On adiabatic perturbations in the ekpyrotic scenario
  In a recent paper arXiv:0910.2230, Khoury and Steinhardt proposed a way to
generate adiabatic cosmological perturbations with a nearly flat spectrum in a
contracting Universe. To produce these perturbations they used a regime in
which the equation of state exponentially rapidly changed during a short time
interval. Leaving aside the singularity problem and the difficult question
about the possibility to transmit these perturbations from a contracting
Universe to the expanding phase, we will show that the methods used in
arXiv:0910.2230 are inapplicable for the description of the cosmological
evolution and of the process of generation of perturbations in this scenario.
