Non-Gaussianities from ekpyrotic collapse with multiple fields

Kazuya Koyama; Shuntaro Mizuno; Filippo Vernizzi; David Wands

doi:10.1088/1475-7516/2007/11/024

paper

Non-Gaussianities from ekpyrotic collapse with multiple fields

arXiv:0708.4321 · doi:10.1088/1475-7516/2007/11/024

Abstract

We compute the non-Gaussianity of the curvature perturbation generated by ekpyrotic collapse with multiple fields. The transition from the multi-field scaling solution to a single-field dominated regime converts initial isocurvature field perturbations to an almost scale-invariant comoving curvature perturbation. In the specific model of two fields, $ϕ_{1}$ and $ϕ_{2}$ , with exponential potentials, $- V_{i} exp (- c_{i} ϕ_{i})$ , we calculate the bispectrum of the resulting curvature perturbation. We find that the non-Gaussianity is dominated by non-linear evolution on super-Hubble scales and hence is of the local form. The non-linear parameter of the curvature perturbation is given by $f_{N L} = 5 c_{j}^{2} /12$ , where $c_{j}$ is the exponent of the potential for the field which becomes sub-dominant at late times. Since $c_{j}^{2}$ must be large, in order to generate an almost scale invariant spectrum, the non-Gaussianity is inevitably large. By combining the present observational constraints on $f_{NL}$ and the scalar spectral index, the specific model studied in this paper is thus ruled out by current observational data.

22 pages, 1figure, version revised after journal publication: sign error corrected in the value of non-linear parameter, fNL. The resulting fNL is large and negative and thus in conflict with current observational bounds

Non-Gaussianities from ekpyrotic collapse with multiple fields

Abstract

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