Flux Flattening in Axion Monodromy Inflation

TL;DR

This work identifies a robust flux-flattening mechanism for axion monodromy inflation arising from the full DBI+CS dynamics of mobile D7-branes in Type IIB/F-theory flux compactifications. By allowing generic background fluxes, the inflaton kinetic term and potential both grow in a way that can produce a plateau-like large-field behavior, yielding $n_s$ in the Planck-compatible range while achieving small tensor modes, with $r$ as low as $\sim 0.04$ in single-field regimes consistent with moduli stabilisation. The authors provide a careful 4D effective theory for the D7-brane position, estimate the relevant scales, and connect the UV data to observable predictions. They also present explicit embeddings, notably a K3×K3 F-theory construction, to realize the necessary shift symmetries and mass hierarchies between the inflaton and heavier moduli, showing that moduli stabilisation can be compatible with a viable inflationary trajectory and small backreaction. Overall, the paper demonstrates that flux-flattening broadens the phenomenological viability of axion-monodromy inflation in string theory and offers concrete pathways for embedding into realistic compactifications.

Abstract

String theory models of axion monodromy inflation exhibit scalar potentials which are quadratic for small values of the inflaton field and evolve to a more complicated function for large field values. Oftentimes the large field behaviour is gentler than quadratic, lowering the tensor-to-scalar ratio. This effect, known as flattening, has been observed in the string theory context through the properties of the DBI+CS D-brane action. We revisit such flattening effects in type IIB flux compactifications with mobile D7-branes, with the inflaton identified with the D7-brane position. We observe that, with a generic choice of background fluxes, flattening effects are larger than previously observed, allowing to fit these models within current experimental bounds. In particular, we compute the cosmological observables in scenarios compatible with closed-string moduli stabilisation, finding tensor-to-scalar ratios as low as r ~ 0.04. These are models of single field inflation in which the inflaton is much lighter than the other scalars through a mild tuning of the compactification data.

Figures (8)

• Figure 1: The single field scalar potential for the canonically normalised inflaton $\hat{\rho}$ for different values of $\Upsilon$ keeping fixed $\hat{G} = 1$.
• Figure 2: Scalar potential $\hat{V}$ for the canonical field $\hat{\rho}$ for two different values of $\hat{\Upsilon}$.
• Figure 3: Spectral index and tensor-to-scalar ratio in terms of $10^{-4} \leqslant \hat{\Upsilon} \leqslant 10^{-2}$ for $N_*=50$ and $N_* = 60$ e-folds.
• Figure 4: Spectral index $n_s$ vs tensor-to-scalar ratio $r$ superimposed over the plot given by the Planck collaboration Ade:2015lrj for the single field model with $10^{-4} \leqslant \hat{\Upsilon} \leqslant 10^{-2}$.
• Figure 5: Scalar potential evaluated in the $({\rm Re}\, T, \varphi)$ plane for the set of parameters (\ref{['parameters']}) where colder colours mean smaller values of $V$.