The BAO+BBN take on the Hubble tension (1907.11594v2)

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 $\Lambda$CDM assumption at the 3.2$\sigma$ 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$\sigma$ 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.

The BAO+BBN Approach to Tackling the Hubble Tension

The paper "The BAO+BBN take on the Hubble tension" authored by Nils Schöneberg, Julien Lesgourgues, and Deanna C. Hooper presents an intriguing examination of the persistent discrepancy known as the Hubble tension. This tension arises from the disparity between the cosmic microwave background (CMB) inferred value of the Hubble constant $H_0$ and the value obtained from local measurements, particularly those involving supernovae calibrated using Cepheids by the SH0ES collaboration.

Core Investigations of the Paper

The research employs Baryon Acoustic Oscillation (BAO) and Big Bang Nucleosynthesis (BBN) data to independently assess the cosmological background history, separate from the commonly used CMB maps and supernovae data. This approach allows for an independent probe of $H_0$ without relying on CMB data, which is significant given the potential model dependencies and biases inherent in the latter.

The paper reveals that, under the standard $\Lambda$CDM cosmological model, the combined BAO and BBN data indicate a 3.2$\sigma$ tension with the SH0ES measurement, corroborating the CMB-derived $H_0$ values. The introduction of an additional radiation density parameter $N_{\rm eff}$, which accounts for extra relics, slightly alleviates this tension, reducing it to 2.6$\sigma$. Despite this reduction, the enhanced $N_{\rm eff}$ fails to substantively resolve the tension.

Methodology and Findings

The authors employ advanced BBN assumptions and constraints from primordial deuterium and helium measurements. They confirm that the BAO+BBN approach robustly supports the CMB-consistent $H_0$ estimates, especially when stringent helium bounds are enforced.

It is noted that the extension involving a floating $N_{\rm eff}$ parameter does not provide a clear resolution to the Hubble tension problem. This is because many of the extended cosmological models require additional post-BBN radiation to bypass the helium constraints, coupled with more complexity at the perturbation level to reconcile these models with CMB and Large Scale Structure (LSS) data.

Implications and Future Directions

This paper accentuates the significance of the BAO and BBN combined probe as a potent approach for discerning the universe's background cosmology independently of CMB data. The research underscores the inadequacy of simple extensions to $\Lambda$CDM in resolving the Hubble tension without introducing extra complexity at the perturbation level or altering post-BBN physics.

Going forward, the paper suggests that future research should focus on refining measurements of primordial element abundances, especially for helium, and improving theoretical constraints on BBN predictions. Progress in these areas could enhance the precision of $N_{\rm eff}$ bounds and, potentially, the $H_0$ inferences derived from BAO+BBN analyses. Furthermore, models proposing additional post-BBN radiation or interactions could see greater scrutiny and experimental validation in attempts to bridge the gap presented by the Hubble tension.

In conclusion, while the BAO+BBN framework contributes valuable insights into cosmological parameter estimation free of CMB biases, the quest to resolve the Hubble tension remains an open challenge, demanding innovative approaches and refinements to current theoretical models.