The standard, scale-invariant, inflationary perturbation spectrum will be
modified if the effects of trans-Planckian physics are incorporated into the
dynamics of the matter field in a phenomenological manner, say, by the
modification of the dispersion relation. The spectrum also changes if we retain
the standard dynamics but modify the initial quantum state of the matter field.
We show that, given {\it any} spectrum of perturbations, it is possible to
choose a class of initial quantum states which can exactly reproduce this
spectrum with the standard dynamics. We provide an explicit construction of the
quantum state which will produce the given spectrum. We find that the various
modified spectra that have been recently obtained from `trans-Planckian
considerations' can be constructed from suitable squeezed states above the
Bunch-Davies vacuum in the standard theory. Hence, the CMB observations can, at
most, be useful in determining the initial state of the matter field in the
standard theory, but it can {\it not} help us to discriminate between the
various Planck scale models of matter fields. We study the problem in the
Schrodinger picture, clarify various conceptual issues and determine the
criterion for negligible back reaction due to modified initial conditions.Comment: revtex4; 17 page