Science

Merging, black holes move at high speed

Merging… movement! The researchers indirectly confirmed the behavior characteristic of black holes, which has so far been theorized in the vast framework of general relativity: when two such objects merge, they receive a strong enough momentum to project a newly formed single black hole into space with extraordinary speed. hole.

A group of researchers led by Vijay Varma from the Institute of Gravitational Physics. Max Planck in Germany and his colleagues discovered a fast-moving black hole with a mass of 60 solar masses by carefully studying data from the LIGO (Laser Interferometer Gravitation-Wave Observatory) in the USA and its counterpart in Italy, Virgo. Thanks to a signal corresponding to this space-time ripple that reached Earth on January 29, 2020, the team was able to follow the course of events: having united, two black holes received a “kick”, throwing a new black hole into a high-speed one.

Mach 4000

The latter flew away at a speed of about 5 million kilometers per hour, with an accuracy of several million, you can read in an article describing the discovery, published in Physical Review Letters. “1,500 km/s is about 0.5% the speed of light, which is pretty fast for a 60-solar black hole,” says Vijay Varma, who was contacted by Science et Avenir. “We can see things differently: it is moving at about 5 million km/h, which is more than Mach 4000! A ball, on the other hand, is moving at about 3,000 km/h. Therefore, a black hole is a thousand times faster. “

As two paired black holes rotate inward and merge, they emit ripples that stretch and compress space. If these gravitational waves are thrown into space in a certain direction, the black hole will eventually recoil, like a gun that fires back after a bullet is fired. To estimate the recoil rate of said black hole, the researchers compared the collected data with various black hole merger predictions. In short, to computer simulations that solve the equations of the theory of gravity formulated by Einstein. They found that the recoil was so great that the black hole was likely thrown out of its focus. “If we assume that this black hole has merged into a globular cluster, it has almost certainly been removed from its original environment,” says Vijay Varma.

Set in motion… then isolated?

Scientists believe that globular clusters — dense clusters of stars and black holes — are the main merger sites for stellar-mass black holes, the same ones that were at the beginning of the black hole merger observed by the team. They also calculated that the chance of an ejected black hole remaining in a globular cluster is only about 0.5%, while a black hole basking in another type of dense medium, such as a nuclear star cluster, only has about 8%. chance to stick. around. “However, it must be taken into account that other environments, such as disks around active galactic nuclei, have also been proposed as sites of repeated mergers and that these environments may have higher escape velocities,” says the astrophysicist.

The fact that black holes travel such distances during their merger is key to understanding how other, much more intriguing black holes formed. And for good reason: if we stick to supernova simulations, stellar black holes with a mass greater than 65 solar masses should not exist. However, LIGO-Virgo has already observed this. “One of the proposed explanations is that these heavy black holes could have formed from already merged stellar-mass black holes. the second merger,” notes Vijay Varma. “Only estimating the frequency of these impacts from more gravitational wave signals can help us solve this mystery.”

Test General Relativity Again and Again

These “kicks” constitute one of the clearest predictions of general relativity made to date. Conversely, if the observations did not fit the theory, the Standard Model might be called into question. Such observations should also be refined with the forthcoming commissioning of the LISA (Laser Interferometer Space Antenna), a detector of low-frequency gravitational waves from space, which, in particular, will observe mergers of supermassive black holes. Thus, the data collected by LISA could help us understand how giant black holes could have been seeded in the primordial universe and, above all, how they could have reached their current sizes.

Note that the LIGO and Virgo data have already revealed signs of small impacts received by black holes. However, this study is the first to report on gravitational waves to observe this great push that was only predicted.

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