High-Scale Axions without Isocurvature from Inflationary Dynamics
  Observable primordial tensor modes in the cosmic microwave background (CMB)
would point to a high scale of inflation $H_{I}$. If the scale of Peccei-Quinn
(PQ) breaking $f_a$ is greater than $\frac{H_{I}}{2\pi}$, CMB constraints on
isocurvature naively rule out QCD axion dark matter. This assumes the potential
of the axion is unmodified during inflation. We revisit models where
inflationary dynamics modify the axion potential and discuss how isocurvature
bounds can be relaxed. We find that models that rely solely on a larger
PQ-breaking scale during inflation $f_I$ require either late-time dilution of
the axion abundance or highly super-Planckian $f_I$ that somehow does not
dominate the inflationary energy density. Models that have enhanced explicit
breaking of the PQ symmetry during inflation may allow $f_a$ close to the
Planck scale. Avoiding disruption of inflationary dynamics provides important
limits on the parameter space.
