Understanding BBN: the physics and its history

Understand

Big-bang nucleosynthesis (BBN), today a pillar of modern cosmology, began with the trailblazing 1948 paper of Alpher, Bethe and Gamow.

• In it, they proposed non-equilibrium nuclear processes in the early Universe ($t \sim 1000\,$sec) and an early radiation-dominated phase to explain the abundances of all the chemical elements.
• Their model was fundamentally flawed, but initiated a complex and interesting path to the modern theory of BBN, which explains only the abundances of the lightest chemical elements (mostly $^4$He) and the discovery of the cosmic microwave background (CMB).
• The purpose of this paper is to clarify the basic physics of BBN, adding some new insights, and to describe how the modern theory developed.