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 2a 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.

Automated summary

The recent measurement of helium-4 from the near-infrared spectroscopy of extremely metal-poor galaxies may indicate a new puzzle in the early Universe. By combining this measurement with the determination of primordial deuterium, researchers have found indications for a nonvanishing lepton asymmetry during the big bang nucleosynthesis era, suggesting cosmological implications beyond lambda cold dark matter. These findings highlight the importance of considering uncertainties in the nuclear reaction network and exploring conditions for a large total lepton asymmetry in the early Universe.