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

• Published in: arXiv.org
• Main Authors: Stein, Nina K, Kinney, William H
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 27.03.2017
• Subjects: Constraint modelling; Cosmic microwave background; Lower bounds; Mathematical analysis; Physics - Cosmology and Nongalactic Astrophysics; Sound; Tensors; Upper bounds; Viability
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1609.08959

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.