A Statistical Approach to Multifield Inflation: Many-field Perturbations Beyond Slow Roll

TL;DR

This paper investigates multifield inflation models from string theory, analyzing their perturbations and identifying conditions where multifield effects are significant but often unobservable, with implications for understanding early universe physics.

Contribution

It provides a numerical and analytical study of six-field inflationary models, revealing the role of many-field effects and their typical unobservability in string-inspired scenarios.

Findings

Many-field effects are often important during inflection point approach.

Most models produce unobservable signatures of multifield evolution.

Scalar masses are of order H, with small non-Gaussianity signatures.

Abstract

We study multifield contributions to the scalar power spectrum in an ensemble of six-field inflationary models obtained in string theory. We identify examples in which inflation occurs by chance, near an approximate inflection point, and we compute the primordial perturbations numerically, both exactly and using an array of truncated models. The scalar mass spectrum and the number of fluctuating fields are accurately described by a simple random matrix model. During the approach to the inflection point, bending trajectories and violations of slow roll are commonplace, and 'many-field' effects, in which three or more fields influence the perturbations, are often important. However, in a large fraction of models consistent with constraints on the tilt the signatures of multifield evolution occur on unobservably large scales. Our scenario is a concrete microphysical realization of quasi-single-field inflation, with scalar masses of order $H$, but the cubic and quartic couplings are typically too small to produce detectable non-Gaussianity. We argue that our results are characteristic of a broader class of models arising from multifield potentials that are natural in the Wilsonian sense.