= Helium sensitivity to the expansion rate
{title2=$T_f\propto(Gg_*)^{1/6}$}
During radiation domination, the weak conversion rate scales as $T^5$ whereas the <Hubble parameter> scales as $\sqrt{Gg_*}\,T^2$. Their crossing fixes the <cosmological weak freeze-out> temperature. A faster expansion raises it, leaving a larger <neutron-to-proton ratio>, and shortens the interval for <free-neutron decay after freeze-out>. Both effects increase the helium-4 mass fraction if the extra component does not independently change reaction rates or inject decay products.
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