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 .