Non-Gaussianities in N-flation

TL;DR

This paper analyzes non-Gaussianities in N-flation, a string-inspired multi-field inflation model, finding that slow evolution suppresses non-Gaussian signals, making it similar to single-field inflation, with potential for larger signals during faster field evolution.

Contribution

It provides a detailed computation of non-Gaussianities in N-flation, including mode evolution effects, and introduces an analytic method to estimate non-Gaussianities during faster field evolution.

Findings

Non-Gaussianities are suppressed during slow evolution, making N-flation similar to single-field models.

Mode evolution after horizon crossing contributes additional, but suppressed, non-Gaussian signals.

Fast field evolution could produce larger non-Gaussianities, relevant during pre-heating.

Abstract

We compute non-Gaussianities in N-flation, a string motivated model of assisted inflation with quadratic, separable potentials and masses given by the Marcenko-Pastur distribution. After estimating parameters characterizing the bi- and trispectrum in the horizon crossing approximation, we focus on the non-linearity parameter $f_{NL}$, a measure of the bispectrum; we compute its magnitude for narrow and broad spreads of masses, including the evolution of modes after horizon crossing. We identify additional contributions due to said evolution and show that they are suppressed as long as the fields are evolving slowly. This renders $\mathcal{N}$-flation indistinguishable from simple single-field models in this regime. Larger non-Gaussianities are expected to arise for fields that start to evolve faster, and we suggest an analytic technique to estimate their contribution. However, such fast roll during inflation is not expected in N-flation, leaving (p)re-heating as the main additional candidate for generating non-Gaussianities.