The first minutes and the light elements
Start with the question of where the helium in the Sun and the deuterium in distant gas came from. Three chapters derive the answer step by step, a method guide recomputes it, and a paper guide reads what the latest reviews conclude.
L1 · Concept
How temperature and expansion speed set helium and deuterium
In the first minutes, cooling decided when nuclei could survive and expansion decided how many neutrons were left to build helium and deuterium.
L1 · Chapter
When neutrons and protons stop turning into each other
Derive the equilibrium neutron-to-proton ratio, compare the weak conversion rate with the expansion rate, and estimate the helium fraction.
L2 · Chapter
Why deuterium waits for the universe to cool
Use nuclear statistical equilibrium to find when deuterons survive the photon bath, then follow why denser matter leaves less deuterium.
L3 · Chapter
How baryon density and extra radiation move the light elements
Define logarithmic sensitivities, infer the baryon density from deuterium, bound extra radiation with helium and combine rate errors.
L3 · Method
Recompute a small network from a frozen rate grid and propagate rate errors
Download the frozen rate grid and Python scripts, rerun the twelve-reaction network, compare with published tables and propagate rate errors.
L3 · Paper guide
Reading the BBN evidence on extra radiation and the lithium problem
Walk through the PDG 2025 nucleosynthesis review and Fields et al. 2020, separating measurements, predictions, conditional bounds and open questions.