Why Must Primordial Non-Gaussianity Be Very Small?
  One-loop correction to the power spectrum in generic single-field inflation
is calculated by using standard perturbation theory. Because of 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.
