---
title: "The first minutes and the light elements"
description: "Follow neutrons from the first second to the first helium, then recompute a small reaction network and read the published lithium and radiation bounds."
canonical_url: "https://sandboxphysics.com/stories/primordial-nucleosynthesis/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/primordial-nucleosynthesis/"
last_modified: "2026-11-14"
---

# The first minutes and the light elements

Sandbox Physics · 2026-11-12

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.

1. L1 · Concept

   ## [How temperature and expansion speed set helium and deuterium](https://sandboxphysics.com/stories/primordial-nucleosynthesis/temperature-expansion-and-light-elements/)

   In the first minutes, cooling decided when nuclei could survive and expansion decided how many neutrons were left to build helium and deuterium.

2. L1 · Chapter

   ## [When neutrons and protons stop turning into each other](https://sandboxphysics.com/stories/primordial-nucleosynthesis/when-neutrons-and-protons-stop-converting/)

   Derive the equilibrium neutron-to-proton ratio, compare the weak conversion rate with the expansion rate, and estimate the helium fraction.

3. L2 · Chapter

   ## [Why deuterium waits for the universe to cool](https://sandboxphysics.com/stories/primordial-nucleosynthesis/why-deuterium-waits/)

   Use nuclear statistical equilibrium to find when deuterons survive the photon bath, then follow why denser matter leaves less deuterium.

4. L3 · Chapter

   ## [How baryon density and extra radiation move the light elements](https://sandboxphysics.com/stories/primordial-nucleosynthesis/baryon-density-radiation-and-abundance-sensitivity/)

   Define logarithmic sensitivities, infer the baryon density from deuterium, bound extra radiation with helium and combine rate errors.

5. L3 · Method

   ## [Recompute a small network from a frozen rate grid and propagate rate errors](https://sandboxphysics.com/stories/primordial-nucleosynthesis/recompute-a-frozen-rate-network/)

   Download the frozen rate grid and Python scripts, rerun the twelve-reaction network, compare with published tables and propagate rate errors.

6. L3 · Paper guide

   ## [Reading the BBN evidence on extra radiation and the lithium problem](https://sandboxphysics.com/stories/primordial-nucleosynthesis/reading-lithium-and-extra-radiation-bounds/)

   Walk through the PDG 2025 nucleosynthesis review and Fields et al. 2020, separating measurements, predictions, conditional bounds and open questions.