On the Quantum-To-Classical Transition for Ekpyrotic Perturbations
  We examine the processes of quantum squeezing and decoherence of density
perturbations produced during a slowly contracting ekpyrotic phase in which
entropic perturbations are converted to curvature perturbations before the
bounce to an expanding phase. During the generation phase, the entropic
fluctuations evolve into a highly squeezed quantum state, analogous to the
evolution of inflationary perturbations. Subsequently, during the conversion
phase, quantum coherence is lost very efficiently due to the interactions of
entropy and adiabatic modes. Moreover, while decoherence occurs, the adiabatic
curvature perturbations inherit their semi-classicality from the entropic
perturbations. Our results confirm that, just as for inflation, an ekpyrotic
phase can generate nearly scale-invariant curvature perturbations which may be
treated as a statistical ensemble of classical density perturbations, in
agreement with observations of the cosmic background radiation.
