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Unitarity-conserving nonminimally coupled inflation and the ACT spectral index

John McDonald*

  • *Contact author: j.mcdonald@lancaster.ac.uk
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Phys. Rev. D 112, 123525 – Published 15 December, 2025

DOI: https://doi.org/10.1103/k3r6-klxs

Abstract

The Atacama Cosmology Telescope (ACT) Collaboration has reported a scalar spectral index ns=0.9743±0.0034. This is substantially larger than the classical prediction of nonminimally coupled inflation models such as Higgs Inflation, ns≈0.965. Here we revisit the unitarity-conserving nonminimally coupled inflation model proposed by R. N. Lerner and J. McDonald [Phys. Rev. D 82, 103525 (2010)]. We show that when the inflaton is a complex nonminimally coupled gauge singlet scalar with additional interactions in the Jordan frame to maintain unitarity, the model predicts ns=0.9730 and r≈9×10−6 for scalar self-coupling λ=0.1.

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Phys. Rev. D 112, 123525– Published 15 December, 2025
  • Received 2 September 2025
  • Accepted 20 November 2025
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