A grounded perspective on new early dark energy using ACT, SPT, and BICEP/Keck

We examine further the ability of the New Early Dark Energy model (NEDE) to resolve the current tension between the Cosmic Microwave Background (CMB) and local measurements of H ₀ and the consequences for inflation. We perform new Bayesian analyses, including the current datasets from the ground-based CMB telescopes Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT), and the BICEP/Keck telescopes, employing an updated likelihood for the local measurements coming from the SH ₀ES collaboration. Using the SH ₀ES prior on H ₀, the combined analysis with Baryonic Acoustic Oscillations (BAO), Pantheon, Planck and ACT improves the best-fit by Δχ ² = -15.9 with respect to ΛCDM, favors a non-zero fractional contribution of NEDE, f _NEDE > 0, by 4.8σ, and gives a best-fit value for the Hubble constant of H ₀ = 72.09 km/s/Mpc (mean 71.49 ± 0.82 with 68% C.L.). A similar analysis using SPT instead of ACT yields consistent results with a Δχ ² = -23.1 over ΛCDM, a preference for non-zero f _NEDE of 4.7σ and a best-fit value of H ₀ = 71.77 km/s/Mpc (mean 71.43 ± 0.85 with 68% C.L.). We also provide the constraints on the inflation parameters r and n _s coming from NEDE, including the BICEP/Keck 2018 data, and show that the allowed upper value on the tensor-scalar ratio is consistent with the ΛCDM bound, but, as also originally found, with a more blue scalar spectrum implying that the simplest curvaton model is now favored over the Starobinsky inflation model.