A black hole spinning at its physical speed limit might be able to split in two, according to new theoretical research. But there’s a catch: the resulting black holes must move far enough apart to avoid immediately falling back together. That leaves only a narrow theoretical possibility, not evidence that black holes have been caught multiplying like cells.
The study, published in Physical Review D, examines the laws that normally prevent such a split. In this unlikely scenario, the black hole’s outer boundary, the event horizon, takes on a pants-like shape across space and time: one incoming black hole branches into two separate tubes, with a resulting black hole emerging from each “leg.”
“In our study, we investigate which physical laws normally prevent a black hole from splitting into two,” coauthor Francesco Sannino, of the University of Naples Federico II in Italy, said in a statement about the research. “We also investigate if certain circumstances may allow a split to happen.”
Unlike black hole mergers, splitting faces a thermodynamic obstacle. Central to the calculations is entropy, commonly described as the degree of disorder in a system. The universe naturally tends toward greater disorder, a tendency reflected in decay. For an everyday analogy, picture a house getting messier unless someone puts energy into tidying it.
For a black hole that isn’t rotating, dividing in two would require entropy to decrease. That makes the split “thermodynamically unfavorable.” The researchers therefore examined whether rapid rotation could get around that restriction, allowing the event horizon to branch into its two-legged shape.
Producing two black holes in the calculation isn’t enough, though. They also have to stay apart. The scenario requires the original black hole to spin at the maximum physically possible rate; slower rotation would leave its offspring unable to separate far enough to avoid merging again. Sannino’s explanation leaves some uncertainty about exactly how narrow that limit is.
“The two resulting black holes must also be able to separate rather than immediately fall back together,” he said. “When we include this requirement, the window largely closes again, except possibly for black holes rotating close to their maximum possible rate.”
This remains a theoretical possibility, not a confirmed event or an established explanation of how a split would happen in nature. The calculations don’t provide a numerical likelihood, either. As Sannino put it, “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.”
If such a split happens, gamma-ray telescopes might be able to detect it. The research also suggests that small black holes are the more likely candidates, rather than giant black holes spawning smaller companions.
