  In the ekpyrotic scenario the Universe is initially collapsing, the energy
density coming from a scalar field with a negative exponential potential. On
the basis of a calculation ignoring the gravitational back-reaction the authors
of the scenario claim that during collapse the vacuum fluctuation creates a
perturbation in the comoving curvature, which has a flat spectrum in accordance
with observation. In this note the back-reaction is included, and it is found
that the spectrum during collapse is strongly scale-dependent with negligible
magnitude. The spectrum is continuous across the bounce if the spacetime is
smooth, making it unlikely that the ekpyrotic scenario can be compatible with
observation.
