  The application of Effective Field Theory (EFT) methods to inflation has
taken a central role in our current understanding of the very early universe.
The EFT perspective has been particularly useful in analyzing the
self-interactions determining the evolution of co-moving curvature
perturbations (Goldstone boson modes) and their influence on low-energy
observables. However, the standard EFT formalism, to lowest order in spacetime
differential operators, does not provide the most general parametrization of a
theory that remains weakly coupled throughout the entire low-energy regime.
Here we study the EFT formulation by including spacetime differential operators
implying a scale dependence of the Goldstone boson self-interactions and its
dispersion relation. These operators are shown to arise naturally from the
low-energy interaction of the Goldstone boson with heavy fields that have been
integrated out. We find that the EFT then stays weakly coupled all the way up
to the cutoff scale at which ultraviolet degrees of freedom become operative.
This opens up a regime of new physics where the dispersion relation is
dominated by a quadratic dependence on the momentum \omega ~ p^2. In addition,
provided that modes crossed the horizon within this energy range, the
prediction of inflationary observables - including non-Gaussian signatures -
are significantly affected by the new scales characterizing it.
