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

by Alessio Notari (Universitat de Barcelona)

Friday 24 May 2024, 11:30 → 13:30 Europe/Rome

Aula Seminari (Laboratori Nazionali di Frascati)

We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos NeffNeff​, as ΔNeff≡Neff−3.044≥0ΔNeff​≡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 ΛΛCDM 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 ΔNeffΔNeff​, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting ΔNeff≤0.39ΔNeff​≤0.39 (95%95% C.L.) for free streaming DR, and a very mild preference for fluid DR, ΔNeff=0.221−0.18+0.088ΔNeff​=0.221−0.18+0.088​ (≤0.46≤0.46, 95%95% C.L.). Applying constraints from primordial element abundances leads to slightly tighter constraints on ΔNeffΔNeff​, but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH00​ES determination of H0H0​ to be around (2.3−2.8)σ(2.3−2.8)σ level, and for free-streaming DR the tension is below 3σ3σ if production occurs after BBN. This lesser degree of tension motivates a combination with SH00​ES in these cases, resulting in a 4.4σ−5σ4.4σ−5σ evidence for dark radiation with ΔNeff≃0.6ΔNeff​≃0.6 and large improvements in χ2χ2 over ΛΛCDM, −18≲Δχ2≲−25−18≲Δχ2≲−25.