Probing the Weak Gravity Conjecture in the Cosmic Microwave Background

TL;DR

This paper investigates how the weak gravity conjecture influences inflation models, showing that potential modulations can reconcile natural inflation with CMB data and predicting observable signatures for future experiments.

Contribution

It demonstrates that axion potential modulations resolve natural inflation's tension with CMB observations and are consistent with the standard cosmological model.

Findings

Modulations in the axion potential fit CMB data well.

Natural inflation with modulations is statistically comparable to ΛCDM.

Predicted small-scale suppression of scalar power spectrum.

Abstract

The weak gravity conjecture imposes severe constraints on natural inflation. A trans-Planckian axion decay constant can only be realized if the potential exhibits an additional (subdominant) modulation with sub-Planckian periodicity. The resulting wiggles in the axion potential generate a characteristic modulation in the scalar power spectrum of inflation which is logarithmic in the angular scale. The compatibility of this modulation is tested against the most recent Cosmic Microwave Background (CMB) data by Planck and BICEP/Keck. Intriguingly, we find that the modulation completely resolves the tension of natural inflation with the CMB. A Bayesian model comparison reveals that natural inflation with modulations describes all existing data equally well as the cosmological standard model $\Lambda$CDM. In addition, the bound of a tensor-to-scalar ratio r > 0.002 correlated with a striking small-scale suppression of the scalar power spectrum occurs. Future CMB experiments could directly probe the modulation through their improved sensitivity to smaller angular scales and possibly the measurement of spectral distortions. They could, thus, verify a key prediction of the weak gravity conjecture and provide dramatic new insights into the theory of quantum gravity.