Is the primordial helium abundance consistent with standard nucleosynthesis?
In plain words
The fraction of mass in helium after the Big Bang is predicted to about one part in a thousand. One survey of tiny star-forming galaxies found less helium than predicted, which could point to an asymmetry between neutrinos and antineutrinos.
Precise statement
Standard BBN with the CMB baryon density gives helium mass fraction $Y_{p} \sim 0.247$, while the EMPRESS survey of extremely metal-poor galaxies reported $Y_{p} \sim 0.237$ (2022, moderate confidence on the exact value), interpretable as an electron-neutrino chemical potential $\xi_{e} \sim 0.05$. Determine $Y_{p}$ with uncertainty below 0.003 from emission-line galaxies and CMB damping tails and state whether it agrees with standard BBN.
What would settle it
Larger samples of metal-poor galaxies with near-infrared helium lines plus CMB-S4-class measurement of $Y_{p}$.
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