Signatures for a Cosmic Flux of Magnetic Monopoles

Any early universe phase transition occurring after inflation has the potential to populate the universe with a flux of magnetic monopoles. Observations of galactic magnetic fields, and models for extragalactic magnetic fields, lead to the conclusion that monopoles of mass $\lsim 10^{15}$ GeV are accelerated in these fields to relativistic velocities. We explore the possible signatures of a cosmic flux of relativistic monopoles impinging on the earth. The electromagnetically-induced signatures of monopoles are reliable. The hadronically-induced signatures are highly model-dependent. Among our findings are (i) the electromagnetic energy losses of monopoles continuously initiate a protracted shower of small intensity; (ii) monopoles may traverse the earth's diameter, making them a probe of the earth's interior structure; (iii) the best search strategy for monopoles is detection of their radio-Cherenkov signal produced by the coherent charge-excess in the $e^+-e^-$ shower --- in fact, underground radio-detectors have the potential to discover a monopole flux (or limit it) several orders of magnitude below the theoretical Parker limit of $10^{-15}$/$\rm{cm}^2$/s/sr; (iv) it is conceivable (but not compelling) that bound states of colored monopoles may be the primary particles initiating the air showers observed above the GZK cutoff.