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 ${\varphi }^{2/3}$ in terms of the canonically normalized field $\varphi$, yielding predictions for the tilt of the power spectrum and the tensor-to-scalar ratio, $n_s\approx 0.98$ and $r\approx 0.04$ with 60 e-foldings of inflation; we note also the possibility of a variant with a candidate inflaton potential proportional to ${\varphi }^{2/5}$. 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.

• Received 5 July 2008

DOI:https://doi.org/10.1103/PhysRevD.78.106003