A Black Hole Might Split in Two by Growing a Pair of Pants

Picture the edge of a black hole traced through time as a pair of trousers. One waist at the top. Two legs below, each holding a newborn black hole of its own.

That shape anchors a theoretical study published in Physical Review D, co-authored by Francesco Sannino of the University of Southern Denmark's Quantum Field Theory Center. The question runs merger physics in reverse. Black holes are known to collide and fuse. Could one ever become two?

Usually, no. The obstacle is entropy, a tally of how many microscopic arrangements a system can have while looking identical from outside. For a black hole that isn't spinning, breaking apart would leave less total entropy than before, and that trade is a losing one.

Spin changes the math. The team's calculations point to a possible loophole for black holes that rotate fast. Then comes the catch. The two pieces also have to fly apart instead of tumbling straight back together, and once the researchers added that requirement, the loophole mostly shut. The holdouts might be black holes whirling at nearly the fastest rate physics allows.

The authors also wandered into stranger territory, including theories with extra spatial dimensions, where the entropy math can favor a split, especially when one fragment ends up tiny next to its partner. A favorable tally still proves nothing on its own.

Sannino said the team can't yet put a single probability on a split, because the answer hinges on how gravity works past Einstein. The researchers suspect tiny primordial black holes break up more readily than far bigger astrophysical ones, and gamma-ray telescopes like Fermi-LAT could hunt for signs of tiny fragments possibly born in mergers.

Black holes in pants exist only in equations. Fermi-LAT could be the one to check the fitting room.

Read the full story at University of Southern Denmark, October 23, 2026

Hot Take: Somewhere out there, a black hole spinning flat out may be stepping into the wrong trousers. Fermi-LAT gets to play Gromit, combing mergers for the evidence.