The Memory Burden

A black hole that forms with mass below 10^15 grams should have evaporated by now through Hawking radiation. This eliminates an entire mass range of primordial black holes as dark matter candidates — unless Hawking evaporation is not the full story.

The memory burden hypothesis proposes that quantum backreaction effects quench evaporation. As a black hole radiates, it must shed not just energy but the information encoded in its quantum state. This “memory burden” grows as evaporation proceeds, eventually suppressing further radiation. Primordial black holes that would have vanished in the standard picture survive to the present.

arXiv:2603.15827 shows that ultra-high-energy cosmic rays provide a powerful probe of this scenario. Memory-burdened black holes still emit some particles — protons and neutrons at extremely high energies. The paper computes this emission including both the Galactic halo contribution and the extragalactic proton component, then confronts the predictions with Pierre Auger Observatory data.

The non-observation of excess ultra-high-energy protons constrains the fraction of dark matter that memory-burdened black holes can constitute, as a function of formation mass and the suppression parameter k. For k greater than approximately 3, the proton constraints are competitive with those from ultra-high-energy gamma rays. Neutron limits from the Galactic plane are comparable to neutrino constraints.

The observation is a multi-messenger constraint on a quantum gravity scenario — cosmic ray detectors constraining Planck-scale physics through the debris of incompletely evaporated objects. The memory that burdens the black hole also leaves a signature in the cosmic ray spectrum, and the signature’s absence is itself information.