Note on the production of scale-invariant entropy perturbation in the
  Ekpyrotic universe
  In the standard entropic mechanism adopted in the simple Ekpyrotic models to
generate the nearly scale-invariant and Gaussian primordial perturbation, the
entropy direction is tachyonically unstable. In this paper, we consider the
stable production of the scale-invariant entropy perturbation in the Ekpyrotic
universe via non-minimal couplings. In this model the non-minimally coupled
massless scalar field serves as a spectator and is stabilized by the introduced
non-minimal couplings. It always corresponds to the entropy field during the
contraction and with appropriate couplings can obtain a scale-invariant
spectrum. This scenario requires additional mechanisms such as curvaton or
modulated preheating to convert the entropy perturbation to the curvature
perturbation after the bounce.
