Non-Gaussian Density Fluctuations from Entropically Generated Curvature Perturbations in Ekpyrotic Models

TL;DR

This paper investigates non-Gaussian density fluctuations in ekpyrotic models, showing how entropic perturbations convert into curvature modes and produce significant non-Gaussianity within observational limits.

Contribution

It provides a detailed analysis of non-Gaussianity generation in ekpyrotic models, highlighting the impact of conversion process dynamics and potential steepness on $f_{NL}$ values.

Findings

Non-Gaussianity parameter $f_{NL}$ ranges from -60 to +80.

Conversion process and potential steepness influence non-Gaussianity.

Values are typically larger than in slow-roll inflation but within observational bounds.

Abstract

We analyze the non-gaussian density perturbations generated in ekpyrotic/cyclic models based on heterotic M-theory. In this picture, two scalar fields produce nearly scale-invariant entropic perturbations during an ekpyrotic phase that are converted into curvature modes {\it after the ekpyrotic phase is complete} and just before the big bang. Both intrinsic non-linearity in the entropy perturbation and the conversion process contribute to non-gaussianity. The range of the non-gaussianity parameter $f_{NL}$ depends on how gradual the conversion process is and the steepness of the scalar field potential during the ekpyrotic phase. Although a wider range is possible, in principle, natural values of the ekpyrotic parameters combined with a gradual conversion process lead to values of $-60 \lesssim f_{NL} \lesssim +80$, typically much greater than slow-roll inflation but within the current observational bounds.