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

Artist's illustration of the black hole

Black holes are one of the most fascinating objects in the Universe. On their surface, known as the “event horizon”

;, gravity is so strong that not even light can escape from them. Usually, black holes are silent, silent creatures that swallow anything that gets too 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, a black hole arises and releases enormous amounts of energy as it settles into its final form. This phenomenon offers astronomers a unique opportunity to observe rapidly evolving black holes and explore gravity in its most extreme form.

Although the colliding black holes produce no light, astronomers can observe what is detected gravitational waves – ripples in the fabric of space and time – bouncing off them. Scientists speculate that, after a collision, the residual black hole’s behavior is key to understanding gravity and should be encoded in the emitted gravitational waves.

Cusp of the black hole

Artistic illustration of the cusp of a black hole. Credit: C. Evans; J.C. Bustillo

In the article published in Communications physics (Nature), a team of scientists led by OzGrav alumni Prof. Juan Calderón Bustillo – now ‘La Caixa Junior Leader – Marie Curie Fellow’ at the Galician Institute of High Energy Physics (Santiago de Compostela, Spain) – revealed how gravitational waves encode the merging shape of black holes as they settle into their final shape.

Graduate student and co-author Christopher Evans of the Georgia Institute of Technology (USA) says, “We ran simulations of black hole collisions using supercomputers and then compared the rapidly changing shape of the residual black hole with the gravitational waves it emits. We have found that these signals are much richer and more complex than is commonly thought, allowing us to learn more about the rapidly changing shape of the final black hole. “

Stages of merger of the black hole

First, both black holes orbit each other, slowly approaching, during the inspiratory phase. Second, the two black holes merge, forming a distorted black hole. Eventually, the black hole reaches its final shape. b: Frequency of gravitational wave signals observed from the top of the collision (furthest left) and from various positions on its equator (rest) as a function of time. The first signal shows the typical “chirp” signal, where the frequency increases as a function of time. The other three show that after the collision (at t = 0), the frequency drops and rises again, producing a second “chirp”. Credit: C. Evans, J. Calderón Bustillo

The gravitational waves from the collision of black holes are very simple signals known as “chirps”. As the two black holes approach, they emit a signal of increasing frequency and amplitude indicating the speed and radius of the orbit. According to Prof. Calderón Bustillo, “the pitch and amplitude of the signal increase as the two black holes get closer and faster. After the collision, the last remaining black hole emits a signal with a constant tone and decreasing amplitude, like the sound of a bell being struck. “This principle is consistent with all gravitational wave observations so far, when studying collision from above.

However, the study found that something completely different happens if the collision is observed from the “equator” of the final black hole. “When we looked at black holes from their equator, we found that the final black hole emits a more complex signal, with a tone that rises and falls a couple of times before dying,” explains the prof. Calderón Bustillo. “In other words, the black hole actually chirps several times.”

Shape of the remaining black hole after the collision

Detail of the shape of the residual black hole after a collision of a black hole, with a “chestnut shape”. The regions of strong gravitational wave emission (in yellow) cluster near its cusp. This black hole rotates making the cusp point for all observers around it. Credit: C. Evans, J. Calderón Bustillo

The team found that this correlates with the shape of the final black hole, which acts as a kind of gravitational wave beacon: “When the two original, ‘parent’ black holes are of different sizes, the final black hole initially appears as a chestnut, with a cusp on one side and a wider, smoother back on the other, “says Bustillo.” It turns out that the black hole emits more intense gravitational waves through its more curved regions, which are those that surround its cusp. This is because the remnant black hole is also spinning and its cusp and its return point repeatedly to all observers, producing more chirps. “

Co-author Prof. Pablo Laguna, former president of the School of Physics at Georgia Tech and now a professor at the University of Texas at Austin, pointed out “while a relationship between gravitational waves and the behavior of the black hole has long been hypothesized, our study provides the first explicit example of this type of relationship. “

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

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