BBN constraints on primordial black holes with a continuous memory-burden crossover

Abstract

Light primordial black holes (PBHs) are disfavored as dark matter if they evaporate through standard Hawking radiation alone. The memory-burden effect can extend their lifetimes by suppressing emission after roughly half the mass is lost. Existing cosmological studies often model the onset of this phase as an instantaneous transition between semi-classical and burden-dominated evaporation. We instead treat the crossover as continuous and compare additive versus multiplicative combinations of the two rates, using a smoothed tanh profile with parameters $(q,δ)$. Monochromatic PBHs are mapped to a decaying scalar field and evolved with Modified AlterBBN during Big Bang nucleosynthesis (BBN). The two prescriptions yield distinct exclusion curves: the additive crossover always gives weaker bounds than the multiplicative one, while both are tighter than the instantaneous transition. For $10^{5}\,\mathrm{g}\lesssim M_i\lesssim 10^{10}\,\mathrm{g}$, the additive case can permit $f_{\mathrm{PBH},0}\sim 10^{-1}$ where the multiplicative case gives $f_{\mathrm{PBH},0}\lesssim 10^{-2}$. Specifying the rate-combination rule is therefore essential when translating memory-burden models into BBN constraints on PBH dark matter.