Helium-4 is the early universe's least negotiable export: its primordial mass fraction Y_p near 0.247 follows from the neutron-to-proton ratio at weak freeze-out, set by known Standard Model physics and the baryon density, with theoretical uncertainty below half a percent. That rigidity makes it the sharpest available probe of the first second: Y_p responds directly to the expansion rate at one second (through N_eff), to neutrino chemistry (through any lepton asymmetry), and to nonstandard energy injection, so a confirmed deviation at even the percent level would be unambiguous new physics. The measurement chain is the soft point: Y_p is extrapolated to zero metallicity from helium emission lines in dwarf galaxies, through emissivity models sensitive to density, temperature, and stellar absorption corrections.
The current ledger is strained from two directions: EMPRESS's extremely metal-poor anchors pull the plateau down toward 0.237 to 0.240, up to 3 sigma below prediction, while independent determinations (Izotov and Thuan's larger samples, CMB-based constraints from Planck damping-tail and ACT) sit consistent with the standard value at coarser precision. The community-wide spread among methods exceeds the quoted individual errors, which is itself the diagnostic: the abundance whose theory is cleanest has measurements whose systematics dominate, and every cosmological application, from N_eff constraints to lepton asymmetry bounds, inherits whichever Y_p its authors adopt.
The standing is foundational metrology under stress: helium constrains the first second only as well as the dwarf-galaxy emission physics is understood, the methods presently disagree beyond their stated errors, and 30-meter-class spectroscopy with resolved density diagnostics is the designated arbiter.