Semiclassical Hawking evaporation is expected to break down at some point in a black hole’s evolution as the effects of quantum gravity become important. In particular, it has been argued that the so-called memory-burden effect could cause black holes to become stabilized by the information that they carry, thereby suppressing the rate at which they undergo Hawking evaporation. It has furthermore been suggested that this opens a new mass window, between ${10}^4\mathrm{g}\lesssim M\lesssim 10^{10}\mathrm{g}$, over which primordial black holes could constitute the dark matter of our Universe. We show for the first time that this is true only if the transition from the semiclassical phase of a black hole to its memory-burdened phase is practically instantaneous. If this transition is instead more continuous, Hawking evaporation will persist at relevant levels throughout the eras of big bang nucleosynthesis and recombination, leading to stringent constraints which rule out the possibility that black holes lighter than $\sim 4\times {10}^{16}\mathrm{g}$ could make up all or most of the dark matter. More broadly, our analysis demonstrates that even if departures from the semiclassical Hawking evaporation occur as proposed, they must be both drastic and abrupt to open viable new mass windows for primordial black hole dark matter.

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• Big bang nucleosynthesis
• Cosmic microwave background
• Dark matter
• Primordial black holes