Strong restriction on inflationary vacua from the local gauge invariance
  III: Infrared regularity of graviton loops
  It has been claimed that the super Hubble modes of the graviton generated
during inflation can make loop corrections diverge. Even if we introduce an
infrared (IR) cutoff at a comoving scale as an ad hoc but a practical way for
the regularization, we encounter the secular growth, which may lead to the
breakdown of perturbative expansion for a sufficiently long lasting inflation.
In this paper, we show that the IR pathology concerning the graviton can be
attributed to the presence of residual gauge degrees of freedom in the local
observable universe as in the case of the adiabatic curvature perturbation. We
will show that choosing the Euclidean vacuum as the initial state ensures the
invariance under the above-mentioned residual gauge transformations. We will
also show that as long as we consider a gauge invariant quantity in the local
universe, we encounter neither the IR divergence nor the secular growth. The
argument in this paper applies to general single field models of inflation up
to a sufficiently high order in perturbation.
