Massive magnetic monopoles: Indirect and direct limits on their number density and flux

The existence of very massive magnetic monopoles is a necessary consequence of most unified theories of the strong, electromagnetic, and weak interactions. Estimates of their masses range from 10¹⁵ to 10¹⁷ that of a proton. Monopoles should have been produced in the "big bang" and survived to the present, either trapped in matter or in flight. By virtue of their huge mass, these monopoles might have eluded previous searches designed to detect monopoles with much smaller mass. I review the indirect limits which can be placed on monopoles because of the existence of large-scale galactic magnetic fields. I also review the experimental searches and reinterpret them to obtain limits for very massive monopoles. If the monopole masses are ≲5×10¹⁴ GeV/c², their concentration must be ≲10^-27 monopoles/nucleon from searches for monopoles trapped in meteorites. This limit is several orders of magnitude lower than the astrophysical limits.