Nuclear-burning qualification of the helium expansion-rate response

A moderately enhanced Hubble parameter raises the primordial helium mass fraction through earlier cosmological weak freeze-out and less free-neutron decay after freeze-out, provided nuclear burning still incorporates nearly all surviving neutrons into helium-4. This provision matters: if expansion carries the gas through the nuclear-burning temperature range faster than the reaction times, neutron-limited helium synthesis no longer applies. More neutrons surviving the bottleneck alone does not prove more final helium in that limit.