Description
In this presentation, I 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. I
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.
