John Ellis, Marcos A. G. García, Keith A. Olive, Sarunas Verner
We analyze the latest cosmic microwave background (CMB) constraints on the scalar spectral index n s and tensor-to-scalar ratio r from 2018, BICEP/Keck 2018, the Atacama Cosmology Telescope Data Release 6 (ACT DR6), and the South Pole Telescope (SPT-3G) data, focusing on their implications for attractor models of inflation. We compare systematically observational bounds with theoretical predictions for both E-model ( α -Starobinsky) and T-model potentials. The observational constraints accommodate E-models with α ≲ 25 , with the canonical Starobinsky model ( α = 1 ) predicting n s = 0.958 – 0.963 for reheating temperatures between 100 − 10 10 GeV , in good agreement with 2018 data and within the 95% CL region determined by the -ACT-SPT combination, but below the 95% confidence region of the -ACT-DESI combination. Higher reheating temperatures from near-instantaneous reheating improve the compatibility. T-models predict slightly lower n s values (0.956–0.961), in some tension with 2018 data, and we find an upper limit of α ≲ 11 in these models. We extend our analysis to generalized α -attractors with monomial potentials V ( φ ) ∝ φ k near the minimum, demonstrating that models with k ≥ 6 naturally predict n s ≃ 0.965 – 0.968 for typical number of e -folds, in better agreement with the ACT DR6 data. We also consider deformed E- and T-models, which allow significantly higher values of n s for low values of α ≃ 1 .