We present a simple mechanism for obtaining large-field inflation,
and hence a gravitational wave signature, from string theory. They
basic structure involved in extending the field range -- monodromies
in the presence of wrapped D-branes combined with symmetries
protecting the flatness of an inflaton potential -- arises in several
classes of string compactifications. In one example, type IIA string
theory on Nil manifolds with a space-time filling D4-brane moving
along an internal circle, we obtain a leading inflationary potential
proportional to phi^(2/3) in terms of the canonically normalized field
phi, yielding predictions for the tilt of the power spectrum and the
tensor-to-scalar ratio, n_s = 0.98 and r = 0.04 with 60 e-foldings of
inflation. In a second example, wrapped branes can also introduce a
monodromy that extends the field range of individual closed-string
axions to beyond the Planck scale. Shift symmetries of the system
naturally control corrections to the axion potential. This suggests a
general mechanism for chaotic inflation driven by monodromy-extended
closed-string axions. We systematically analyze this possibility and
show its compatibility with moduli stabilization in e.g. warped
Calabi-Yau manifolds. In this broad class of models, the potential is
linear in the canonical inflaton field, predicting a tensor to scalar
ratio r = 0.07 accessible to upcoming cosmic microwave background
(CMB) observations.
|