= Baryon-density consistency between nucleosynthesis and the microwave background
{title2=$\eta_{10}\simeq274\,\Omega_bh^2$}
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.
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