  We investigate the sensitivity of a universe's nuclear entropy after Big Bang
nucleosynthesis (BBN) to variations in both the baryon-to-photon ratio and the
temporal evolution of cosmological expansion. Specifically, we construct
counterfactual cosmologies to quantify the degree by which these two parameters
must vary from those in our Universe before we observe a substantial change in
the degree of fusion, and thus nuclear entropy, during BBN. We find that, while
the post-BBN nuclear entropy is indeed linked to baryogenesis and the
Universe's expansion history, the requirement of leftover light elements does
not place strong constraints on the properties of these two cosmological
processes.
