The Gravitational Constant at Nucleosynthesis: Abundance Bounds Against Drift Extrapolations in the Frame

In the first minutes of the universe the expansion rate set the neutron-to-proton ratio at freeze-out and thereby the helium-4 abundance, which is measured at Yp = 0.245 plus or minus 0.003. Because the expansion rate scales as the square root of the gravitational constant, the abundance translates into an allowed window for G at nucleosynthesis of roughly 0.89 to 1.04 times its present value, under the standard sensitivity of the helium yield to the expansion rate. A power-law drift of G sized to saturate today's lunar-ranging bound would instead imply a ratio of about 1.9 - excluded by orders of magnitude. The abundance window is the deep-time fence that forces any variation of G into late-time or saturating profiles, a constraint the frame's density-driven coupling is shaped to respect. Computed from published inputs, not measured.