Past exam of the mathematics course of the University of Cambridge / 2010 / iii / Paper 53 / 2 / i / g / Solution

A faster expansion at fixed temperature makes the weak reaction rate fall below $H$ sooner, at higher temperature, leaving a larger neutron-to-proton ratio. It also shortens the time before the deuterium bottleneck ends, reducing neutron decay. The helium-4 abundance increases, assuming nuclear rates and the temperature history of the particle bath are otherwise unmodified.