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

TL;DR

This paper revisits a unitarity-conserving non-minimally coupled inflation model, showing it predicts a spectral index consistent with recent ACT observations by considering a complex scalar inflaton with additional interactions.

Contribution

It demonstrates that a complex non-minimally coupled scalar inflaton with unitarity-preserving interactions can match the observed spectral index and tensor-to-scalar ratio.

Findings

Predicted spectral index n_s = 0.9730

Tensor-to-scalar ratio r ≈ 9 × 10^{-6}

Model remains unitarity-conserving during inflation

Abstract

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 $\lambda = 0.1$.