Monodromy in the CMB: Gravity Waves and String Inflation
Eva Silverstein, Alexander Westphal
TL;DR
This work demonstrates a string-theoretic mechanism for large-field inflation via monodromy of wrapped D4-branes on twisted tori (Nil manifolds), yielding an inflaton potential that scales as $V_{ m R}(\phi) \propto \phi^{2/3}$ in the principal setup. By carefully balancing curvature effects, moduli stabilization, and symmetry considerations, the authors show that a super-Planckian field excursion can be realized without destabilizing the compactification, predicting a tensor-to-scalar ratio $r$ around $0.04$ and a spectral tilt $n_s$ near $0.978$ for $N\approx 60$ e-folds. A variant configuration yields a different fractional power, $V(\tilde{\phi}) \propto \tilde{\phi}^{2/5}$, while a separate analysis of $\alpha'$ and loop corrections argues they do not jeopardize slow-roll in the controlled regime. The results offer a concrete UV-complete pathway to observable gravity waves from string inflation and suggest broader applicability of monodromy-based field-range extension in other metric-flux compactifications.
Abstract
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, $n_s\approx 0.98$ 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 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.