One Asymmetry, Two Measurements: Nucleosynthesis and Microwave-Background Consistency

The baryon-to-photon ratio is measured twice, by unrelated physics separated by hundreds of thousands of years: by the light-element abundances from the first minutes and by the acoustic peaks of the microwave background from year 380,000. This paper checks that a single value satisfies both, which is the final consistency gate any baryogenesis mechanism, in any cosmology, must pass. At the microwave-background value eta = 6.12e-10, the predicted primordial deuterium abundance is D/H = 2.60e-05, against the observed 2.53e-05; the predicted helium mass fraction is 0.247. Shifting eta to 5.8e-10 raises deuterium to 2.83e-05, and 6.6e-10 lowers it to 2.30e-05, both outside the tight observational interval. A bounce between production and nucleosynthesis would show up here as a mismatch, because entropy production after production but before the first minutes changes eta between the two measurements; the observed agreement bounds any such late entropy release tightly. Computed from stated assumptions, not measured.