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(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)approximate to 0.98 and r approximate to 0.04 with 60 e-foldings of inflation; we note also the possibility of a variant with a candidate inflaton potential proportional to phi(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.