Reduced Hubble tension in dark radiation models after DESI 2024

Abstract We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos N eff , as Δ N eff ≡ N eff -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 N eff , with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+ ), setting Δ N eff ≤ 0.39 (95% C.L.) for free streaming DR, and a very mild preference for fluid DR, Δ N eff = 0.221 +0.088 -0.18 (≤ 0.46, 95% C.L.). Applying constraints from primordial element abundances leads to tighter constraints on Δ N eff , but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH 0 ES determination of H 0 to be less than 3 σ and as low as 2 σ , and for free-streaming DR the tension is below 3 σ if production occurs after BBN. This lesser degree of tension motivates a combination with SH 0 ES in these cases, resulting in a 4.4 σ -5 σ evidence for dark radiation with Δ N eff ≃ 0.6 and large improvements in χ 2 over ΛCDM, -18 ≲ Δ χ 2 ≲ -25. Upcoming data releases by DESI and other CMB and LSS surveys will decisively confirm or disfavour this conclusion.