UV-Protected Inflation

TL;DR

The paper tackles the issue that Natural Inflation typically requires a decay constant $f$ well above the Planck scale, raising UV sensitivity concerns. It introduces a non-minimal coupling between the inflaton's kinetic term and the Einstein tensor, yielding gravitationally enhanced friction that permits a sub-Planckian $f$ while preserving shift symmetry and avoiding extra degrees of freedom. Through a detailed analysis of the action, strong-coupling scales, and slow-roll parameters, it shows that the enhanced friction suppresses the slow-roll parameters and establishes scale hierarchies ensuring UV-protection, with quantum gravity corrections remaining negligible. The result is a robust, perturbative inflationary framework with $f\ll M_p$, offering a viable alternative to multi-axion or large-$f$ scenarios and potentially broader applicability to axion-like inflaton models.

Abstract

In Natural Inflation, the Inflaton is a pseudo-Nambu-Goldstone boson which acquires a mass by explicit breaking of a global shift symmetry at scale f. In this case, for small field values, the potential is flat and stable under radiative corrections. Nevertheless, slow roll conditions enforce f>>M_p making the validity of the whole scenario questionable. In this letter, we show that a coupling of the Inflaton kinetic term to the Einstein tensor allows f<<M_p by enhancing the gravitational friction acting on the Inflaton during inflation. This new unique interaction, a) keeps the theory perturbative in the whole inflationary trajectory, b) preserves the tree-level shift invariance of the pseudo-Nambu-Goldstone Boson and c) avoids the introduction of any new degrees of freedom with respect the standard Natural Inflation.