Planck Limits on Non-canonical Generalizations of Large-field Inflation Models

TL;DR

This paper analyzes how Planck satellite data constrains non-canonical large-field inflation models with reduced sound speed, showing that many such models are now highly limited or potentially ruled out, except for certain infrared-type variants.

Contribution

It provides the first comprehensive constraints on Dirac-Born-Infeld (DBI) inflation models with reduced sound speed using recent Planck and BICEP/Keck data.

Findings

Reduced sound speed lowers tensor-scalar ratio, improving data fit.

Planck constraints limit the viability of many DBI models.

Infrared-type DBI models remain unconstrained by these bounds.

Abstract

In this paper, we consider two case examples of Dirac-Born-Infeld (DBI) generalizations of canonical large-field inflation models, characterized by a reduced sound speed, $c_{S} < 1$. The reduced speed of sound lowers the tensor-scalar ratio, improving the fit of the models to the data, but increases the equilateral-mode non-Gaussianity, $f^\mathrm{equil.}_\mathrm{NL}$, which the latest results from the Planck satellite constrain by a new upper bound. We examine constraints on these models in light of the most recent Planck and BICEP/Keck results, and find that they have a greatly decreased window of viability. The upper bound on $f^\mathrm{equil.}_\mathrm{NL}$ corresponds to a lower bound on the sound speed and a corresponding lower bound on the tensor-scalar ratio of $r \sim 0.01$, so that near-future Cosmic Microwave Background observations may be capable of ruling out entire classes of DBI inflation models. The result is, however, not universal: infrared-type DBI inflation models, where the speed of sound increases with time, are not subject to the bound.