On Non-Minimal Couplings to Gravity and Axion Isocurvature Bounds

TL;DR

This paper investigates how non-minimal couplings of the inflaton and Peccei-Quinn fields to gravity influence axion isocurvature bounds during inflation, analyzing their interplay in different gravity formulations and inflation models.

Contribution

It provides a comprehensive analysis of the combined effects of inflaton and Peccei-Quinn non-minimal couplings on isocurvature bounds in both metric and Palatini gravity.

Findings

Non-minimal inflaton coupling tightens isocurvature constraints.

Peccei-Quinn coupling can alleviate bounds but is limited by inflation dynamics.

Results are demonstrated in Palatini Higgs and Starobinsky inflation models.

Abstract

For axions present during inflation, it has been shown that a non-minimal coupling $\xi_\sigma$ of the inflaton to gravity worsens isocurvature bounds, while a non-minimal coupling $\xi_\rho$ of the radial Peccei-Quinn field can alleviate them. We analyze the simultaneous presence of both couplings and determine when one effect dominates the other, in both the metric and Palatini formulations of gravity. The two tendencies interpolate smoothly, but introducing a non-minimal inflaton coupling reduces the viable interval of $\xi_\rho$ in which isocurvature bounds can be alleviated while avoiding backreaction on the inflationary dynamics. We illustrate our findings in Palatini Higgs inflation and Starobinsky inflation.