Warped Radion Inflation
  We show that the radion in a warped geometry bounded by two branes can have a
potential suitable for inflation. Our construction is based upon a solution
known in string theory as the linear dilaton, in which the back-reaction from a
bulk scalar \Phi is exactly accounted for. The radion, stabilized by \Phi, is
much heavier than the TeV scale and its couplings to the standard model are
much more suppressed than in the usual Randall-Sundrum solution. We present a
new formalism for obtaining approximate time-dependent solutions, based on
perturbing the exact solution to the coupled Einstein and scalar field
equations in the bulk. It allows the radion potential to be computed directly
in terms of the brane potentials for \Phi. We show that simple exponential
potentials on the branes can lead to a 4D radion potential with a flattened
hilltop form, yielding inflation with a spectral index of typically n_s=0.96
and no higher than 0.99. With more complicated brane potentials, the descent
from the hilltop can be a linear potential, giving a tensor-to-scalar ratio as
large as r=0.07 with n_s=0.974. The couplings of the radion to the standard
model particles are dictated by general covariance, so the details of reheating
are explicitly calculable, leading to a reheat temperature of at least 10^7
GeV. The quantum corrections to the inflaton potential from its couplings to
matter are also calculable and are shown to be small, so that the prediction
for the shape of the potential is under theoretical control, even with
superPlanckian field excursions.
