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Taken individually, magnetic monopoles and axions are both well-motivatedaspects of physics beyond the Standard Model. We demonstrate that by virtue ofthe Witten effect, their interplay is furthermore nontrivial; monopoles breakthe corresponding axial symmetries explicitly and result in an axion potentiallacking the usual instanton-derived exponential suppression factor. As axionsare distinguished by their extremely weak interactions, any regime where theireffects are enhanced may be of interest. As an application of these findings,we demonstrate that a phenomenologically acceptable population ofgrand-unification monopoles in the early Universe can efficiently suppress boththe QCD axion dark matter abundance and CMB isocurvature contribution.

GUT monopoles, the Witten effect and QCD axion phenomenology