Unitarity-Conserving Non-Minimally Coupled Inflation and the ACT Spectral Index

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 non-minimally coupled inflation models such as Higgs Inflation, nₛ ≈ 0.965. Here we revisit the unitarity-conserving non-minimally coupled inflation model proposed in [1]. We show that when the inflaton is a complex non-minimally 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 $λ= 0.1$.