Ultralight Black Holes as Sources of High-Energy Particles

Abstract: The \textit{memory burden} effect, stating that the amount of information stored within a system contributes to its stabilization, is particularly significant in systems with a high capacity for information storage such as black holes (BHs). In these systems, the evaporation process is halted, at the latest, after approximately half of the BH's initial mass has been radiated away. Consequently, light primordial BHs (PBHs) of mass $m_{\rm PBH} \lesssim 10^{15}$\,g, which are expected to have fully evaporated by present time, may remain viable candidates for dark matter (DM). In this scenario, we demonstrate that their mergers would continue to occur today, leading to the formation of ``young'' BHs that resume evaporating, producing ultrahigh-energy cosmic rays detectable by current experiments. The emission spectrum would be thermal in all Standard Model particle species, offering a clear and distinguishable signature. Current measurements of the isotropic neutrino flux at Earth are in tension with light PBHs as DM candidates within the mass range $7\times10^3\lesssim m_{\rm PBH}/{\rm g}\lesssim 4\times 10^8$. Contrarily to existing bounds, our results depend uniquely on the mass of the remnant and not on the specific model building details of the stabilized phase. We also discuss the potential for refining these constraints through gamma-ray and cosmic-ray observations, as well as gravitational wave detections.