Black hole mergers generate chirps in complex gravitational wave emissions

Artistic representation of the newborn black hole which has a distorted shape with a cusp along with the emissions of gravitational waves
An article published in the journal “Communications Physics” reports a study on black hole mergers that shows the relationship between the gravitational signal emitted by that event and the shape of the black hole produced by it. A team of researchers led by Juan Calderón Bustillo – Marie Curie Fellow of the Galician Institute of High Energy Physics in Santiago de Compostela, Spain, created computer simulations of these mergers establishing that the shape of the black hole produced, distorted during the moments while it’s settling and similar to a chestnut, influence the characteristics of gravitational waves. The “chirps”, as the multiple frequency peaks produced in the gravitational emissions, could be detected if the line of sight were parallel to the merger’s orbital plane.

In the very few years that passed since the astronomy of gravitational waves moved from a theoretical concept to actual detections with the study of their results, many researchers started studying the events discovered thanks to the LIGO and Virgo detectors. Almost all of those events were mergers of two black holes, which we can’t see directly, with the consequence that even nailing the merging moment we wouldn’t be able to see how the shape of the newly generated black hole settles. However, Juan Calderón Bustillo’s team found a way to reconstruct that event in simulations, and above all found a way to reconstruct it from the characteristics of the gravitational waves.

By examining the gravitational emissions of mergers between two black holes, the researchers noted that there can be multiple frequency peaks, nicknamed chirps. Christopher Evans, a student at the Georgia Institute of Technology and one of the authors of this study, explained that the team ran simulations of black holes using supercomputers and then compared the shape of the rapidly changing black hole produced to the gravitational waves it emits. The result is the discovery that these signals are much richer and more complex than is commonly thought, allowing to learn more about the final black hole’s shape changes.

The image (Courtesy Christopher Evans, Juan Calderon Bustillo. All rights reserved) shows an artistic representation of the newborn black hole which has a distorted shape with a cusp along with the emissions of gravitational waves. Basically, the black hole’s shape initially resembles a chestnut with a cusp on one side while the other is wider and smoother. In the area around the cusp, more intense gravitational waves are emitted, a kind of cosmic beacon that produces various chirps.

The ability to detect complete gravitational wave emissions depends on the line of sight. If it’s parallel to the merger’s orbital plane, in the future the detectors should be sensitive enough to receive the various chirps. The teams of the LIGO and Virgo collaborations keep on improving their instruments and new, even more sophisticated gravitational detectors, are being designed. Gravitational-wave astronomy will continue to develop, and in the next few years, it will be possible to better study extreme phenomena such as black hole mergers.

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