as they reached Earth on January 29, 2020. These waves revealed details of how the black holes merged, hinting at the possibility of a big push.
As the black holes rotated around each other, the plane in which they rotated rotated, or precessed, much like a top wobbles as it rotates. Precessing black holes are expected to get a bigger push when they merge.
So Varma and his colleagues dug into the data to determine if the black hole had gotten kicked. To estimate the recoil rate, the researchers compared the data to various predicted versions of black hole mergers created from computer simulations solving the equations of general relativity, Einstein’s theory of gravity (SN: 2/3/21). The recoil was so great that the black hole was likely ejected from its home and thrown to the edge of space.
Dense groups of stars and black holes, called globular clusters, are one of the places where black holes are thought to coalesce and merge.
The chance of a discarded black hole remaining within a globular cluster is only about 0.5 percent, the team calculated. For a black hole in another type of dense medium called a nuclear star cluster, the probability of staying here is about 8 percent.
A big black hole escape could have big consequences. LIGO and Virgo detect mergers of stellar-mass black holes, which form when a star explodes in a supernova and collapses into a black hole. Scientists want to see if black holes that have coalesced into tight clusters can coalesce again after going through several rounds of merging.
If so, this could help explain some of the remarkably bulky black holes previously observed in mergers (SN: 9/2/20). But if merged black holes are usually rocketed away from home, that would make multiple mergers less likely.
“The impacts are very important for understanding how heavy stellar-mass black holes form,” says Varma.
Astronomers have previously received evidence that gravitational waves give a big push to supermassive black holes – much larger monsters that are at the centers of galaxies (SN: 3/28/17). But this conclusion is based on observations of light, not gravitational waves.
“Gravitational waves are, in a sense, cleaner and easier to interpret,” says astrophysicist Manuela Campanelli of the Rochester Institute of Technology in New York, who was not involved in the new study.
The LIGO and Virgo data have already revealed some evidence that black holes are getting hit a little. The new study is the first to report the use of gravitational waves to detect a black hole that has taken a big hit.
This big hit isn’t a surprise, says Campanelli. Earlier theoretical predictions by Campanelli and colleagues suggested that such powerful strikes were possible. “It’s always interesting when someone can measure through observations what you predicted based on calculations.”
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