Dark Radiation with Baryon Acoustic Oscillations from DESI 2024 and the H₀ tension

We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos Nₑff, as Δ Nₑff≡ Nₑff-3.044≥ 0, in light of the recent measurements of Baryon Acoustic Oscillations (BAO) by the DESI collaboration. We analyze one-parameter extensions of the ΛCDM model where dark radiation (DR) is free streaming or behaves as a perfect fluid, due to self-interactions. We report a significant relaxation of upper bounds on Δ Nₑff, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting Δ Nₑff≤ 0.39 ($95%$ C.L.) for free streaming DR, and a very mild preference for fluid DR, Δ Nₑff = 0.221+0.088-0.18 (≤ 0.46, $95%$ C.L.). Applying constraints from primordial element abundances leads to slightly tighter constraints on Δ Nₑff, but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH₀ES determination of H₀ to be around (2.3-2.8)σ level, and for free-streaming DR the tension is below 3σ if production occurs after BBN. This lesser degree of tension motivates a combination with SH₀ES in these cases, resulting in a 4.4σ-5σ evidence for dark radiation with Δ Nₑff 0.6 and large improvements in χ² over ΛCDM, -18 Δ χ² -25.