Inflation and ACT DR6

Abstract

The recent data from the Atacama Cosmology Telescope (ACT DR6) has shifted the preferred value of the scalar spectral index to ns = 0.9743±0.0034, placing well-established models such as the Starobinsky potential near the 2σ boundary of the r-ns plane. We address this tension through two approaches. First, we consider the Power-Law Plateau potential within standard Einstein gravity and show that its predictions lie within the 1σ ACT contour for a broad range of the free parameters. Second, we embed the Starobinsky potential within Einstein-Gauss-Bonnet gravity, where the non-minimal coupling between the inflaton and the Gauss-Bonnet term shifts the predictions into the 1σ ACT region for both tanh and exponential coupling functions. In both cases, the running of the scalar spectral index is consistent with the ACT ns-αs contour.