Ancient primordial black holes may reveal hidden fifth dimension of the cosmos

A new study proposes that our universe may sit on a four-dimensional brane within a hidden fifth-dimensional bulk only a micron wide. Tiny primordial black holes, remnants from the moments after the Big Bang, could have gravity that leaks into this dark dimension, effectively becoming five-dimensional. The authors model two formation pathways—collapse of early-universe overdensities and collapse of cosmic-string loops—and find that in both cases the black holes would either rapidly transition to or be born as five-dimensional objects.

These five-dimensional black holes would lose mass via Hawking radiation far more slowly, allowing some to survive for the 13.8-billion-year age of the universe. The paper speculates that the evaporation of such a black hole might explain a high-energy particle detected by the KM3NeT detector in 2023, though this remains unproven. If confirmed, the findings could offer a novel probe of spacetime structure and dark matter composition.