Black holes are generally expected to merge when they collide, but new theoretical research suggests that, under extreme conditions, one black hole might split into two smaller ones. The possibility appears most plausible for black holes rotating very close to their maximum possible speed, while models involving additional spatial dimensions may permit fragmentation under other circumstances. No such event has been confirmed, but gamma-ray observations could eventually provide indirect evidence.
The study, titled “When black holes admit pant topology,” was published in Physical Review D. It examines the physical laws that normally keep black holes intact and whether particular conditions could allow them to disintegrate.
“In our study, we investigate what physical laws normally prevent a black hole from splitting into two. We also investigate whether certain circumstances can cause splitting to occur,” says study co-author Francesco Sannino, a professor of physics at Italy’s Federico II University of Naples and the Center for Quantum Field Theory at the University of Southern Denmark.
The research team also included Giacomo Cacciapaglia, a researcher at France’s National Center for Scientific Research (CNRS) and the Institute for Physics and Autoenergetics, a joint research institute between CNRS and Sorbonne University in Paris. Manuel Del Piano and Vania Vellucci, from the Center for Quantum Field Theory at the University of Southern Denmark, were also contributors.
Why black holes normally cannot split
One of the main obstacles to black hole splitting comes from thermodynamics, the branch of physics that describes how energy behaves and how physical systems change.
A central concept in thermodynamics is entropy, which measures how many microscopic configurations a system can have while still appearing the same from the outside. Black holes have entropy, and that property places important constraints on what can happen to them.
For a non-rotating black hole, splitting into two smaller black holes would reduce the total entropy. That makes the process thermodynamically unfavorable and helps explain why black holes are generally expected to remain intact.
Extreme rotation could open a narrow possibility
The researchers’ calculations suggest that rapid rotation could create a narrow exception to the physical principles that normally prevent a black hole from splitting. In theory, a black hole spinning fast enough might overcome the usual thermodynamic limitation.
There is, however, another significant obstacle: the two resulting black holes would need to move far enough apart to avoid quickly merging again.
“But the two resulting black holes must be able to separate, rather than quickly coming back together. Including this requirement almost closes the window again, except for black holes that rotate at speeds close to their maximum possible speed,” Sannino says.
Why the study refers to “pants topology”
The study’s unusual title refers to the shape traced by a black hole’s boundary through space-time if a split were to occur. The boundary, known as the event horizon, would begin as a single structure and then divide into two branches.
Viewed over time, that structure resembles a pair of pants, with one upper section dividing into two “legs.” This is the origin of the term “pants topology.”
Additional dimensions could change the picture
The researchers also examined more speculative scenarios involving additional dimensions beyond the three spatial dimensions experienced in everyday life.
Some theoretical models with extra dimensions contain black hole-like objects that can become unstable and begin to fragment. These models suggest that the familiar rules governing black holes may work differently under unusual conditions.
In some cases, splitting can even be thermodynamically favorable, particularly when one of the two resulting black holes is much smaller than the other.
However, a process being allowed or favored by thermodynamics does not necessarily mean that it will occur. Researchers must still determine whether the fragmentation is physically possible and what conditions would be required.
Could astronomers detect a splitting black hole?
Scientists are continuing to investigate how likely such events might be. The answer could depend on aspects of gravity that extend beyond Einstein’s theory of general relativity, making the question especially difficult to resolve.
“We still cannot give a single probability that a black hole will split up. It depends on how gravity behaves beyond Einstein’s theory, and we are working to quantify this,” Sannino says. “We do not yet know how a small primordial black hole would compare with a much larger astronomical object. We expect that they are more prone to fragmentation than physical black holes. We may be able to detect fragmentation indirectly. Gamma-ray telescopes such as Fermi LAT can search for signals from small black hole fragments that may form during mergers.”
There are currently no confirmed observations of black hole fragmentation. Signals from small fragments could nevertheless offer a potential way to investigate the idea and provide clues about how gravity behaves under the universe’s most extreme conditions.
Sannino is director of the Quantum Field Theory Center (ℏQTC) at the Faculty of Mathematics and Computer Science (IMADA) at the University of Southern Denmark. He is also affiliated with the Danish Institute for Advanced Study and the Department of Physics “Ettore Pancini” at the University of Naples Federico II. In addition, he is a visiting professor at Durham University.
Source: www.sciencedaily.com