Sourcing curvature modes with entropy perturbations in non-singular
  bouncing cosmologies
  The observed temperature fluctuations in the cosmic microwave background can
be traced back to primordial curvature modes that are sourced by adiabatic
and/or entropic matter perturbations. In this paper, we explore the entropic
mechanism in the context of non-singular bouncing cosmologies. We show that
curvature modes are naturally generated during `graceful exit,' i.e., when the
smoothing slow contraction phase ends and the universe enters the bounce stage.
Here, the key role is played by the kinetic energy components that come to
dominate the energy density and drive the evolution towards the cosmological
bounce.
