  A characteristic of D-brane inflation is that fluctuations in the inflaton
field can propagate at a speed significantly less than the speed of light. This
yields observable effects that are distinct from those of single-field slow
roll inflation, such as a modification of the inflationary consistency relation
and a potentially large level of non-Gaussianities. We present a numerical
algorithm that extends the inflationary flow formalism to models with general
speed of sound. For an ensemble of D-brane inflation models parameterized by
the Hubble parameter and the speed of sound as polynomial functions of the
inflaton field, we give qualitative predictions for the key inflationary
observables. We discuss various consistency relations for D-brane inflation,
and compare the qualitative shapes of the warp factors we derive from the
numerical models with analytical warp factors considered in the literature.
Finally, we derive and apply a generalized microphysical bound on the inflaton
field variation during brane inflation. While a large number of models are
consistent with current cosmological constraints, almost all of these models
violate the compactification constraint on the field range in four-dimensional
Planck units. If the field range bound is to hold, then models with a
detectable level of non-Gaussianity predict a blue scalar spectral index, and a
tensor component that is far below the detection limit of any future
experiment.
