Intuitive understanding of non-gaussianity in ekpyrotic and cyclic models

TL;DR

This paper provides an intuitive semi-analytic estimate of non-gaussianity in ekpyrotic and cyclic models, showing it is significantly larger than in inflationary models and can serve as a key observational test.

Contribution

It introduces a physically intuitive method to estimate the bispectrum in ekpyrotic and cyclic models, highlighting an intrinsic non-gaussianity contribution linked to the equation of state.

Findings

Intrinsic f_{NL} is about 100 times larger than in inflation.

Other contributions can increase |f_{NL}| but not decrease it significantly.

Non-gaussianity correlates with the scalar spectral index, aiding observational tests.

Abstract

It has been pointed out by several groups that ekpyrotic and cyclic models generate significant non-gaussianity. In this paper, we present a physically intuitive, semi-analytic estimate of the bispectrum. We show that, in all such models, there is an intrinsic contribution to the non-gaussianity parameter f_{NL} that is determined by the geometric mean of the equation of state w_{ek} during the ekpyrotic phase and w_{c} during the phase that curvature perturbations are generated and whose value is O(100) or more times the intrinsic value predicted by simple slow-roll inflationary models, f_{NL}^{intrinsic} = O(0.1). Other contributions to f_{NL}, which we also estimate, can increase |f_{NL}| but are unlikely to decrease it significantly, making non-gaussianity a useful test of these models. Furthermore, we discuss a predicted correlation between the non-gaussianity and scalar spectral index that sharpens the test.