Probing the Weak Gravity Conjecture in the Cosmic Microwave Background
  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.
