Solving the Tension between High-Scale Inflation and Axion Isocurvature
  Perturbations
  The BICEP2 experiment determined the Hubble parameter during inflation to be
about $10^{14}$ GeV. Such high inflation scale is in tension with the QCD axion
dark matter if the Peccei-Quinn (PQ) symmetry remains broken during and after
inflation, because too large axion isocurvature perturbations would be
generated. The axion isocurvature perturbations can be suppressed if the axion
acquires a sufficiently heavy mass during inflation. We show that this is
realized if the PQ symmetry is explicitly broken down to a discrete symmetry
and if the breaking is enhanced during inflation. We also show that, even when
the PQ symmetry becomes spontaneously broken after inflation, such a
temporarily enhanced PQ symmetry breaking relaxes the constraint on the axion
decay constant.
