Natural Inflation with a periodic non-minimal coupling

TL;DR

This paper proposes a non-minimal coupling to gravity in natural inflation models, which respects the inflaton's symmetry and improves compatibility with observational data, even for sub-Planckian periodicity scales.

Contribution

It introduces a simple, symmetry-respecting non-minimal coupling to gravity that enhances natural inflation's agreement with cosmological observations.

Findings

Improved fit to Planck 2018 data with the new coupling.

Satisfactory inflationary predictions for periodicity scales around the Planck mass.

The non-minimal coupling maintains the inflaton's symmetry and reduces tension with observations.

Abstract

Natural inflation is an attractive model for primordial inflation, since the potential for the inflaton is of the pseudo Nambu-Goldstone form, $V(\phi)=\Lambda^4 [1+\cos (\phi/f)]$, and so is protected against radiative corrections. Successful inflation can be achieved if $f \gtrsim {\rm few}\, M_{P}$ and $\Lambda \sim m_{GUT}$ where $\Lambda$ can be seen as the strong coupling scale of a given non-abelian gauge group. However, the latest observational constraints put natural inflation in some tension with data. We show here that a non-minimal coupling to gravity $\gamma^2(\phi) R$, that respects the symmetry $\phi\rightarrow \phi+2 \pi f$ and has a simple form, proportional to the potential, can improve the agreement with cosmological data. Moreover, in certain cases, satisfactory agreement with the Planck 2018 TT, TE, EE and low P data can be achieved even for a periodicity scale of approximately $M_p$.