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

The Atacama Cosmology Telescope (ACT) collaboration has reported a scalar spectral index $n_s=0.9743\pm 0.0034$. This is substantially larger than the classical prediction of non-minimally coupled inflation models such as Higgs Inflation, $n_s\approx 0.965$. 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 $n_s=0.9730$ and $r\approx 9\times {10}^{-6}$ for scalar self-coupling $\lambda =0.1$.