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 ϕ^2/3 in terms of the canonically normalized field ϕ, 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; we note also the possibility of a variant with a candidate inflaton potential proportional to ϕ^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.