Ultralight black holes as sources of high-energy particles

The memory burden effect, the idea that the amount of information stored within a system contributes to its stabilization, is particularly relevant for systems with a large information storage capacity, such as black holes. In these objects, the evaporation process halts, at the latest, once approximately half of the initial mass has been radiated away. As a result, light primordial black holes (PBHs) with mass $m_{\mathrm{PBH}}\lesssim 10^{15}g$, which are traditionally assumed to have fully evaporated by the present time, may instead survive and constitute viable dark matter candidates. Ongoing mergers of such PBHs would give rise to “young” black holes that resume their evaporation, emitting ultrahigh-energy particles potentially detectable by current experiments. The resulting emission spectrum would be thermal across all Standard Model particle species, offering a clear and distinctive signature. We demonstrate that, if the memory burden effect activates after PBHs have lost around half of their initial mass, current measurements of the neutrino flux at Earth place strong constraints on such dark matter candidates for $m_{\mathrm{PBH}}\lesssim 10^{9}g$. This suggests that the memory burden must set in at earlier stages of evaporation. Unlike existing bounds, our results depend solely on the mass of the remnant, and not on model-dependent details of the stabilized phase. We also discuss the potential for refining these constraints through observations of gamma rays, cosmic rays, and gravitational waves.