Suppressing the QCD Axion Abundance by Hidden Monopoles
  We study the Witten effect of hidden monopoles on the QCD axion dynamics, and
show that its abundance as well as isocurvature perturbations can be
significantly suppressed if there is a sufficient amount of hidden monopoles.
When the hidden monopoles make up a significant fraction of dark matter, the
Witten effect suppresses the abundance of axion with the decay constant smaller
than $10^{12}$ GeV. The cosmological domain wall problem of the QCD axion can
also be avoided, relaxing the upper bound on the decay constant when the
Peccei-Quinn symmetry is spontaneously broken after inflation.
