  Modifications to the primordial power spectrum of inflationary density
perturbations have been studied recently using a boundary effective field
theory approach. In the approximation of a fluctuating quantum field on a fixed
background, the generic effect of new physics is encoded in parameters of order
H/M. Here, we point out that the back-reaction on the metric can be neglected
only when these parameters obey certain bounds that may put them beyond the
reach of observation.
