Natural Inflation After Planck 2018
Nina K. Stein, William H. Kinney (Univ. at Buffalo, SUNY)

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 and , 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 Natural Inflation can be brought into agreement with current observational constraints. However, bringing unmodified Natural Inflation into the 68\% confidence region requires values of below the scale of electroweak symmetry breaking. The addition of a SHOES prior on the Hubble Constant only worsens the fit.
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