Abstract

Recent measurements from the Atacama Cosmology Telescope (ACT), especially when combined with baryon acoustic oscillation data from the Dark Energy Spectroscopic Instrument (DESI), favor a scalar spectral index that is significantly higher than the Planck 2018 value, placing increasing tension on a wide class of conventional inflationary models. Motivated by this discrepancy, we study the squared–quartic Hilltop inflation scenario with potential V(ϕ)=V<inf>0</inf>[1−λ(ϕ/Mp)4]², focusing on both minimally and non-minimally coupled realizations of gravity. We introduce a non-minimal interaction between the inflaton and curvature, ξϕ ² R , and perform a comprehensive slow-roll analysis in both the Jordan and Einstein frames. Analytic expressions for the slow-roll parameters, the scalar spectral index n<inf>s</inf> , the tensor-to-scalar ratio r , and the running of the spectral index are derived and confronted with current observational constraints. For representative parameters in the Einstein frame, the model yields n<inf>s</inf> ≃ 0.9743 with r ≃ 0.00904, consistent with the joint Planck-ACT-DESI bounds. The associated inflationary energy scale remains compatible with high-scale inflation with V<inf>0</inf>≃O(10⁻³)M<inf>p</inf>. We further clarify the relationship between the Jordan- and Einstein-frame formulations, showing however, the Einstein-frame description provides a more transparent and efficient realization responsible for reconciling hilltop inflation with the latest CMB data. Our results demonstrate that non-minimally coupled squared-quartic Hilltop inflation offers a theoretically well-motivated and observationally viable framework capable of accommodating the ACT-preferred scalar spectral index while maintaining a mind suppressed tensor signal.