Moment transport equations for the primordial curvature perturbation

TL;DR

This paper extends the moment transport approach to multiple inflationary fields, deriving differential equations for primordial curvature perturbation moments, enabling efficient computation of non-Gaussianity parameters like fNL in complex models.

Contribution

It generalizes the moment transport equations to an arbitrary number of fields, providing a scalable numerical method for calculating primordial non-Gaussianity.

Findings

Efficient calculation of fNL in models with ~100 fields.

Agreement with existing analytic results.

Scalable O(M^3) computational complexity.

Abstract

In a recent publication, we proposed that inflationary perturbation theory can be reformulated in terms of a probability transport equation, whose moments determine the correlation properties of the primordial curvature perturbation. In this paper we generalize this formulation to an arbitrary number of fields. We deduce ordinary differential equations for the evolution of the moments of zeta on superhorizon scales, which can be used to obtain an evolution equation for the dimensionless bispectrum, fNL. Our equations are covariant in field space and allow identification of the source terms responsible for evolution of fNL. In a model with M scalar fields, the number of numerical integrations required to obtain solutions of these equations scales like O(M^3). The performance of the moment transport algorithm means that numerical calculations with M >> 1 fields are straightforward. We illustrate this performance with a numerical calculation of fNL in Nflation models containing M ~ 10^2 fields, finding agreement with existing analytic calculations. We comment briefly on extensions of the method beyond the slow-roll approximation, or to calculate higher order parameters such as gNL.