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Inflation and Decoupling

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.