  Decoupling of heavy modes in effective low energy theory is one of the most
fundamental concepts in physics. It tells us that modes must have a negligible
effect on the physics of gravitational backgrounds with curvature radius larger
than their wavelengths. Despite this, there exist claims that trans-Planckian
modes put severe bound on the duration of inflation even when the Hubble
parameter is negligible as compared to the Planck mass. If true, this would
mean that inflation violates the principle of decoupling or at least requires
its reformulation. We clarify the fundamental misconception on which these
bounds are based and respectively refute them. Our conclusion is that inflation
fully falls within the validity of a reliable effective field theory treatment
and does not suffer from any spurious trans-Planckian problem.
