Big Bang Nucleosynthesis (BBN) occurred during the first ~20 minutes of the universe,
when temperatures fell from ~10¹² K to ~10⁹ K. A network of nuclear reactions converted
free neutrons and protons into light nuclei: D, ³He, ⁴He, ⁷Li.
The ⁴He mass fraction Y_p ≈ 0.245 is primarily set by the n/p ratio at freeze-out,
controlled by the neutron lifetime and Neff.
The D/H ratio is a sensitive baryometer — higher η destroys more deuterium via p+D→³He.
Observations agree with ΛCDM (η ≈ 6.1×10⁻¹⁰) to ~1%.