Indications for a Nonzero Lepton Asymmetry from Extremely Metal-Poor Galaxies

The recent measurement of helium-4 from the near-infrared spectroscopy of extremely metal-poor galaxies by the Subaru Survey may point to a new puzzle in the early Universe. We exploit this new helium measurement together with the percent-level determination of primordial deuterium, to assess indications for a nonvanishing lepton asymmetry during the big bang nucleosynthesis era, paying particular attention to the role of uncertainties in the nuclear reaction network. A cutting-edge Bayesian analysis focused on the role of the newly measured extremely metal-poor galaxies, jointly with information from the cosmic microwave background, suggests the existence of a nonzero lepton asymmetry at around the $2\sigma$ level, providing a hint for cosmology beyond lambda cold dark matter. We discuss conditions for a large total lepton asymmetry to be consistently realized in the early Universe.

Figure 1

68% and 95% two-dimensional probability distribution of the primordial chemical potential of neutrinos, ${\mu }_{\nu }$, normalized to the neutrino temperature $T_{\nu }$, and the number of extrarelativistic degrees of freedom in the early Universe, $\mathrm{\Delta }{N}_{\mathrm{eff}}$, from a state-of-the-art analysis of BBN and CMB data. The red and blue contours indicate the results for two different sets of nuclear uncertainties; magenta lines the $\mathrm{\Lambda }\mathrm{CDM}$ prediction.

Figure 2

Probability density function (p.d.f.) for the primordial light elements analyzed in this Letter. In the left panel, the p.d.f. for helium-4, $Y_P$, as precisely predicted in the SM according to two different sets of nuclear uncertainties and adopting the determination of the cosmological baryon-to-photon ratio from the fit of CMB data within $\mathrm{\Lambda }\mathrm{CDM}$ (color code similar to Fig. 1). In the same panel, the outcome from the joint fit to BBN and CMB likelihoods in the BSM scenario where ${\xi }_{\nu }$ and $\mathrm{\Delta }{N}_{\mathrm{eff}}$ are consistently allowed to differ from their $\mathrm{\Lambda }\mathrm{CDM}$ limit. In the right panel, the same set of p.d.f.s is shown for the deuterium. In both panels, vertical dark green bands correspond to the $1\sigma$ interval for the BBN measurements employed in the analysis. In the left one, the PDG 2021 recommended value for helium-4 is also reported, in agreement with the SM prediction.