5. Theoretical Physics (CSN4)

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

by Alessio Notari (Universitat de Barcelona)

Aula Seminari (Laboratori Nazionali di Frascati)

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.