# Planck limits on non-canonical generalizations of large-field inflation models

## 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 {sub 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 {sup equil.}{sub 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 {sup equil.}{sub NL} corresponds to a lower bound on the sound speed and a corresponding lower bound on the tensor-scalar ratio of r ∼ 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.

- Authors:

- Dept. of Physics, University at Buffalo, the State University of New York, Buffalo, NY 14260-1500 (United States)

- Publication Date:

- OSTI Identifier:
- 22679937

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Journal of Cosmology and Astroparticle Physics; Journal Volume: 2017; Journal Issue: 04; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BORN-INFELD THEORY; INFLATIONARY UNIVERSE; RELICT RADIATION; SATELLITES; SOUND WAVES; VELOCITY; VISIBLE RADIATION

### Citation Formats

```
Stein, Nina K., and Kinney, William H., E-mail: ninastei@buffalo.edu, E-mail: whkinney@buffalo.edu.
```*Planck limits on non-canonical generalizations of large-field inflation models*. United States: N. p., 2017.
Web. doi:10.1088/1475-7516/2017/04/006.

```
Stein, Nina K., & Kinney, William H., E-mail: ninastei@buffalo.edu, E-mail: whkinney@buffalo.edu.
```*Planck limits on non-canonical generalizations of large-field inflation models*. United States. doi:10.1088/1475-7516/2017/04/006.

```
Stein, Nina K., and Kinney, William H., E-mail: ninastei@buffalo.edu, E-mail: whkinney@buffalo.edu. Sat .
"Planck limits on non-canonical generalizations of large-field inflation models". United States.
doi:10.1088/1475-7516/2017/04/006.
```

```
@article{osti_22679937,
```

title = {Planck limits on non-canonical generalizations of large-field inflation models},

author = {Stein, Nina K. and Kinney, William H., E-mail: ninastei@buffalo.edu, E-mail: whkinney@buffalo.edu},

abstractNote = {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 {sub 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 {sup equil.}{sub 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 {sup equil.}{sub NL} corresponds to a lower bound on the sound speed and a corresponding lower bound on the tensor-scalar ratio of r ∼ 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.},

doi = {10.1088/1475-7516/2017/04/006},

journal = {Journal of Cosmology and Astroparticle Physics},

number = 04,

volume = 2017,

place = {United States},

year = {Sat Apr 01 00:00:00 EDT 2017},

month = {Sat Apr 01 00:00:00 EDT 2017}

}