The Non-Minimal Ekpyrotic Trispectrum

TL;DR

This paper develops a formalism to predict non-Gaussian features in multi-field ekpyrotic cosmological models, revealing that the conversion process induces detectable non-Gaussianity with distinctive negative trispectrum signatures.

Contribution

It derives third-order evolution equations for multi-field models with non-canonical kinetic terms and applies them to ekpyrotic scenarios, highlighting unique non-Gaussian signatures.

Findings

Entropy perturbations have negligible bi- and trispectra during ekpyrosis.

Conversion to curvature perturbations induces observable non-Gaussianity.

The trispectrum parameter g_{NL} is typically negative and large in magnitude.

Abstract

Employing the covariant formalism, we derive the evolution equations for two scalar fields with non-canonical field space metric up to third order in perturbation theory. These equations can be used to derive predictions for local bi- and trispectra of multi-field cosmological models. Our main application is to ekpyrotic models in which the primordial curvature perturbations are generated via the non-minimal entropic mechanism. In these models, nearly scale-invariant entropy perturbations are generated first due to a non-minimal kinetic coupling between two scalar fields, and subsequently these perturbations are converted into curvature perturbations. Remarkably, the entropy perturbations have vanishing bi- and trispectra during the ekpyrotic phase. However, as we show, the conversion process to curvature perturbations induces local non-Gaussianity parameters $f_{NL}$ and $g_{NL}$ at levels that should be detectable by near-future observations. In fact, in order to obtain a large enough amplitude and small enough bispectrum of the curvature perturbations, as seen in current measurements, the conversion process must be very efficient. Interestingly, for such efficient conversions the trispectrum parameter $g_{NL}$ remains negative and typically of a magnitude ${\cal O}(10^2) - {\cal O}(10^3),$ resulting in a distinguishing feature of non-minimally coupled ekpyrotic models.