Black holes are the most attractive objects in the universe. The gravitational pull on their surface, known as the “event horizon”, is so strong that even light cannot escape from them. In general, black holes are quiet and silent creatures that swallow anything that comes close to them; However, when two black holes collide and merge, they create one of the most devastating events in the universe: one second, very distorted. Black hole When it is born and transforms into its final form, it emits a lot of energy. This phenomenon gives astronomers a unique opportunity to observe rapidly changing black holes and explore gravity in its most intense form.

Although collisions with black holes do not produce light, astronomers can observe the findings. Gravitational waves – Fabrics of place and time – They are expelled. Scientists speculate that the nature of the black hole remaining after a collision is important for understanding gravity and encoding it in the gravitational waves emitted.

Image of a black hole art artist. Attribution: c. Evans; J. C. Bustillo

In the article published in Communications Physics (Nature), Osgrav alumnus prof. Encode the shape of the merge as the black holes become their final shape.

Christopher Evans, a graduate student and co – author at the Georgia Institute of Technology (USA), says: “We used supercomputers to simulate black hole collisions and compare the shape of the remaining black hole to gravitational waves. We found that these signals are much richer and more complex than previously thought, allowing us to learn more about the shape of the final black hole. ”

First, the black holes orbit each other, slowly approaching the excitation phase. Second, the two black holes combine to form a deformed black hole. Finally, the black hole reaches its final shape. b: The frequency of gravitational-wave signals from the top of the collision (left) at different points in its equator (rest) as a function of time. The first signal shows a normal “chirping” signal, in which the frequency is raised as a function of time. The other three show that after a collision (at t = 0) the frequency decreases and rises again, forming a second “chirp”. Attribution: c. Evans, J .; Calderon Bustillo

Gravitational waves that collide with black holes are very simple signals known as “chirps”. When two black holes approach each other, they emit a signal of frequency and magnitude indicating the speed and distance of the orbit. Prof. According to Calderon Bastilo, “The pitch and amplitude of a signal increase as two black holes approach faster and faster. After a collision, the remaining black hole emits a signal with a constant pitch and decreasing amplitude – like the sound of a bell ringing. “When studying collisions from above, this principle applies to all gravitational-wave observations to date.

However, the study found that when the collision is observed from the “equator” of the last black hole, something completely different happens. “When we observed black holes from their equator, we found that the last black hole emits a more complex signal, a pitch that goes up and down a few times before it dies,” he said. Calderon Bustillo explains. “In other words, the black hole laughs many times.”

Details of the shape of a black hole remaining after a black hole collision, a ‘chestnut shape’. Areas of the cluster (yellow) of strong gravitational-wavelength. Rotating this black hole gives cusp points to all the observers around. Attribution: c. Evans, J .; Calderon Bustillo

The team found that this was related to the shape of the last black hole, which acts like a kind of gravitational-wave lighthouse: “When two real, ‘parent’ black holes are different in size, the last black hole initially appears to be a chestnut, with a cone on one side and a soft one on the other.” “The black hole emits more intense gravitational waves through its most curved regions, which surround it. This is because the remaining black hole rotates, and its outer surface is repeatedly pointed back and forth to all observers, forming multiple shells. ”

He is a former chairman of the School of Physics at Georgia Tech and now a professor at the University of Texas at Austin. Pablo Laguna points out that “the relationship between gravitational waves and the behavior of the final black hole has been long suppressed. , Our study provides the first clear example of this type of relationship. ”

Reference: 8 October 2020, Communications Physics.
DOI: 10.1038 / s42005-020-00446-7