A general mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models

Abstract

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 $f_{NL}\sim 5$.

8 pages, 1 figure, accepted for publication in PRD

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• Higher order derivative coupling to gravity and its cosmological implications
• The Cosmological OTOC: Formulating new cosmological micro-canonical correlation functions for random chaotic fluctuations in Out-of-Equilibrium Quantum Statistical Field Theory
• Quantum Circuit Complexity of Primordial Perturbations
• Gravitational-wave background in bouncing models from semi-classical, quantum and string gravity
• Fingerprints of a Non-Inflationary Universe from Massive Fields
• Cosmological initial data for numerical relativity
• Distinguishing cosmological models through quantum signatures of primordial perturbations
• Cuscuton Bounce Beyond the Linear Regime: Bispectrum and Strong Coupling Constraints