[Paper Review] The primordial helium abundance and the number of neutrino families

Antonio Peimbert, M. Peimbert+1 more|Redalyc (Universidad Autónoma del Estado de México)|Aug 6, 2016

Dark Matter and Cosmic Phenomena4 references3 citations

TL;DR

This study re-evaluates the primordial helium abundance $Y_P$ using updated recombination coefficients from Porter et al. (2013) and high-resolution H II region data, yielding $Y_P = 0.2446 \pm 0.0029$. Combining this with Big Bang nucleosynthesis models, it derives $N_{\text{eff}} = 2.90 \pm 0.22$, consistent with three active neutrino families and a neutron mean lifetime of $\tau_n = 872 \pm 14$ seconds, challenging earlier high-$Y_P$ determinations that implied a fourth neutrino family.

ABSTRACT

Based on observations of HII regions and the new computations of the recombination coefficients of the He I lines by Porter et al. (2013) we obtain a primordial helium abundance by mass of $Y_P = 0.2446\pm0.0029$. We consider thirteen sources of error for the $Y_P$ determination, some of them are mainly due to systematic effects, while the rest are mainly due to statistical effects. We compare our results with other determinations of $Y_P$ present in the literature. Combining our $Y_P$ value with computations of primordial nucleosynthesis we find a number of neutrino species $N_{eff} = 2.90\pm0.22$, and a neutron mean life $τ_ν = 872\pm14(s)$.

Motivation & Objective

• To improve the precision and reduce systematic errors in the determination of the primordial helium abundance $Y_P$ using H II regions.
• To incorporate updated effective recombination coefficients for He I lines from Porter et al. (2013) to refine $Y_P$ calculations.
• To assess the consistency of $Y_P$ with standard Big Bang nucleosynthesis (SBBN) and constrain the number of neutrino families $N_{\text{eff}}$ and neutron mean lifetime $\tau_n$.
• To resolve discrepancies between recent $Y_P$ determinations, particularly the tension with Izotov et al. (2014) which implied a fourth neutrino family.
• To validate the robustness of $Y_P$ by applying tailor-made physical models to individual low-metallicity H II regions, minimizing systematic uncertainties.

Proposed method

• Utilizes high-resolution spectroscopic data from five low-metallicity H II regions (SBS 0335–052, I Zw 18, Haro 29, etc.) to derive individual helium abundances.
• Applies tailor-made photoionization models for each H II region to account for unique physical conditions, including electron temperature and density structures.
• Incorporates updated effective recombination coefficients for He I lines from Porter et al. (2013), which correct case B calculation errors and improve atomic physics accuracy.
• Performs a comprehensive error budget analysis considering 13 sources of uncertainty, distinguishing between statistical and systematic effects.
• Combines the derived $Y_P$ with Big Bang nucleosynthesis (SBBN) predictions using the relation $\Delta N_{\text{eff}} = 75 \Delta Y$ to infer $N_{\text{eff}}$ and $\tau_n$.
• Cross-validates results with cosmic microwave background (CMB) constraints from Planck, combining $Y_P$ from H II regions and CMB to derive combined $\tau_n$ and $N_{\text{eff}}$ values.

Experimental results

Research questions

• RQ1 What is the most accurate determination of the primordial helium abundance $Y_P$ when updated atomic physics parameters are applied?
• RQ2 How do systematic errors—particularly from reddening, ionization correction factors, and recombination coefficients—affect $Y_P$ determinations?
• RQ3 Does the derived $Y_P$ value support the standard model prediction of three active neutrino families ($N_{\text{eff}} = 3.046$) or indicate new physics such as a fourth neutrino family?
• RQ4 What is the implied neutron mean lifetime $\tau_n$ from the $Y_P$ value, and how does it compare with laboratory measurements?
• RQ5 Why is the $Y_P$ value from this work in significant tension (>3σ) with the result from Izotov et al. (2014), which implied $N_{\text{eff}} \approx 3.58$?

Key findings

• The updated primordial helium abundance is $Y_P = 0.2446 \pm 0.0029$, derived using improved He I recombination coefficients and tailored models for individual H II regions.
• The result is consistent with the standard Big Bang nucleosynthesis prediction of $N_{\text{eff}} = 3.046$, yielding $N_{\text{eff}} = 2.90 \pm 0.22$, which disfavors a fourth neutrino family at high significance.
• The derived neutron mean lifetime is $\tau_n = 872 \pm 14$ seconds, consistent with both bottle and beam laboratory measurements and with the Planck CMB constraint of $\tau_n = 907 \pm 69$ s.
• The $Y_P$ value is in good agreement with Aver et al. (2015) but in significant tension (>3σ) with Izotov et al. (2014), whose higher $Y_P = 0.2551 \pm 0.0022$ implies $N_{\text{eff}} = 3.58 \pm 0.25$ at 68% CL.
• The study confirms that systematic errors dominate $Y_P$ uncertainty, and that tailored modeling of individual H II regions is essential for precision cosmology.
• Combining $Y_P$ from H II regions with CMB data yields $\tau_n = 872 \pm 14$ s and $N_{\text{eff}} = 2.90 \pm 0.22$, reinforcing consistency with the standard model of cosmology.