  We investigate the validity of effective field theory methods and the
decoupling of heavy fields during inflation. Considering models of inflation in
which the inflaton is coupled to a heavy (super-Hubble) degree of freedom
initially in its vacuum state, we find that violations of decoupling are absent
unless there is a breakdown of the slow-roll conditions. Next we allow for a
temporary departure from inflation resulting in a period of non-adiabaticity
during which effective field theory methods are known to fail. We find that the
locality of the event and energy conservation lead to a tight bound on the size
of the effects of the heavy field. We discuss the implications for the power
spectrum and non-gaussianity, and comment on the connection with recent studies
of the dynamics of multi-field inflation models. Our results further motivate
the use of effective field theory methods to characterize cosmic inflation, and
focus the question of observability of additional degrees of freedom during
inflation to near the Hubble scale or below - as anticipated from the Wilsonian
notions of decoupling and naturalness.
