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arXiv · 1404.1265

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

Anna Ijjas·Jean-Luc Lehners·Paul J. Steinhardt
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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 $f_{NL} \sim 5$.

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Anna Ijjas, Jean-Luc Lehners, Paul J. Steinhardt. 2014-06-06. A general mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models. https://doi.org/10.1103/physrevd.89.123520

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