ACT's hint of positive spectral tilt favors Gauss-Bonnet inflation

Recent ACT measurements hint that the running of the primordial spectral index is positive — a result that would eliminate many standard inflationary models. Testing three frameworks, standard scalar-field inflation and F(R) gravity both find it difficult to produce a positive running while remaining consistent with Planck constraints. Einstein-Gauss-Bonnet gravity, already constrained by the neutron-star merger GW170817, accommodates a positive running easily, making it a observationally viable candidate if future CMB experiments confirm the ACT trend.