Intuitive understanding of non-gaussianity in ekpyrotic and cyclic models
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 fNL that is determined by the geometric mean of the equation of state wek during the ekpyrotic phase and wc 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, fNLintrinsic = O(0.1). Other contributions to fNL, which we also estimate, can increase |fNL| 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.