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In the standard entropic mechanism adopted in the simple Ekpyrotic models togenerate the nearly scale-invariant and Gaussian primordial perturbation, theentropy direction is tachyonically unstable. In this paper, we consider thestable production of the scale-invariant entropy perturbation in the Ekpyroticuniverse via non-minimal couplings. In this model the non-minimally coupledmassless scalar field serves as a spectator and is stabilized by the introducednon-minimal couplings. It always corresponds to the entropy field during thecontraction and with appropriate couplings can obtain a scale-invariantspectrum. This scenario requires additional mechanisms such as curvaton ormodulated preheating to convert the entropy perturbation to the curvatureperturbation after the bounce.

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