Scientists propose that ancient primordial black holes could exist in a hidden fifth dimension

Physicists at Lehman College in New York have proposed a new theoretical framework suggesting that a class of extremely ancient, microscopic black holes from the early universe may have been five-dimensional objects, and that their decay could explain the most powerful particle ever detected, which has puzzled scientists for years.

The study, published in Physical Review D, examines what are known as primordial black holes, hypothetical objects thought to have formed from density fluctuations in the very early universe. Unlike the supermassive black holes at the centres of galaxies or those created when massive stars collapse, primordial black holes would be extraordinarily small—some potentially as lightweight as an asteroid yet as compressed as a black hole.

The researchers, working with physicist Dieter Lüst, explored what would happen to such objects in a theoretical universe containing an additional spatial dimension beyond the three we experience. Their calculations found that while primordial black holes of certain masses would initially behave as ordinary four-dimensional objects, their tiny size would make that configuration unstable, forcing a transition to a five-dimensional state.

Crucially, five-dimensional primordial black holes would evaporate far more slowly than their four-dimensional counterparts under the standard Hawking radiation framework, slowly enough that some could still exist today, 13.8 billion years after the Big Bang. The evaporation of such an object might produce a burst of extremely high-energy neutrinos, particles that interact almost not at all with ordinary matter and travel unobstructed across cosmic distances. 

This matches the profile of the most powerful particle ever recorded, detected by the KM3NeT telescope in the Mediterranean in 2023 at an energy of 220 petaelectronvolts: a level that no known conventional source could explain. Neither primordial black holes nor the proposed extra dimension have been directly observed.