Constraints on Single-Field Inflation

TL;DR

This paper examines constraints on single-field inflation models within an effective field theory framework, highlighting how current and future observational limits, especially on the tensor-to-scalar ratio, impact model viability and the potential for observable non-Gaussianity.

Contribution

The work provides a unified analysis of various single-field inflation models using EFT, identifying key parameters that constrain model viability and exploring models compatible with future observational bounds.

Findings

Constraints are mainly determined by three EFT parameters.

Most models are limited by current data, but some remain viable.

Weakly-broken galileon inflation can produce low tensor-to-scalar ratios and observable non-Gaussianity.

Abstract

Many alternatives to canonical slow-roll inflation have been proposed over the years, one of the main motivations being to have a model, capable of generating observable values of non-Gaussianity. In this work, we (re-)explore the physical implications of a great majority of such models within a single, effective field theory framework (including novel models with large non-Gaussianity discussed for the first time below.) The constraints we apply---both theoretical and experimental---are found to be rather robust, determined to a great extent by just three parameters: the coefficients of the quadratic EFT operators $(\delta N)^2$ and $\delta N \delta E$, and the slow-roll parameter $\varepsilon$. This allows to significantly limit the majority of single-field alternatives to canonical slow-roll inflation. While the existing data still leaves some room for most of the considered models, the situation would change dramatically if the current upper limit on the tensor-to-scalar ratio decreased down to $r < 10^{-2}$. Apart from inflationary models driven by plateau-like potentials, the single-field model that would have a chance of surviving this bound is the recently proposed slow-roll inflation with weakly-broken galileon symmetry. In contrast to \textit{canonical} slow-roll inflation, the latter model can support $r < 10^{-2}$ even if driven by a convex potential, as well as generate observable values for the amplitude of non-Gaussianity.