On Cosmological Low Entropy After the Big Bang: Universal Expansion and Nucleosynthesis

TL;DR

This paper explores how variations in the universe's expansion and baryon-to-photon ratio affect nuclear entropy after Big Bang nucleosynthesis, finding that these factors are loosely constrained by light element remnants.

Contribution

It constructs counterfactual cosmologies to quantify the sensitivity of post-BBN nuclear entropy to key cosmological parameters.

Findings

Post-BBN nuclear entropy is linked to baryogenesis and expansion history.

Constraints from light element remnants are weak.

Significant variations in parameters do not drastically alter nuclear entropy.

Abstract

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.