Unitarity-conserving nonminimally coupled inflation and the ACT spectral index

The Atacama Cosmology Telescope (ACT) Collaboration has reported a scalar spectral index n s = 0.9743 ± 0.0034 . This is substantially larger than the classical prediction of nonminimally coupled inflation models such as Higgs Inflation, n s ≈ 0.965 . Here we revisit the unitarity-conserving nonminimally coupled inflation model proposed by R. N. Lerner and J. McDonald []. 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 n s = 0.9730 and r ≈ 9 × 10 − 6 for scalar self-coupling λ = 0.1 .