Natural Inflation and Quantum Gravity

TL;DR

The paper investigates how quantum-gravity constraints, particularly the Weak Gravity Conjecture, challenge high-scale inflation and proposes a parametrically controlled class of Natural Inflation models built from higher-dimensional gauge dynamics. By employing multi-axion (bi- and tri-axion) extranatural constructions and a Chern-Simons framework, the authors achieve effective super-Planckian decay constants $f_{\text{eff}}$ with sub-Planckian fundamental scales, while maintaining theoretical control and UV consistency. The models yield sizable tensor-to-scalar ratios around $r \sim 0.1$ and predict modulations in the scalar power spectrum that could be observable in future CMB data, with the extra dimension dynamics rigorously stabilized via Goldberger-Wise-type mechanisms. Collectively, the work provides a viable, UV-conscious route to high-scale inflation with concrete phenomenological signatures.

Abstract

Cosmic Inflation provides an attractive framework for understanding the early universe and the cosmic microwave background. It can readily involve energies close to the scale at which Quantum Gravity effects become important. General considerations of black hole quantum mechanics suggest nontrivial constraints on any effective field theory model of inflation that emerges as a low-energy limit of quantum gravity, in particular the constraint of the Weak Gravity Conjecture. We show that higher-dimensional gauge and gravitational dynamics can elegantly satisfy these constraints and lead to a viable, theoretically-controlled and predictive class of Natural Inflation models.