Primordial Nucleosynthesis and the First Three Minutes: A Precision Test of Cosmic Expansion

 Big Bang nucleosynthesis (BBN) represents the earliest epoch from which we have direct observational evidence, occurring roughly 10 seconds to 20 minutes after the initial expansion. This study provides an updated analysis of the predicted abundances of deuterium, helium-4, and lithium-7 as a function of the baryon-to-photon ratio. By comparing these theoretical predictions to high-resolution spectroscopic observations of metal-poor stars and quasar absorption systems, we demonstrate remarkable agreement for deuterium and helium-4, confirming the standard model’s prediction of a universal cosmic expansion rate. However, we also reaffirm the persistent "lithium problem" — a factor of 3–4 discrepancy between predicted and observed lithium-7 abundance — and evaluate whether non-standard physics (e.g., decaying dark matter or variations in the fine-structure constant) can resolve this anomaly without disrupting other BBN successes.