[arugfchm] Quantitative Big Bang nucleosynthesis

How difficult is it to compute that the universe will be roughly 75% hydrogen and 25% helium after the Big Bang?

Difficulty probably depends on what assumptions you are allowed to make, including whether one may use measured (instead of calculated from first principles) strengths of subatomic interactions.

Create a game in which the player experiments with creating universes of different compositions by playing with the fundamental constants of the universe.