Note on the production of scale-invariant entropy perturbation in the Ekpyrotic universe

Understand

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.