IR Divergences in Inflation and Entropy Perturbations
  We study leading order perturbative corrections to the two point correlation
function of the scalar field describing the curvature perturbation in a
slow-roll inflationary background, paying particular attention to the
contribution of entropy mode loops. We find that the infrared divergences are
worse than in pure de Sitter space: they are power law rather than logarithmic.
The validity of perturbation theory and thus of the effective field theory of
cosmological perturbations leads to stringent constraints on the coupling
constants describing the interactions, in our model the quartic
self-interaction coupling constant of the entropy field. If the self coupling
constant is larger than some critical value which depends in particular on the
duration of the inflationary phase, then perturbation theory breaks down. Our
analysis may have implications for the stability of de Sitter space: the
quantum effects which lead to an instability of de Sitter space will be larger
in magnitude in the presence of entropy fluctuations.
