Monodromy in the CMB: Gravity Waves and String Inflation

We present a simple mechanism for obtaining large-field inflation, and hence a gravitational wave signature, from string theory compactified on twisted tori. For nil manifolds, we obtain a leading inflationary potential proportional to {phi}{sup 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{sub s}{approx_equal}0.98 and r{approx_equal}0.04 with 60 e-foldings of inflation; we note also the possibility of a variant with a candidate inflaton potential proportional to {phi}{sup 2/5}. The basic mechanism involved in extending the field range--monodromy in D-branes as they move in circles on the manifold--arises in a more general class of compactifications, though our methods for controlling the corrections to the slow-roll parameters require additional symmetries.