Ekpyrotic Perturbations With Small Non-Gaussian Corrections
  The entropic mechanism for producing nearly scale-invariant density
perturbations in a contracting ekpyrotic universe relies on having an unstable
scalar potential. Here we develop a variant of this mechanism (recently
proposed by Qiu, Gao and Saridakis, and by Li), in which there exists a
non-trivial coupling between adiabatic and entropic fields, and where an
unstable potential is not required. In the model nearly scale-invariant entropy
perturbations are generated first. Remarkably, we find that the bispectrum of
these perturbations vanishes, with the values of the non-Gaussianity parameters
of local, equilateral and orthogonal type all exactly zero. Subsequently, the
entropy perturbations can be converted into curvature perturbations by a
variety of mechanisms. The bispectrum of the curvature perturbations depends on
the non-linearity of the conversion process and is thus more model-dependent -
however, for an efficient conversion process the final bispectrum remains
small. The only distinguishing feature compared to single-field slow-roll
inflationary models is an absence of primordial gravitational waves. Thus the
present model provides a perfect match to current data from the PLANCK
satellite.
