Two black holes can collide and merge into one. But could the opposite happen: could a single black hole split into two smaller ones? That question is explored in a new theoretical study: "When Black Holes Admit Pair-of-pants Topologies", published in the journal Physical Review D .
"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", says Francesco Sannino, co-author of the study and a professor of physics at University of Naples Federico II in Italy and Quantum Field Theory Center at University of Southern Denmark.
Co-authors are Giacomo Cacciapaglia, a Centre Nacional de la Recherche Scientifitque (CNRS)-researcher at the Laboratoire de Physique Théorique et Hautes Énergies, a joint CNRS–Sorbonne Université laboratory in Paris, and Manuel Del Piano and Vania Vellucci, both from the Quantum Field Theory Center, University of Southern Denmark.
Why black holes do not normally split
Black holes do not normally split. One important obstacle preventing this comes from thermodynamics.
A black hole has what physicists call entropy. Put simply, entropy is a measure of how many different microscopic ways a physical system can be arranged while still appearing the same from the outside. For an ordinary, non-rotating black hole, splitting into two smaller black holes would reduce the total entropy. That makes the process thermodynamically unfavourable.
But things might be different with rapidly rotating black holes.
A trouser-like shape
The team's calculations show that rapidly rotating black holes provide a possible loophole allowing for a split to happen.
- 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, says Francesco Sannino.
If such a split did occur, the black hole's outer boundary, known as the horizon, would form a trouser-like shape through time: one section dividing into two "legs". This explains the study's unusual term "pair-of-pants topologies".
Other, more exotic possibilities
The authors also explore more exotic possibilities. In theories involving additional dimensions of space, physicists already know of black-hole-like objects that can become unstable and begin to break apart.
Under some of these conditions, splitting can become thermodynamically favourable, particularly when one resulting black hole is much smaller than the other. But that still does not demonstrate that the fragmentation will actually happen.
- 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, says Francesco Sannino.
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Paper title: When black holes admit pair-of-pants topologies. Authors: Giacomo Cacciapaglia, Manuel Del Piano, Francesco Sannino, and Vania Vellucci
Phys. Rev. D 114, 084072 – Published 22 October, 2026. DOI: https://doi.org/10.1103/cx69-t466
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Professor Francesco Sannino is Director of the Quantum Field Theory Center (ℏQTC), affiliated with the Department of Mathematics and Computer Science (IMADA). 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 a Visiting Professor at Durham University.
