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

The helium yield is primarily neutron-limited, so small changes in cosmological baryon density have only a weak effect compared with their effect on residual deuterium. Increasing the baryon-to-photon ratio makes deuterium destruction less effective relative to its formation and releases the deuterium bottleneck at a slightly higher temperature. Nuclear reactions also proceed faster. Fewer neutrons decay before capture, so a small increase in baryon density slightly increases helium-4; a decrease slightly lowers it.