Black Holes Spinning Near Their Maximum Rate Could Split in Two, New Calculations Suggest
A theoretical study finds a narrow loophole in the laws of thermodynamics that might let a rapidly rotating black hole divide into two smaller ones, leaving a possible gamma-ray signature.
Two black holes can collide and merge into one. Physicists have now asked whether the reverse could ever happen: whether a single black hole could split into two smaller ones. A new theoretical study in Physical Review D concludes that, for black holes spinning close to their maximum possible rate, the answer may be a cautious maybe.
The paper, titled "When Black Holes Admit Pair-of-pants Topologies," comes from Francesco Sannino of the University of Naples Federico II and the Quantum Field Theory Center at the University of Southern Denmark, with Giacomo Cacciapaglia of the CNRS and Sorbonne Université in Paris, and Manuel Del Piano and Vania Vellucci of the University of Southern Denmark. "In our study, we investigate which physical laws normally prevent a black hole from splitting into two," Sannino said. "We also investigate if certain circumstances may allow a split to happen."
The main obstacle is thermodynamics. A black hole has entropy, a measure of how many microscopic arrangements look the same from the outside. For an ordinary, non-rotating black hole, splitting into two smaller holes would lower the total entropy, which the laws of physics disfavor.
The team's calculations show that rapid rotation opens a possible loophole. But the two new black holes must also be able to move apart rather than immediately fall back together. When that requirement is added, the window mostly closes again. "Except possibly for black holes rotating close to their maximum possible rate," Sannino said.
If such a split occurred, the black hole's outer boundary, the horizon, would trace a pants-like shape through time, with one section dividing into two legs. That picture gives the study its title.
The authors also look at more exotic settings. In theories with extra dimensions of space, physicists already know of black-hole-like objects that can become unstable and start to break apart. Under some of those conditions, splitting becomes thermodynamically favorable, especially when one resulting black hole is much smaller than the other. That alone does not show that fragmentation would actually happen.
The researchers are careful about what they can claim. "We cannot yet give a single probability for a black hole to split," Sannino said. "It depends on how gravity behaves beyond Einstein's theory, and we are working to quantify this." He expects small primordial black holes to be more prone to splitting than much larger astrophysical ones.
There may be a way to look for evidence. "We may detect a split indirectly," Sannino said. "Gamma-ray telescopes such as Fermi-LAT can search for signals from tiny black-hole fragments that might form during mergers." The work, which is also on arXiv (2606.24642), is a calculation, not an observation, and no one has seen a black hole divide. It does sharpen a basic question about what the laws of gravity and thermodynamics permit, and gives observers a specific signal to hunt for.
