A general mechanism for producing scale-invariant perturbations and
  small non-Gaussianity in ekpyrotic models
  We explore a new type of entropic mechanism for generating density
perturbations in a contracting phase in which there are two scalar fields, but
only one has a steep negative potential. This first field dominates the energy
density and is the source of the ekpyrotic equation of state. The second field
has a negligible potential, but its kinetic energy density is coupled to the
first field with a non-linear sigma-model type interaction. We show that for
any ekpyrotic equation of state it is possible to choose the potential and the
kinetic coupling such that exactly scale-invariant (or nearly scale-invariant)
entropy perturbations are produced. The corresponding background solutions are
stable, and the bispectrum of the entropy perturbations vanishes as no
non-Gaussianity is produced during the ekpyrotic phase. Hence, the only
contribution to non-Gaussianity comes from the non-linearity of the conversion
process during which entropic perturbations are turned into adiabatic ones,
resulting in a local non-Gaussianity parameter $f_{NL} \sim 5$.
