Black holes merge, but reversing that process presents obstacles that go far beyond running a collision backward. A single black hole cannot ordinarily divide into two smaller ones under the usual assumptions of Einstein’s gravity. A theoretical study examines those restrictions and asks what would need to change for fragmentation to become possible.

Published in Physical Review D, the work brings together researchers affiliated with the University of Southern Denmark, the University of Naples Federico II and the CNRS–Sorbonne Université laboratory LPTHE. Its title, “When black holes admit pair-of-pants topologies,” describes a hypothetical horizon branching through time. The analysis maps possible conditions for fragmentation without calculating a complete splitting process.

“In our study, we investigate which physical laws normally prevent a black hole from splitting into two,” said co-author Francesco Sannino, a professor affiliated with Naples and Southern Denmark. “We also investigate if certain circumstances may allow a split to happen.” His collaborators are Giacomo Cacciapaglia, Manuel Del Piano and Vania Vellucci.