The BAO+BBN take on the Hubble tension

TL;DR

The paper investigates whether BAO+BBN data can constrain the H0–N_eff degeneracy at the background level, independent of CMB and SN data. Using galaxy and Ly-α BAO measurements together with BBN predictions for deuterium and helium across several BBN codes, the authors quantify the H0 in ΛCDM and ΛCDM+N_eff scenarios. They find that in ΛCDM, BAO+BBN yields H0 around 68 km/s/Mpc, in 3.2σ tension with SH0ES but in agreement with Planck; allowing N_eff to vary reduces the tension to about 2.6σ but remains constrained by helium priors. The work argues that extra radiation cannot easily solve the Hubble tension at the background level without additional perturbation-level physics or post-BBN radiation production, highlighting the importance of improved BBN and helium measurements for future constraints.

Abstract

Many attempts to solve the Hubble tension with extended cosmological models combine an enhanced relic radiation density, acting at the level of background cosmology, with new physical ingredients affecting the evolution of cosmological perturbations. Several authors have pointed out the ability of combined Baryon Acoustic Oscillation (BAO) and Big Bang Nucleosynthesis (BBN) data to probe the background cosmological history independently of both CMB maps and supernovae data. Using state-of-the-art assumptions on BBN, we confirm that combined BAO, deuterium, and helium data are in tension with the SH0ES measurements under the $Λ$CDM assumption at the 3.2$σ$ level, while being in close agreement with the CMB value. We subsequently show that floating the radiation density parameter $N_\mathrm{eff}$ only reduces the tension down to the 2.6$σ$ level. This conclusion, totally independent of any CMB data, shows that a high $N_\mathrm{eff}$ accounting for extra relics (either free-streaming or self-interacting) does not provide an obvious solution to the crisis, not even at the level of background cosmology. To circumvent this strong bound, (i) the extra radiation has to be generated after BBN to avoid helium bounds, and (ii) additional ingredients have to be invoked at the level of perturbations to reconcile this extra radiation with CMB and LSS data.

Figures (5)

• Figure 1: Primordial abundances $Y_P$ (colored) and $y_\mathrm{DP}$ (grey) as a function of $(\omega_\mathrm{b}, \Delta N_\mathrm{eff})$ with the PArthENoPE-standard BBN model, with superimposed measurements of Aver et al. Aver:2015iza for $Y_P$ (blue) and Cooke et al. Cooke:2017cwo for $y_\mathrm{DP}$ (black). We can see that a $\Delta N_\mathrm{eff} = N_\mathrm{eff}-3.046$ around zero is preferred, and deviations from it are mostly constrained by the $Y_P$ measurement.
• Figure 2: 68% and 95% confidence levels on $\Omega_m$ and $H_0$ with various data sets: BAO+BBN for galaxy BAO (blue), Lyman-$\alpha$ BAO (green), and combined (red). Additionally displayed is the Riess et al 2019 measurement (orange), and the Planck 2018 measurement (purple). Left: Minimal $\Lambda$CDM model. Right: The $\Lambda$CDM model extended with $N_\mathrm{eff}$ .
• Figure 3: 68% and 95% confidence levels on $\Omega_m$ and $H_0$ for the minimal $\Lambda$CDM model and our combined BAO+BBN data set under various assumptions: Left: BBN predictions taken from PArthENoPE-standard (red), PArthENoPE-Marcucci (green), and PRIMAT (blue). Right: Helium abundance taken from Aver et al. Aver:2015iza (red), Peimbert et al. Peimbert:2016bdg (green), and Izotov et al. Izotov:2014fga (blue).
• Figure 4: 68% and 95% confidence levels on $\Omega_m$, $H_0$ and $N_\mathrm{eff}$ for the $\Lambda$CDM+$N_\mathrm{eff}$ model with various data sets: BAO+BBN for galaxy BAO (blue), Lyman-$\alpha$ BAO (green), and combined (red). Additionally displayed is the Riess et al 2019 measurement (orange), and the Planck 2018 measurement (purple).
• Figure 5: 68% and 95% confidence levels on $\Omega_m$ and $H_0$ for the $\Lambda$CDM+$N_\mathrm{eff}$ model and our combined BAO+BBN data set under various assumptions: Left: BBN predictions taken from PArthENoPE-standard (red), PArthENoPE-Marcucci (green) and PRIMAT (blue). Right: Helium abundance taken from Aver et al. Aver:2015iza (red), Peimbert et al. Peimbert:2016bdg (green), and Izotov et al. Izotov:2014fga (blue).