  It has recently been suggested that Planck scale physics may effect the
evolution of cosmological fluctuations in the early stages of cosmological
inflation in a non-trivial way, leading to an excited state for modes whose
wavelength is super-Planck but sub-Hubble. In this case, the issue of how this
excited state back-reacts on the background space-time arises. In fact, it has
been suggested that such back-reaction effects may lead to tight constraints on
the magnitude of possible deviations from the usual predictions of inflation.
In this note we discuss some subtle aspects of this back-reaction issue and
point out that rather than preventing inflation, the back-reaction of
ultraviolet fluctuations may simply lead to a renormalization of the
cosmological constant driving inflation.
