Study examines whether rapidly spinning black holes can split

A theoretical study published in Physical Review D asks whether a single black hole could split into two smaller black holes, according to researchers from the University of Southern Denmark, the University of Naples Federico II, CNRS and Sorbonne Université whose work the media covered Oct. 23, 2026, and Oct. 25, 2026.

The paper When Black Holes Admit Pair-of-pants Topologies was conducted by researchers associated with the University of Southern Denmark and collaborating institutions across Italy and France. Francesco Sannino, a professor at the University of Naples Federico II and the University of Southern Denmark, co-authored the study. His co-authors include Giacomo Cacciapaglia, a researcher at the Centre Nacional de la Recherche Scientifitque (CNRS) working at the Laboratoire de Physique Théorique et Hautes Énergies in Paris, and Manuel Del Piano and Vania Vellucci, both based at the Quantum Field Theory Center at the University of Southern Denmark. The study examines whether established physical constraints allow black-hole splitting.

Standard thermodynamics normally blocks a single black hole from splitting into two because dividing an ordinary nonrotating black hole into smaller holes would lower total entropy, making the process unfavorable. Entropy counts the microscopic arrangements of a system even while the system appears unchanged from the outside. For ordinary black holes, that entropy loss is the central obstacle to division. This principle holds under standard thermodynamic rules regardless of how the division might otherwise be imagined.

Rapid rotation alters the entropy calculation because a spinning black hole stores energy in its rotation, which changes the conditions governing possible final states. That stored rotational energy may make splitting thermodynamically possible in some circumstances. Yet, satisfying entropy requirements alone does not guarantee the physical process can occur. Only rotation close to the theoretical maximum might still permit it.

If splitting happened, the event horizon would evolve into a trouser-like shape, with one section dividing into two legs, giving the study its pair-of-pants topology term. The event horizon is the outer boundary of a black hole. This geometric picture describes how that boundary changes over time during a hypothetical division into two separate objects. The trouser shape reflects the moment one region becomes two.

Rapidly rotating black holes near their maximum spin open a narrow loophole where the horizon could take a pair-of-pants shape. Yet demanding that the two new holes separate instead of reuniting mostly closes that window. The resulting black holes would need sufficient separation to avoid immediately falling back together, and after including this condition, most possibilities disappeared. Francesco Sannino says the separation condition is what makes splitting so difficult.

Kip Thorne’s 1970s theory sets the maximum black hole spin at 99.8% of the speed of light, limited by photons from the accretion disk pushing back on the spin. This figure is one of two competing maximum spin rates debated for decades. The other comes from a 2004 model involving magnetic jets. Current observations cannot yet decide between them.

Charles Gammie of the University of Illinois Urbana-Champaign and co-authors proposed in 2004 that maximum spin is 93.75% of the speed of light, held back by highly magnetized jets that brake the rotation. This alternative limit competes with Thorne’s model. The two figures represent different ideas about what ultimately caps a black hole’s rotation.

Current telescopes cannot determine actual black hole spin rates, according to coverage published July 16, 2026. The Event Horizon Telescope’s 20 microarcseconds resolution cannot distinguish between black holes spinning at the two proposed maximum rates. Synthetic radio images from simulations of Sgr A* show nearly identical accretion rates, jets and polarization signals at either maximum spin limit. That similarity leaves the spin question unresolved.

The Black Hole Explorer (BHEX), a NASA Small Explorer mission planned for the coming decade, would place a radio telescope in Earth orbit to work with Event Horizon Telescope components such as the Green Bank Telescope or ALMA and observe Sgr A*’s photon ring. The mission concept appeared in coverage dated July 16, 2026. BHEX aims to extend the Event Horizon Telescope into space, creating an interferometer large enough to measure the photon ring and help resolve the maximum spin debate. A photon ring is a halo of light formed by photons orbiting close to the black hole.

In gravity models with extra spatial dimensions, splitting can become more favorable when the resulting objects have markedly different sizes, especially when one resulting black hole is much smaller than the other. These models are theoretical possibilities rather than confirmed descriptions of the universe. The authors examine exotic gravity models where black-hole-like objects grow unstable, but even there the result remains a theoretical possibility rather than an observed phenomenon. The work does not prove fragmentation will occur even in these settings.

The authors expect small primordial black holes to be more prone to splitting than larger astrophysical black holes. Fermi-LAT gamma-ray telescopes could search for signals from small fragments formed during mergers. Francesco Sannino says detection would be indirect and depends on gravity beyond Einstein’s theory, which the team is working to quantify. The search would target gamma-ray signatures associated with hypothetical fragments.

The team cannot name one probability for black hole splitting, since the answer depends on how gravity operates outside Einstein’s theory, which the researchers are still working to measure. That uncertainty means the study identifies conditions under which splitting might be allowed, not how often it would happen in nature. No general probability exists for the event. Quantifying that dependence remains part of the team’s ongoing work.

No telescope has seen a black hole split into two, and the researchers cannot fix a general probability on that event. Even for black holes rotating near their theoretical maximum, the work does not show that a real black hole would fragment. The narrow opening remains a mathematical possibility rather than a demonstrated physical outcome. The result stays in the realm of theory for now.

Francesco Sannino said, “We cannot yet give a single probability for a black hole to split.” He made the remark when discussing why the team cannot quantify the likelihood of a black hole splitting.

The study published in Physical Review D examines whether established physical constraints allow black-hole splitting, but it remains unconfirmed theory rather than observation. No telescope has detected the process. The results point to conditions under which division might be allowed, not to a confirmed natural phenomenon.