Spontaneous breaking of the Peccei-Quinn symmetry during warm inflation
  We show that, for values of the axion decay constant parametrically close to
the GUT scale, the Peccei-Quinn phase transition may naturally occur during
warm inflation. This results from interactions between the Peccei-Quinn scalar
field and the ambient thermal bath, which is sustained by the inflaton field
through dissipative effects. It is therefore possible for the axion field to
appear as a dynamical degree of freedom only after observable CMB scales have
become super-horizon, thus avoiding the large-scale axion isocurvature
perturbations that typically plague such models. This nevertheless yields a
nearly scale-invariant spectrum of axion isocurvature perturbations on small
scales, with a density contrast of up to a few percent, which may have a
significant impact on the formation of gravitationally-bound axion structures
such as mini-clusters.
