Big Bang Nucleosynthesis: The First 20 Minutes of the Universe

A 20-minute nuclear reactor

Between about 1 second and 3 minutes after the Big Bang, temperature fell from roughly 10¹⁰ K to 10⁹ K — the narrow window in which nuclear fusion could occur. The neutron-to-proton ratio, frozen at about 1:7 when the weak interactions that had kept protons and neutrons in equilibrium decoupled, is the single number that fixes the final element abundances: it determines how much hydrogen fuses into helium-4 versus how much is left over as free hydrogen.

Predicted primordial abundances (by mass)
⁴He / H ≈ 24-25%   (4 × neutron fraction ≈ 4 × 1/8 ≈ 25%)
D / H   ≈ 2.5×10⁻⁵  (sensitive to baryon density)
³He / H ≈ 10⁻⁵
⁷Li / H ≈ 5×10⁻¹⁰   (lithium problem: measured ~3× lower)

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Why fusion stops: free neutrons run out

As the universe expands and cools past the ~3-minute mark, free neutrons have either decayed into protons or already been locked into helium-4 nuclei, and the raw material for further fusion is exhausted. The abundances calculated in the table above then freeze in place, unchanged for the next 13.8 billion years except for tiny modifications from later stellar processing — this theoretical prediction of roughly 75% hydrogen and 25% helium by mass matches observed primordial element abundances to extraordinary precision, one of the three great pillars of evidence for the Big Bang model alongside Hubble expansion and the cosmic microwave background.

Deuterium: the cleanest baryon-density probe

Deuterium abundance is the cleanest probe of the universe's baryon density: more baryons mean more deuterium gets burned into helium-4, so less deuterium survives. Observations of D/H in high-redshift absorption systems give Ω_b h² = 0.0224 — a value that independently matches the Planck satellite's measurement of the same quantity from the cosmic microwave background, a striking cross-check between two completely different epochs and methods.

The unsolved lithium problem

One puzzle remains open: standard Big Bang nucleosynthesis predicts roughly three times more lithium-7 than is actually observed in old, metal-poor halo stars. Either those stars have destroyed some of their primordial lithium through internal mixing processes, or there is new physics at play. The lithium problem is still unresolved.

Frequently asked questions

Why did nucleosynthesis stop after about 3 minutes?

The universe's temperature fell from about 10 billion to 1 billion kelvin during the nucleosynthesis window, and as free neutrons ran out — many having already decayed into protons or fused into helium-4 — the raw material for further fusion was exhausted, freezing the final element abundances in place.

Why is deuterium the cleanest probe of the universe's baryon density?

More baryons mean more deuterium gets burned into helium-4, so less deuterium survives — making the observed D/H ratio in high-redshift gas a sensitive, direct tracer of baryon density. It gives a value (Ω_b h² = 0.0224) that independently matches the Planck satellite's measurement from the cosmic microwave background.

What is the lithium problem?

Standard Big Bang nucleosynthesis predicts about three times more lithium-7 than is actually observed in old, metal-poor halo stars. Either those stars have destroyed some of their primordial lithium through internal processes, or there is new physics beyond the standard model at play — the discrepancy remains unresolved.

Try it live

Everything above runs in your browser — open Big Bang Nucleosynthesis, drag the baryon density slider, and scrub the time slider from 0.01 seconds to 17 minutes to watch element abundances lock in live. Nothing is installed, nothing is uploaded.

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