A new theoretical study from physicists at the University of Southern Denmark (SDU) and the University of Naples Federico II suggests that, under narrow conditions, a black hole could do something it has never been observed to do: split into two. The work, published in Physical Review D, doesn't claim to have found a real-world example — it maps out the physics that would have to hold for a split to even be possible.
Led by SDU and Naples physicist Francesco Sannino, with colleagues Giacomo Cacciapaglia, Manuel Del Piano and Vania Vellucci, the team started from a basic rule of black hole physics: because a black hole's event horizon can never shrink, ordinary splitting is thermodynamically forbidden — it would reduce the total horizon area, the black hole equivalent of losing entropy. "In our study, we investigate which physical laws normally prevent a black hole from splitting into two," Sannino said in a statement.
A narrow loophole
The team's calculations show that a black hole spinning near the maximum rate general relativity allows could open a loophole, producing what the authors call a "pair-of-pants" topology: the horizon narrows and forks into two separate legs over time, each becoming its own black hole. Even then, the split only works if the two resulting black holes can actually fly apart rather than immediately merging back together — a requirement that closes off all but the most extreme spins. The researchers also found that in theories with extra spatial dimensions, splitting can become thermodynamically favorable, particularly when one resulting fragment is far smaller than the other.
The authors are careful to note they cannot yet assign a probability to any of this; the odds depend on details of gravity beyond Einstein's theory that remain untested. They expect any splitting, if it happens at all, to favor small primordial black holes — relics of the early universe — over the much larger black holes astronomers routinely observe merging via gravitational-wave detectors like LIGO.
If a split ever did occur, catching it directly would be extraordinarily difficult. The team points to one indirect possibility: gamma-ray telescopes such as NASA's Fermi-LAT could, in principle, search for faint high-energy signatures from tiny black-hole fragments formed this way, according to a summary of the work in Space.com. For now, the result stands as a theoretical curiosity — a reminder that even black holes, long treated as the universe's point of no return, may have more exotic behavior hiding in the mathematics.
