The Dark Dimension, the Swampland, and the Dark Matter Fraction Composed of Primordial Black Holes

TL;DR

This paper explores how the existence of a hypothesized dark dimension affects primordial black hole evaporation and their potential role as dark matter, extending the mass range for viable black hole dark matter candidates.

Contribution

It demonstrates that the presence of a dark dimension modifies Hawking radiation rates, expanding the viable primordial black hole mass range for dark matter composition by three orders of magnitude.

Findings

Hawking radiation slows down in the dark dimension.

Primordial black holes with masses between 10^{14} and 10^{21} grams could constitute all dark matter.

PBHs around 10^{12} grams might explain the Galactic 511 keV gamma-ray line.

Abstract

Very recently, it was suggested that combining the Swampland program with the smallness of the dark energy and confronting these ideas to experiment lead to the prediction of the existence of a single extra-dimension (dubbed the dark dimension) with characteristic length-scale in the micron range. We show that the rate of Hawking radiation slows down for black holes perceiving the dark dimension and discuss the impact of our findings in assessing the dark matter fraction that could be composed of primordial black holes. We demonstrate that for a species scale of ${\cal O}(10^{10}~{\rm GeV})$, an all-dark-matter interpretation in terms of primordial black holes should be feasible for masses in the range $10^{14} \leq M_{BH}/{\rm g}\leq 10^{21}$. This range is extended compared to that in the 4D theory by 3 orders of magnitude in the low mass region. We also show that PBHs with $M_{\rm BH} \sim 10^{12}~{\rm g}$ could potentially explain the well-known Galactic 511 keV gamma-ray line if they make up a tiny fraction of the total dark matter density.