Cosmologically safe QCD axion as a present from extra dimension
  We propose a QCD axion model where the origin of PQ symmetry and suppression
of axion isocurvature perturbations are explained by introducing an extra
dimension. Each extra quark-antiquark pair lives on branes separately to
suppress PQ breaking operators. The size of the extra dimension changes after
inflation due to an interaction between inflaton and a bulk scalar field, which
implies that the PQ symmetry can be drastically broken during inflation to
suppress undesirable axion isocurvature fluctuations.
