  During inflation, primordial energy density fluctuations are created from
approximate de Sitter vacuum quantum fluctuations redshifted out of the horizon
after which they are frozen as perturbations in the background curvature. In
this paper we demonstrate that there exists an intrinsic theoretical
uncertainty in the inflationary predictions for the curvature perturbations due
to the failure of the well known prescriptions to specify the vacuum uniquely.
Specifically, we show that the two often used prescriptions for defining the
initial vacuum state -- the Bunch-Davies prescription and the adiabatic vacuum
prescription (even if the adiabaticity order to which the vacuum is specified
is infinity) -- fail to specify the vacuum uniquely in generic inflationary
spacetimes in which the total duration of inflation is finite. This conclusion
holds despite the absence of any trans-Planckian effects or effective field
theory cutoff related effects. We quantify the uncertainty which is applicable
to slow roll inflationary scenarios as well as for general FRW spacetimes and
find that the uncertainty is generically small. This uncertainty should be
treated as a minimal uncertainty that underlies all curvature perturbation
calculations.
