  The ``trans-Planckian'' challenge in cosmology appears when we trace the
present physical wavelengths of fluctuations backwards in time. They become
smaller and smaller until crossing the Planck scale where conventional QFT is
challenged, so that unknown ultraviolet physics may be traced in the observable
cosmological fluctuations. Usually this issue is addressed in the inflationary
context, but trans-Planckian reasoning is much broader. We examine this logic
in a simple example of scalar quantum field theory in the expanding and
contracting Milne universes, where wavelengths of the eigenmodes are red- or
blue-shifted. Trans-Planckian modifications of QFT should result in a
UV-dependent VeV of the energy momentum tensor of a scalar field in the Milne
universe. On the other hand, the Milne universe is another coordinate systems
of flat Minkowski space-time, and the covariant energy momentum tensor should
be the same (but vacuum-dependent) in different coordinates of flat space time.
We explicitly demonstrate that in conventional QFT the energy momentum tensor,
choosing the adiabatic vacuum, is identical to zero in Minkowski coordinates,
and remains zero in the contracting Milne universe (due to non-trivial
cancellations of contributions from particles which appear in the accelerating
frame and from vacuum polarization there). In contrast to this, the
trans-Planckian modification of the energy momentum tensor is not motivated. We
provide a similar argument for the expanding Milne universe, where the energy
momentum tensor in the conformal vacuum is non-zero. Similar arguments are
applicable for other cosmological models where the curvature is much lower than
Planckian which leads to conflicts with trans-Planckian considerations.
