Natural Inflation After Planck 2018

TL;DR

This paper assesses the viability of Natural Inflation models against recent observational data, finding that standard models are disfavored unless specific post-inflationary conditions are assumed, such as prolonged reheating.

Contribution

It provides high-precision constraints on Natural Inflation using Planck 2018, BICEP/Keck, and BAO data, exploring the impact of post-inflationary history on model viability.

Findings

Natural Inflation with a cosine potential is disfavored at >95% confidence with standard post-inflationary dynamics.

Protracted reheating with ar w > 1/3 can reconcile Natural Inflation with observations.

Adding a Hubble constant prior worsens the fit to observational data.

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.