When two black holes collide, they merge into a single, larger one, and this happens regularly across the universe. A new theoretical paper looks at the opposite process, a single black hole dividing into two smaller ones, and concludes that physics may not completely forbid it.
The conditions would need to be extreme, though. Even then, the opening is so small that the authors devote a large part of the paper to explaining why it usually stays closed.
Titled “When Black Holes Admit Pair-of-pants Topologies,” the study appears in Physical Review D. It examines the physical laws that hold a black hole together and asks whether any set of circumstances could allow it to split.
“In our study, we investigate which physical laws normally prevent a black hole from splitting into two. We also investigate if certain circumstances may allow a split to happen,” said Francesco Sannino, a co-author of the study and a professor of physics at the University of Naples Federico II in Italy and the Quantum Field Theory Center at the University of Southern Denmark.
Thermodynamics stands in the way first
The biggest obstacle to a split comes from thermodynamics, the area of physics that describes how energy behaves and how systems change over time.
Entropy is the concept at work. It measures how many microscopic configurations a system can take on while appearing identical from the outside. Black holes carry entropy, and that places firm limits on what can happen to them.
Consider a black hole with no rotation. Dividing it into two smaller black holes would reduce the total entropy, so the process is thermodynamically unfavorable. That alone explains a lot about why black holes don’t normally fall apart.
Fast rotation creates an opening, and gravity shuts most of it
The picture shifts for a black hole that spins at an exceptionally high speed. According to the team’s calculations, rapid rotation may create a narrow gap in the rules. In principle, a black hole rotating quickly enough could get around the usual thermodynamic barrier.
Splitting only solves part of the problem. The two resulting objects would also need to move far enough apart to avoid merging again right away.
“But the two resulting black holes must also be able to separate rather than immediately fall back together. When we include this requirement, the window largely closes again, except possibly for black holes rotating close to their maximum possible rate,” Sannino said.
The unusual title refers to the shape a black hole’s boundary, known as the horizon, would trace through space and time as it splits. It begins as a single structure and then divides into two branches. Viewed across time, it resembles a pair of trousers, with one upper section separating into two “legs.” That’s where “pair-of-pants topologies” comes from.
More dimensions, different math
The researchers also considered more unusual scenarios taken from theories where space has more than the three dimensions of everyday life.
In some of those models, physicists have already identified black hole-like objects that can become unstable and begin to break apart. This hints that the usual rules may not apply in the same way under more exotic conditions.
In certain cases, a split could even be thermodynamically favorable, especially when one of the two resulting black holes is far smaller than the other.
A process that thermodynamics allows or favors still might not happen. The team needs to determine whether this kind of fragmentation can physically take place and what conditions it would require.
No probability yet, but a place to search
The researchers are now working to estimate how likely these events are. The answer may hinge on how gravity behaves beyond Einstein's theory, which is a difficult problem on its own.
“We cannot yet give a single probability for a black hole to split. It depends on how gravity behaves beyond Einstein’s theory, and we are working to quantify this. We expect small primordial black holes to be more prone to splitting than much larger astrophysical black holes. We may detect a split indirectly: gamma-ray telescopes such as Fermi-LAT can search for signals from tiny black-hole fragments that might form during mergers,” Sannino said.
Nobody has confirmed observing a black hole split. Signals from tiny fragments would let astronomers test the idea, and any such evidence would reveal something about how gravity behaves in some of the most extreme conditions in the universe.
The research team
Joining Sannino as co-authors are Giacomo Cacciapaglia, a researcher with the French National Centre for Scientific Research (CNRS) at the Laboratoire de Physique Théorique et Hautes Énergies, a joint CNRS and Sorbonne Université laboratory in Paris, along with Manuel Del Piano and Vania Vellucci, who are both from the Quantum Field Theory Center at the University of Southern Denmark.
Sannino is director of the Quantum Field Theory Center (ℏQTC), which is affiliated with the Department 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, and he is a visiting professor at Durham University.