Black Holes in an Eccentric Orbit – ScienceDaily

When black holes collide in the universe, the collision shakes space and time: the amount of energy released during the merger is so great that it causes space-time to oscillate, similar to waves on the surface of water. These gravitational waves propagate throughout the universe and can still be measured thousands of light-years away, as was the case on May 21, 2019, when the gravitational-wave observatories LIGO (USA) and Virgo (Italy) picked up such a signal. Named GW190521 after the date of its discovery, the gravitational wave event has since sparked debate among experts because it differs markedly from previously measured signals.

The signal was initially interpreted to mean that the collision involved two black holes moving in near-circular orbits around each other. “Such binary systems can be generated by a number of astrophysical processes,” explains Professor Sebastiano Bernuzzi, a theoretical physicist from the University of Jena, Germany. Most of the black holes discovered by LIGO and Virgo, for example, are of stellar origin. “This means that they are the remnants of massive stars in binary star systems,” adds Bernuzzi, who led the current study. These black holes orbit each other in near-circular orbits, just as the original stars once did.

One black hole captures a second

“GW190521 behaves very differently,” explains Rossella Gamba. The publication’s lead author is currently doing her Ph.D. at Jena Research Training Group 2522 and is part of the Bernuzzi team. “Its shape and explosion-like structure are very different from previous observations.” So, Rossella Gamba and her colleagues set out to find an alternative explanation for the unusual gravitational-wave signal. Using a combination of the latest analytical methods and numerical simulations on supercomputers, they calculated various models of the cosmic collision. They come to the conclusion that it must have occurred on a severely eccentric path rather than a semi-circular one. A black hole initially moves freely in an environment relatively full of matter, and as soon as it approaches another black hole, it can be “captured” by the other’s gravitational field. This also leads to the formation of a binary system, but here the two black holes do not rotate in a circle, but rather move eccentrically, in tumbling motions around each other.

“Such a scenario explains the observations much better than any other hypothesis presented to date. The probability is 1:4,300,” says Matteo Bresci, PhD student and co-author of the study, who developed the infrastructure for the analysis. Postdoctoral researcher Dr. Gregorio Carullo adds, “Although we do not currently know how common these dynamic motions of black holes are, we do not expect them to occur frequently.” He adds that this makes the current findings even more exciting. However, more research is needed to clarify beyond a reasonable doubt the processes that created GW190521.

Group work in the research training group

For the current project, the two teams in Turin and Jena (as part of the Gina Research Training Group 2522 “Dynamics and Criticality in Quantum and Gravitational Systems” funded by the German Research Foundation) developed a general relativistic framework for eccentric mergers of black holes and verified the analytical predictions using simulations of Einstein’s equations. For the first time, models of dynamical encounters have been used in the analysis of gravitational wave monitoring data.

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Materials Introduction of Friedrich Schiller University Jena. Original by Ute Schönfelder. Note: Content can be modified by style and length.

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