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 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 CDM 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 Nₑff, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting Nₑff 0. 39 (95\% C. L. ) for free streaming DR, and a very mild preference for fluid DR, Nₑff = 0. 221^+0. 088-₀. ₁₈ (0. 46, 95\% C. L. ). Applying constraints from primordial element abundances leads to slightly tighter constraints on 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) level, and for free-streaming DR the tension is below 3 if production occurs after BBN. This lesser degree of tension motivates a combination with SH₀ES in these cases, resulting in a 4. 4-5 evidence for dark radiation with Nₑff 0. 6 and large improvements in ² over CDM, -18 ² -25.