Warping the Weak Gravity Conjecture

TL;DR

The paper investigates whether open-string axions in warped throat geometries can realize large-field inflation in a way that remains consistent with the Weak Gravity Conjecture. By analyzing Wilson line inflatons and brane-position inflatons (related by T-duality) in type IIB warped compactifications, it derives decay-constant expressions and the UV cutoff $\\Lambda = M_{Pl} g_4$, showing that warping can yield super-Planckian $f$ within the perturbative regime but at the cost of a too-small string scale for observable inflation. The main result is that these explicit open-string constructions, while compatible with the WGC, cannot field-scale up to the inflationary energies needed for observable tensor modes due to $M_s \ll M_{Pl}$ in perturbative regimes. The work then discusses implications for the Generalized Weak Gravity Conjecture and outlines possible resolutions or required departures from perturbative control, including warped suppression of instantons, new stringy instantons, or potential revisions to the conjecture.

Abstract

The Weak Gravity Conjecture, if valid, rules out simple models of Natural Inflation by restricting their axion decay constant to be sub-Planckian. We revisit stringy attempts to realise Natural Inflation, with a single open string axionic inflaton from D-branes in a warped throat. We show that warping allows the requisite super-Planckian axion decay constant to be achieved consistently with the Weak Gravity Conjecture. However, there is a tension between large axion decay constant and high string scale, where the requisite high string scale is difficult to achieve in all attempts to realise large field inflation using perturbative string theory. We comment on the Generalized Weak Gravity Conjecture in the light of our results.