The Swampland Conjecture and F-term Axion Monodromy Inflation

TL;DR

This work investigates whether F-term axion monodromy can realize large-field inflation in string theory when full moduli stabilization is included. By analyzing representative closed- and open-string models, it demonstrates that backreaction of stabilized moduli drives a logarithmic relation between the inflaton excursion and the proper field distance, yielding a critical distance Θ_c of order the Planck scale. The authors show that attempts to tune λ via fluxes or open-string moduli fail because flux quantization and mass-hierarchy requirements spoil parametric control, and a tower of KK states inevitably destabilizes the effective theory before trans-Planckian excursions can be achieved. These results provide evidence in favor of the Refined Swampland Conjecture and imply a stringent bound on observable large-field inflation in string theory, with r typically below ~10^{-3} for any controllable scenario.

Abstract

We continue the investigation of F-term axion monodromy inflation in string theory, while seriously taking the issue of moduli stabilization into account. For a number of closed and open string models, we show that they suffer from serious control issues once one is trying to realize trans-Planckian field excursions. More precisely, the flux tuning required to delay the logarithmic scaling of the field distance to a trans-Planckian value cannot be done without leaving the regime where the employed effective supergravity theory is under control. Our findings are consistent with the axionic extension of the Refined Swampland Conjecture, stating that in quantum gravity the effective theory breaks down for a field excursion beyond the Planck scale. Our analysis suggests that models of F-term axion monodromy inflation with a tensor-to-scalar ratio $r\ge O(10^{-3})$ cannot be parametrically controlled.

Figures (3)

• Figure 1: The backreacted potential $V_{\rm back}(\Theta)$ (after adding a constant uplift) depending on the proper field distance.
• Figure 2: We plotted on the left the backreacted potential $V_{\rm back}(\theta)$ including the uplift and on the right a slice of the potential $V(\theta, s_0(\tau),\tau)$. Both pictures show the destabilization of the inflationary valley.
• Figure 3: The plot depicts schematically a typical potential $V (\Theta)$ for an inflaton $\Theta$ achieved via axion monodromy. Above some critical value $\Theta_c$ the backreaction of the inflaton onto the other moduli deforms the polynomial potential to a Starobinsky-like plateau. However, in this regime the effective theory breaks down according to the swampland conjecture. The refined version of the conjecture (RSC) sets $\Theta_c \sim 1$, reducing the controllable inflaton field range to sub-Planckian distances.