Solution
= Solution
A larger $\eta$ increases the deuterium fraction at fixed <temperature>, so the bottleneck ends at a higher <temperature> and earlier time. With weak decoupling unchanged, fewer free neutrons then beta-decay before nuclear reactions bind them. Since primordial helium production is neutron-limited, the <baryon-density effect on primordial helium> makes $Y_{\rm He}$ larger than the standard value.