Usually black holes only merge. But can one black hole split into two smaller ones? This question is the subject of a new theoretical study published in the journal Physical Review D. Its authors are Francesco Sannino of the University of Naples Federico II and the Center for Quantum Field Theory at the University of Southern Denmark, Giacomo Cacciapaglia of the Paris laboratory LPTHE (a joint unit of CNRS and Sorbonne), and Manuel Del Piano and Vania Vellucci of the University of Southern Denmark.
Why black holes do not divide
Usually thermodynamics prevents this. A black hole has entropy - roughly speaking, a measure of how many different microscopic ways a system can be arranged so that from the outside it looks the same. For an ordinary non-rotating black hole, decay into two smaller ones would reduce the total entropy, and therefore would be thermodynamically unfavorable.
The situation may be different for rapidly rotating black holes. The team's calculations showed that they offer a possible loophole for decay. But there is a second requirement: the two resulting black holes must be able to fly apart rather than immediately fall back onto each other. Taking this condition into account, the window of opportunity again nearly closes - except for black holes rotating close to the maximum possible speed.
If such a decay occurred, the outer boundary of the black hole - the horizon - would over time take the shape of trousers: one part divides into two legs. Hence the unusual term in the title of the work - pair-of-trousers topology.
More exotic scenarios
The authors also considered other options. In theories with extra dimensions of space, physicists already know of objects similar to black holes that can lose stability and begin to decay. Under certain conditions, decay becomes thermodynamically favorable, especially if one of the new black holes is much smaller than the other. But this does not yet prove that fragmentation will actually occur.
We cannot yet name a single probability of black hole decay, says Sannino. It depends on how gravity behaves beyond Einstein's theory, and we are working to estimate this. We expect that small primordial black holes will decay more often than much larger astrophysical ones. Decay can be sought indirectly: gamma-ray telescopes such as Fermi-LAT are capable of searching for signals from tiny black hole fragments that may arise during mergers.
Let us emphasize: this concerns theoretical calculations, not an observed event. The authors do not claim that black hole decay has already been recorded, and they do not offer recommendations.