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Nonminimal couplings to gravity and axion isocurvature bounds

Claire Rigouzzo1,* and Sebastian Zell2,3,†

  • *Contact author: claire.rigouzzo@kcl.ac.uk
  • †Contact author: sebastian.zell@lmu.de
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Phys. Rev. D 113, 123502 – Published 2 June, 2026

DOI: https://doi.org/10.1103/ctll-ggr6

Abstract

For axions present during inflation, it has been shown that a nonminimal coupling ξσ of the inflaton to gravity worsens isocurvature bounds [arXiv:2504.02952], while a nonminimal coupling ξρ of the radial Peccei-Quinn field can alleviate them [Revisiting isocurvature bounds on the minimal QCD axion, J. High Energy Phys. 12 (2025) 028.]. 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 nonminimal inflaton coupling reduces the viable interval of ξρ 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.

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Phys. Rev. D 113, 123502– Published 2 June, 2026
  • Received 23 January 2026
  • Accepted 2 March 2026
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