Has ACT measured radiative corrections to the tree-level Higgs-like inflation? (2504.06002v1)

Abstract: Starobinsky inflation and non-minimally coupled Higgs inflation have been among the most favored models of the early universe, as their predictions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ fall comfortably within the constraints set by Planck and BICEP/Keck. However, new results from the Atacama Cosmology Telescope (ACT) suggest a preference for higher values of $n_s$, introducing tension with the simplest realizations of these models. In this work, being agnostic about the nature of the inflaton, we show that incorporating one-loop corrections to a Higgs-like inflationary scenario leads to a shift in the predicted value of $n_s$, which brings Higgs-like inflation into better agreement with ACT observations. Remarkably, we find that this can be achieved with non-minimal couplings $\xi < 1$, in contrast to the large values typically required in conventional Higgs inflation, thereby pushing any unitarity-violation scale above the Planck scale. The effect is even more significant when the model is formulated in the Palatini approach, where the modified field-space structure naturally enhances deviations from the metric case. These findings highlight the importance of quantum corrections and gravitational degrees of freedom in refining inflationary predictions in light of new data.