  One-loop correction to the power spectrum in generic single-field inflation
is calculated extracting the most important operator, which also generates
primordial non-Gaussianity, using soft effective field theory. Due to the
enhancement inversely proportional to the observed red-tilt of the spectral
index of curvature perturbation, the correction turns out to be much larger
than previously anticipated. As a result, the primordial non-Gaussianity must
be much smaller than the current observational bound in order to warrant the
validity of cosmological perturbation theory.
