Constraints on Relic Magnetic Black Holes
  We present current direct and astrophysical limits on the cosmological
abundance of black holes with extremal magnetic charge. Because they don't
Hawking radiate, much lighter primordial black holes could exist today if they
are extremal. The dominant constraints come from white dwarf destruction for
intermediate masses, and intergalactic gas heating for heavier black holes.
Extremal magnetic black holes may catalyze proton decay, and thus we derive
robust limits -- independent of the catalysis cross section -- from the above
as well as from white dwarf heating. We discuss other bounds such as those from
neutron star heating, solar neutrino production, binary formation and
annihilation into gamma rays, and magnetic field destruction. We note that
stable magnetically charged black holes can assist in the formation of neutron
star mass black holes.
