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We discuss and clarify the validity of effective single field theories ofinflation obtained by integrating out heavy degrees of freedom in the regimewhere adiabatic perturbations propagate with a suppressed speed of sound. Weshow by construction that it is indeed possible to have inflationarybackgrounds where the speed of sound remains suppressed and slow-roll persistsfor long enough. In this class of models, heavy fields influence the evolutionof adiabatic modes in a manner that is consistent with decoupling of physicallow and high energy degrees of freedom. We emphasize the distinction betweenthe effective masses of the isocurvature modes and the eigenfrequencies of thepropagating high energy modes. Crucially, we find that the mass gap thatdefines the high frequency modes increases with the strength of the turn, evenas the naive heavy (isocurvature) and light (curvature) modes become morestrongly coupled. Adiabaticity is preserved throughout, and the derivedeffective field theory remains in the weakly coupled regime, satisfying allcurrent observational constraints on the resulting primordial power spectrum.In addition, these models allow for an observably large equilateralnon-Gaussianity.

Heavy fields, reduced speeds of sound and decoupling during inflation