Converting entropy to curvature perturbations after a cosmic bounce
  We study two-field bouncing cosmologies in which primordial perturbations are
created in either an ekpyrotic or a matter-dominated contraction phase. We use
a non-singular ghost condensate bounce model to follow the perturbations
through the bounce into the expanding phase of the universe. In contrast to the
adiabatic perturbations, which on large scales are conserved across the bounce,
entropy perturbations can grow significantly during the bounce phase. If they
are converted into adiabatic/curvature perturbations after the bounce, they
typically form the dominant contribution to the observed temperature
fluctuations in the microwave background, which can have several beneficial
implications. For ekpyrotic models, this mechanism loosens the constraints on
the amplitude of the ekpyrotic potential while naturally suppressing the
intrinsic amount of non-Gaussianity. For matter bounce models, the mechanism
amplifies the scalar perturbations compared to the associated primordial
gravitational waves.
