  As a simple model for unknown Planck scale physics, we assume that the
quantum modes responsible for producing primordial curvature perturbations
during inflation are placed in their instantaneous adiabatic vacuum when their
proper momentum reaches a fixed high energy scale M. The resulting power
spectrum is derived and presented in a form that exhibits the amplitude and
frequency of the superimposed oscillations in terms of H/M and the slow roll
parameter epsilon. The amplitude of the oscillations is proportional to the
third power of H/M. We argue that these small oscillations give the lower bound
of the modifications of the power spectrum if the notion of free mode
propagation ceases to exist above the critical energy scale M.
