Primordial Black Holes, Charge, and Dark Matter: Rethinking Evaporation Limits

Abstract

Limits on the dark matter fraction of small mass primordial black holes from Hawking radiation are predominantly derived from the assumption of a Schwarzschild black hole evaporating. However, astrophysical black holes are usually much more realistically modelled by the rotating Kerr black hole solution. Meanwhile, electromagnetically charged black holes are astrophysically of little importance due to their fast neutralisation in the present universe. Dark matter is not just a possible solution to issues of astrophysics and cosmology, but also to issues of the standard model of particle physics. Extensions of this model thus can lead to charges present in the early universe which remain preserved in the charge of primordial black holes - even when the corresponding particles have disappeared from the particle content of the present epoch of the universe. Here, we report on a thorough proof-of-concept that such charges can greatly change evaporation limits for primordial black hole dark matter. Special emphasis is placed on (near-)extremal black holes, for which this effect is especially pronounced.

Figures (2)

• Figure 1: The general features of the HW evaporation model, and its dependence on the dark QED parameters $e_\chi$ and $m_\chi$. Figure (a) summarizes the three core features of a black hole's trajectory in the parameter space $(Y=Q^2/M^2, M = \mu M_\text{s})$, where $M_\text{s}$ is chosen such that $\mu\in[0,1]$ for the chosen parameter range: (1) Starting parameters in the light green region are dominated by charge loss and the Schwinger effect. (2) Starting parameters in the light blue region are dominated by mass loss and the Hawking effect. (3) Trajectories approach an attractor curve separating the two regions. Figures (b) and (c) show how a semi-analytic approximation of this attractor curve (derived in Santiago:2025rzb) depends on the dark electron's charge $e_\chi$ and mass $m_\chi$, respectively.
• Figure 2: The minimal mass for a charged black hole to have a lifetime of at least the age of the universe and its dependence on $e_\chi$ and $m_\chi$. The black region at the top is ruled out by condition \ref{['eq:summ_conditions']}nobreak(ii). $M_{z_0}$ is the mass scale corresponding to $\mu=z_0$, where the approximated attractor curve reaches $Y=1$. On the left, this is shown in non-dimensionalized mass units $\mu_\textrm{univ}$, on the right, in solar masses $M_\odot$. The dotted line in figure \ref{['fig:Muniv-contour']} corresponds to the limit of conditon \ref{['eq:summ_conditions']}nobreak(i).