LAUSR

Abstract We present current direct and astrophysical limits on the cosmological abundance of black holes with extremal magnetic charge. Such black holes do not Hawking radiate, allowing those normally too light to survive to the present to do so. The dominant constraints come from white dwarf destruction for low and intermediate masses (2 × 10−5 g – 4 × 1012 g) and Galactic gas cloud heating for heavier masses (> 4 × 1012 g). Extremal magnetic black holes may catalyze proton decay. We derive robust limits — independent of the catalysis cross section — from the effect this has on white dwarfs. We discuss other bounds from neutron star heating, solar neutrino production, binary formation and annihilation into gamma-rays, and magnetic field destruction. Stable magnetically charged black holes can assist in the formation of neutron star mass black holes.