Ripples in the fabric of space and time provide new clues to the shape of black holes

Illustration of a black hole artist

Black holes are one of the most amazing things in the universe. On its surface, known as the “event horizon,” the gravity is so strong that even light cannot escape from it. Black holes are usually quiet and silent creatures that swallow anything that comes close to them; However, when two black holes collide and merge together, they produce one of the most catastrophic events in the universe: in a split second, severe distortion occurs. Black hole It generates and releases massive amounts of energy as it settles into its final form. This phenomenon gives astronomers a unique opportunity to observe rapidly changing black holes and explore gravity in its most extreme form.

Although colliding black holes do not produce light, astronomers can observe what has been discovered Gravitational waves Ripples in the fabric of space and time – bounce off them. Scientists speculate that after a collision, the behavior of the remaining black hole is key to understanding gravity and should be encoded in the gravitational waves emitted.

Black hole protrusion

An artist’s illustration of a black hole. Credit: C. Evans; JC Bustillo

In the article published in Communication Physics (Nature), a team of scientists led by Osgraph graduate Professor Juan Calderon Bustillo – now “La Caixa Junior Leader – Marie Curie Fellow” at the Galician Institute for High Energy Physics (Santiago de Compostela, Spain) – revealed how gravitational waves encode the shape of merging black holes When it settles into its final form.

“We simulated collisions of black holes using supercomputers, and then compared the rapidly changing shape of the remaining black hole to the gravitational waves it emits,” says Christopher Evans, graduate student and co-author from Georgia Institute of Technology (USA). We discovered that these signals are richer. Complex than what is usually thought, allowing us to learn more about the greatly changed shape of the final black hole. “

The stages of a black hole merger

First, both black holes rotate around each other, slowly approaching, during the inspiring phase. Second, the two black holes merge, forming a distorted black hole. Finally, the black hole reaches its final shape. B: The frequency of the gravitational wave signals observed from above the collision (far left) and from different positions on the equator (rest) as a function of time. The first signal shows a typical “chirping” signal, with the frequency increasing as a function of time. The other three show that after the collision (at t = 0) the frequency decreases and rises again, resulting in a second “chirp”. Credit: C. Evans, J. Calderon Bustillo

The gravitational waves from colliding with black holes are very simple signals known as “chirping”. As the two black holes approach each other, they emit a signal with an increasing frequency and amplitude that indicates the velocity and radius of the orbit. According to Professor Calderon Bustillo, “The signal increases in intensity and amplitude as the two black holes approach faster and faster. After the collision, the last remaining black hole emits a signal of steady tone and decaying amplitude – like the sound of a bell striking.” This principle is consistent with all observations of gravitational waves to date, when studying collisions from above.

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However, the study found that something completely different would happen if collision was observed from the “equator” of the final black hole. Professor Calderon Bustillo explains: “When we observed black holes from the equator, we found that the last black hole emits a more complex signal, with a tone rising and falling several times before it dies.” In other words, a black hole makes a sound multiple times.

What remains of a black hole after collision

Details of the shape of the black hole remaining after colliding with a “chestnut shape”. Strong gravitational wave emitting regions (yellow) near their cusp. This black hole spins making it the reflection point for all observers around it. Credit: C. Evans, J. Calderon Bustillo

The team discovered that this correlates with the shape of the final black hole, which acts as a kind of gravitational wave beacon: “When the original ‘original’ black holes are of different sizes, the final black hole initially looks like a chestnut, with a bump on one side and a wider and smoother back on the other side. Bustillo says. “It turns out that the black hole emits more intense gravitational waves through its most curved regions, which are the ones that surround its tiniest. This is because the remaining black hole also rotates and its protrusion and back are repeatedly pointing to all observers, resulting in multiple chirps.”

Co-author Professor Pablo Laguna, former chair of the School of Physics at Georgia Tech and current professor at the University of Texas at Austin, noted that “while the relationship between gravitational waves and the behavior of a final black hole has been foreseen for a long time, our study provides the first clear example of this type. From relationships. “

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

Phil Schwartz

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