New Clues to the Shape of Black Holes Offered by Ripples in the Fabric of Space and Time

A collision of Two Black Holes

One of the most fascinating objects in the Universe is Black holes. Gravity is so strong at the surface of the black hole, surface known as the “event horizon,” that not even light can escape from them. Black holes are silent creatures and usually quiet that swallow anything getting too close to them but when two black holes merge and collide, one of the most catastrophic events in the Universe is produced within a fraction of a second. This catastrophic event results in the birth of a highly-deformed black hole. The event releases tremendous amounts of energy as it settles to its final form. This phenomenon gives astronomers a unique chance to explore gravity in its most extreme form and observe rapidly changing black holes.

Astronomers can observe the detected gravitational waves as colliding black holes do not produce light and gravitational forces ripples in the fabric of space and time. Scientists speculate that the behavior of the remnant black hole is key to understanding gravity which is best suited after a collision and this behavior should be encoded in the emitted gravitational waves.

Simulations Performed by Scientists

In the article published in Communications Physics (Nature) has revealed how gravitational waves encode the shape of merging black holes as they settle to their final form.  The study was done by a team of scientists from the Galician Institute for High Energy Physics (Santiago de Compostela, Spain) led by OzGrav alumnus Prof. Juan Calderón Bustillo.

Scientists performed simulations of black-hole collisions using supercomputers from Georgia Institute of Technology (USA) and then compared the rapidly changing shape of the remnant black hole to the gravitational waves it emits. And these signals discovered are rich and complex than their assumptions which allows them to learn more about the vastly changing shape of the final black hole.

Chirpings in Black Hole

The gravitational waves known as “chirps” from colliding black holes are very simple signals. A signal is emitted as the two black holes approach each other which is of increasing frequency and amplitude that indicates the speed and radius of the orbit. Prof. Calderón Bustillo has deduced that the pitch and amplitude of the signal increases as the two black holes approach faster and faster. He added that when the collision has occurred, then the final remnant black hole emits a signal with a constant pitch and decaying amplitude which is similar to the sound of a bell being struck. When the collision is studied from the top, the consistency is observed with the principle of all gravitational-wave observations so far.

However, if the collision is observed from the “equator” of the final black hole, completely different was found by the study. Prof. Calderón Bustillo explained that when they observed black holes from their equator, they found that the final black hole emits a more complex signal and before the signal dies, the pitch goes up and down a few times. In simpler words, the black hole chirps several times.

The shape defines Intensity and Chirping

The team observed that the chirping of a black hole is related to the shape of the final black hole acting as a gravitational-wave lighthouse. Bustillo said that when two original parent black holes are of different sizes, the resultant black holes look like a chestnut, which is wider and has a cusp on one side, while is smooth on the other side. Intense gravitational waves through its most curved regions are emitted which are those surrounding its cusp. The remnant black hole’s cusp and back repeatedly point to all observers and are also spinning, which is the reason for intense gravitational force and production of multiple chirps.

Co-author Prof. Pablo Laguna, now Professor at the University of Texas at Austin and former chair of the School of Physics at Georgia Tech pointed out that their our study provides the first explicit example of the relation between the gravitational waves and the behavior of the final black hole, which was previously long conjectured.