Adiabatic Ekpyrosis: Scale-Invariant Curvature Perturbations from a
  Single Scalar Field in a Contracting Universe
  The universe can be made flat and smooth by undergoing a phase of ultra-slow
(ekpyrotic) contraction, a condition achievable with a single, canonical scalar
field and conventional general relativity. It has been argued, though, that
generating scale-invariant density perturbations, requires at least two scalar
fields and a two-step process that first produces entropy fluctuations and then
converts them to curvature perturbations. In this paper, we identify a loophole
in the argument and introduce an ekpyrotic model based on a single, canonical
scalar field that generates nearly scale-invariant curvature fluctuations
through a purely "adiabatic mechanism" in which the background evolution is a
dynamical attractor. The resulting spectrum can be slightly red with
distinctive non-gaussian fluctuations.
