  We construct a model of hybrid inflation within a controlled five-dimensional
effective field theory framework. The inflaton and waterfall fields are
realized as naturally light moduli of the 5D compactification. At the quantum
level, waterfall loops must be cut off at a scale considerably lower than the
inflaton field transit in order to preserve slow-roll dynamics without
fine-tuning. We accomplish this by a significant warping, or redshift, between
the extra-dimensional regions in which the inflaton and waterfall fields are
localized. The mechanisms we employ have been separately realized in string
theory, which suggests that a string UV completion of our model is possible. We
study a part of the parameter space in which the cosmology takes a standard
form, but we point out that it is also possible for some regions of space to
end inflation by quantum tunneling. Such regions may provide new cosmological
signals, which we will study in future work.
