Study finds rapid rotation may open a black-hole split loophole, near-maximal spin the exception

Giacomo Cacciapaglia, Francesco Sannino, Manuel Del Piano and Vania Vellucci published “When Black Holes Admit Pair-of-pants Topologies” in Physical Review D in 2026.

Black holes ordinarily do not split because dividing one hole into two cuts entropy, and entropy counts the microscopic arrangements that leave a hole’s exterior appearance unchanged. For an ordinary black hole that does not rotate, dividing into two smaller black holes would lower total entropy, making the process thermodynamically unfavorable.

The team’s calculations show that rapidly rotating black holes might open a loophole permitting a split, since rotation changes the thermodynamic balance that ordinarily blocks fission. Extra conditions narrow the range of cases where splitting might work. Francesco Sannino said separation must replace rejoining, leaving near-maximal spin as the main exception to the rule against division. Rotation alters the energy accounting that would otherwise forbid a single hole from becoming two.

A split would form a pants-like horizon through time, with one section dividing into two branches. The horizon marks the black hole’s outer boundary, and its evolution from a single part into two legs gives the study its pair-of-pants name. The paper’s title, “When Black Holes Admit Pair-of-pants Topologies,” refers directly to that picture of one section splitting into two branches over time.

The authors also examine theories with extra dimensions of space, where black-hole-like objects can turn unstable. With extra dimensions of space, splitting may become thermodynamically favorable under some conditions, particularly when one of the two resulting black holes is much smaller than the other. In those higher-dimensional cases, an imbalance in mass between the two products can make splitting energetically preferable. The authors note that this preference does not show fragmentation will actually happen.

Francesco Sannino said no one can yet give a single probability for a black hole split because the answer depends on gravity beyond Einstein’s theory. “We cannot yet give a single probability for a black hole to split,” Sannino said. Researchers are working to quantify the probability, though the calculations stay theoretical.

Sannino said small primordial black holes split more easily, while large astrophysical black holes split less easily. That difference suggests early-universe objects may matter more for fission than the massive black holes found in galaxies and mergers.

Sannino said Fermi-LAT could detect a split indirectly through signals from small merger fragments. The authors offer this as a proposed indirect observational route. The authors mention Fermi-LAT as a possible way to look for evidence, not as confirmation that a split has occurred.

Francesco Sannino holds physics professorships at the University of Naples Federico II in Italy and at the Quantum Field Theory Center at the University of Southern Denmark, and he co-authored the study. Giacomo Cacciapaglia works as a researcher for the Centre National de la Recherche Scientifique at the Laboratoire de Physique Théorique et Hautes Énergies in Paris, a joint laboratory involving CNRS and Sorbonne Université. Manuel Del Piano and Vania Vellucci are both affiliated with the Quantum Field Theory Center at the University of Southern Denmark.

The paper’s DOI is 10.1103/cx69-t466 and the arXiv identifier is 2606.24642v3. These identifiers locate the peer-reviewed publication and its preprint version.

On May 27, 2026, the collaboration’s fifth gravitational-wave catalog, GWTC-5.0, contained 267 confirmed events from detector data spanning nearly ten years. Sharan Banagiri, a research fellow at Monash University, led the population analysis of the catalog. The catalog records the spins of merging black holes. Banagiri’s work underlies the spin and mass subpopulation findings drawn from the catalog data.

In the catalog, most merging black holes have spin below roughly 0.5, while the scale’s theoretical maximum is 1. A distinct subpopulation spins at roughly 0.7. Spin serves as the clearest fingerprint for telling different black hole assembly processes apart, setting slow typical spins against a faster subpopulation. Spin describes how fast a black hole rotates around its own axis.

A hierarchical merger takes place in a dense stellar cluster when the cluster’s gravity retains a black hole produced by a prior collision, which later encounters another black hole. Objects formed in those second-generation encounters take on high spin from the merger itself. These repeat collisions in dense clusters are one route to building rapidly rotating black holes.

High-spin systems show up for primary masses roughly between 10 and 20 solar masses and again above roughly 45 solar masses, while the 20-to-45 solar mass region has comparatively little high-spin signature. Banagiri’s team puts the start of the high-mass subpopulation at roughly 46 solar masses, with an uncertainty of about 12. The estimate marks the point where heavier second-generation objects paired with lighter companions appear in the data.

About 9 to 40 percent of mergers show positively aligned effective inspiral spin, according to catalog estimates, while somewhere between 30 and 46 percent of mergers show negative effective inspiral spin. Positive skew reflects isolated binary formation, while negative values reflect dynamical formation channels in clusters.

Stellar evolution theory expects a pair-instability gap beginning above roughly 40 to 50 solar masses and extending below several hundred. But the GWTC-5.0 primary mass distribution shows no clear sign of this gap and instead runs smoothly beyond 100 solar masses. The article suggests hierarchical mergers might fill the gap from above, past where ordinary collapse can reach.

The collaboration anticipates no fewer than 68 new events for GWTC-6.0, drawing on public alert counts. Every added detection should sharpen the boundaries between subpopulations and the correlations among mass, spin, and redshift.