The effective field theory of multi-field inflationary fluctuations

TL;DR

This paper develops an effective field theory framework for multi-field inflationary fluctuations, simplifying the analysis of interactions and mixing, and comparing it to traditional models to enhance understanding of inflationary phenomenology.

Contribution

It introduces a new EFT approach for multi-field inflation that does not rely on extra symmetries and systematically explores the phenomenology of such models.

Findings

Derived simple forms of multi-field mixing and interactions

Identified cubic interactions dictated by spacetime symmetries

Compared EFT predictions with traditional multi-field models

Abstract

We build an effective field theory of multi-field inflationary fluctuations based on the adiabatic perturbation and on any number of matter fluctuations in the non-adiabatic sector, without imposing extra symmetries on the latter. Focusing on terms with at most two derivatives in fields' fluctuations, we argue that taking the decoupling limit -- in which gravitational interactions are neglected -- is justified in a quasi de Sitter spacetime with slow-varying Hubble scale. With these working hypotheses, we find simple forms of multi-field mixings (quadratic order) and interactions (cubic order). We explain how to break degeneracies amongst various terms, and we compare the predictions of the effective field theory to those of non-linear sigma models of inflation and more general multi-field Lagrangian in the traditional model approach. We stress that several multi-field cubic interactions are dictated by non-linearly realised spacetime symmetries and are therefore given in terms of parameters already present in the quadratic action. We propose various directions to systematically explore the phenomenology generic to multi-field inflation and beyond the lamppost of known models.