Endpoint of PBH evaporation near the Planck scale

Determine which endpoint occurs for Hawking evaporation of primordial black holes as their mass approaches the Planck scale: (i) formation of a stable Planck-scale relic, (ii) survival of a naked spacetime singularity, or (iii) continued evaporation at trans-Planckian energies and temperatures that eliminates the singularity.

Background

The paper reviews the uncertain fate of ultralight primordial black holes (PBHs) as their mass approaches the Planck scale. Three distinct outcomes are considered in the literature: stable Planck-scale relics, naked singularities, or complete evaporation continuing into trans-Planckian regimes. Identifying the correct endpoint is central to understanding whether relics could constitute dark matter and to predicting observational signatures such as high-energy emission or charged relic detection.

This uncertainty directly impacts the viability of various experimental searches discussed in the paper, including direct detection of charged relics and searches for explosive end stages of evaporation. A resolution would significantly constrain cosmological scenarios and particle physics at Planckian energies.