The effective number of relativistic species in the early universe, N_eff, counts how many light particle species contributed to the radiation energy density during Big Bang nucleosynthesis and recombination. The standard model predicts N_eff = 3.044 — three neutrino species, plus a small correction from neutrino heating during electron-positron annihilation. Any deviation from this number signals new physics: additional light particles, modified neutrino interactions, or exotic radiation components.
Goldstein and Hill (arXiv:2603.13226) combine primordial helium and deuterium abundances, CMB data from multiple experiments, and baryon acoustic oscillation measurements to achieve N_eff = 2.990 ± 0.070 — a 2% determination, the tightest constraint to date. The result is consistent with exactly three neutrino species and leaves room for excess contributions below 0.107 at 95% confidence.
The precision matters because it closes doors. Models that add a light sterile neutrino (N_eff ≈ 4) are excluded by many standard deviations. Models that propose extra radiation to resolve the Hubble tension (which require ΔN_eff ≈ 0.2-0.4) are increasingly constrained — the 95% upper limit of 0.107 on excess contributions is below what most such models need. The measurement doesn't resolve the Hubble tension; it eliminates one class of solutions.
The methodology deliberately avoids large-scale CMB polarization data, which allows consistent integration of current CMB and BAO datasets without the systematic uncertainties that polarization measurements introduce. This is measurement discipline: accepting a slightly larger error bar to ensure the systematic floor is below the statistical uncertainty.