The Baryon-to-Photon Ratio in a Bounced Cosmology: A Consistency Window from Two Independent Eras

The baryon-to-photon ratio is measured twice, by two methods separated by hundreds of thousands of years of cosmic history: once by the acoustic pattern in the microwave background, and once by the light-element abundances, principally deuterium. The allowed entropy production at the bounce is limited to about 2 percent, corresponding to a dilution factor no larger than 1.02 at the stated confidence. At a factor of 1.2 the deuterium prediction would already fall to 1.89e-05. The bounce in this framework must therefore be close to adiabatic: a violent, entropy-generating turnaround is excluded not by taste but by the agreement of two abundance-era measurements. Computed from stated assumptions, not measured.