Inflationary models with Gauss–Bonnet coupling in light of ACT observations

Abstract Recent analyses combining Atacama Cosmology Telescope (ACT) data with other cosmological datasets report a higher scalar spectral index $$n_s$$ n s , creating tension with a wide range of inflationary models. Since a Gauss–Bonnet term with a coupling function $$\xi (\phi ) = 3\lambda /[4V(\phi )]$$ ξ ( ϕ ) = 3 λ / [ 4 V ( ϕ ) ] leaves $$n_s$$ n s nearly unchanged (up to a field rescaling) while reducing the tensor-to-scalar ratio r by a factor $$(1-\lambda )$$ ( 1 - λ ) , so choosing $$(1-\lambda )$$ ( 1 - λ ) sufficiently small effectively removes r as a limiting observable, making it easier for inflationary models to satisfy the latest observational constraints and alleviating this tension. Applying this mechanism to chaotic inflation, E-models, T-models, and hilltop inflation, we find that broad regions of parameter space become consistent with the latest ACT-based cosmic microwave background (CMB) constraints. These results demonstrate that Gauss–Bonnet couplings can help bring a broad class of inflationary models into agreement with current CMB measurements.