Baryon-density consistency between nucleosynthesis and the microwave background (${\eta }_{10}\simeq 274{\Omega }_b{h}^2$)

For the measured mean cosmic microwave background temperature, the baryon-to-photon ratio and cosmological baryon density obey the displayed conversion. Big Bang nucleosynthesis, especially deuterium, measures this density during the first minutes, while acoustic peak ratios measure it near recombination. Agreement tests the thermal history and approximate conservation of baryon number and photon entropy between these epochs. It is not a claim that every observed light-element abundance has no residual tension.