  In the inflationary universe, there can be light fields other than the
inflaton. We explore a possibility that such light fields source the primordial
perturbations, while minimally affecting the inflaton dynamics. We show that
during inflation, fluctuations of the light fields can be converted to
adiabatic curvature perturbations, which accumulate and become significant by
the end of the inflationary era. An additional goal of this work is to
distinguish between light fields which can/cannot be ignored during inflation.
Such criteria become useful for examining cosmological scenarios with multiple
fields. As concrete examples, our results are applied to D-brane inflation
models. We consider effects from KK modes (oscillation modes) of wrapped branes
in monodromy-driven large-field models, and angular directions of throat
geometries in warped D-brane inflation.
