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

TL;DR

This work revisits unitarity-conserving non-minimally coupled inflation with a complex gauge-singlet inflaton in light of the ACT $n_s$ result. It shows that a Jordan-frame completion can remove unitarity-violating pieces while preserving a safe Einstein-frame potential, leading to robust predictions $n_s \approx 0.973$ and $r \approx 9\times 10^{-6}$ for $\lambda=0.1$ under instantaneous reheating and Einstein-frame renormalisation. The findings align with ACT data and remain largely insensitive to $\lambda$, while connecting to SM extensions through a Higgs portal and potentially enabling a first-order electroweak phase transition and primordial gravitational waves. The model thus offers a unitary, phenomenologically viable inflationary scenario with testable implications for future CMB observations and beyond-Standard Model physics.

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 $λ= 0.1$.