Natural Inflation After Planck 2018

Abstract: We calculate high-precision constraints on Natural Inflation relative to current observational constraints from Planck 2018 + BICEP/Keck(BK15) Polarization + BAO on $r$ and $n_S$, including post-inflationary history of the universe. We find that, for conventional post-inflationary dynamics, Natural Inflation with a cosine potential is disfavored at greater than 95\% confidence out by current data. If we assume protracted reheating characterized by $\overline{w}>1/3,$ Natural Inflation can be brought into agreement with current observational constraints. However, bringing unmodified Natural Inflation into the 68\% confidence region requires values of $T_{\mathrm{re}}$ below the scale of electroweak symmetry breaking. The addition of a SHOES prior on the Hubble Constant $H_0$ only worsens the fit.