Big Bang Nucleosynthesis

Protons and neutrons were available before efficient nuclear assembly. What changed was the balance between reactions that could bind them together, radiation that could break fragile nuclei apart, and expansion that steadily reduced temperature and density.[1]

These are approximate stages, not exact boundaries shared by every reaction.[1]

At high temperatures, weak interactions repeatedly converted neutrons into protons and protons into neutrons. Because a neutron is slightly heavier, cooling favored a smaller neutron fraction. Around the first second, interconversion became too slow to maintain equilibrium—a gradual process called freeze-out.[2]

The approximate ratio fell from about one neutron for six protons near freeze-out to one for seven when efficient nuclear assembly began. Nearly all the surviving neutrons then entered helium-4 nuclei, each containing two protons and two neutrons.[2]

Free-neutron beta decay contributed to that decline. It produces a proton, an electron, and an electron antineutrino. A free neutron’s mean lifetime is about fifteen minutes, corresponding to a half-life of about ten minutes.[3]

Deuterium, or hydrogen-2, is a nucleus containing one proton and one neutron. It provides the essential bridge to larger light nuclei. But in the early plasma, energetic photons could break newly formed deuterium apart almost as soon as it appeared.[4]

Deuterium’s binding energy is about 2.2 MeV. Nevertheless, it did not survive efficiently until the thermal energy scale k_BT had fallen to roughly 0.08–0.1 MeV, around a billion kelvins. The reason is the enormous number of photons per baryon: even a small high-energy fraction could keep destroying it.[4]

Once deuterium could survive, several connected reactions rapidly built helium. The symbols below name nuclei: D is deuterium, T is tritium or hydrogen-3, and γ is a photon.[5]

The superscript counts nucleons: ³He has three, while ⁴He has four.[5]

Why much of the lithium began as beryllium

At the baryon density favored by observations, much of the primordial mass-seven material was made as beryllium-7. It later captured electrons and converted into lithium-7. That delayed conversion is part of what astronomers mean by the primordial lithium yield.[6]

The later electron-capture reaction is ⁷Be + e⁻ → ⁷Li + ν_e, with an electron neutrino emitted. The ionization state of the early plasma delays this process, so a neutral beryllium atom’s laboratory half-life is not the appropriate cosmic clock.[6]

Why the reactions did not fill the periodic table

There are no stable nuclei with mass numbers five or eight, interrupting easy steps beyond helium. Extended calculations produce extraordinarily small heavy-element traces, but almost no carbon, nitrogen, or oxygen compared with later stellar production.[7]

The broad outcome was approximately 75% hydrogen-1 and 25% helium-4 by ordinary-matter mass, with small amounts of other light nuclei. These percentages describe baryonic material; they do not include dark matter or dark energy.[8]

Deuterium is especially useful as a baryometer: in standard BBN, a higher baryon density generally allows more deuterium to be burned into heavier light nuclei, leaving less behind. Its residual abundance therefore constrains the cosmic supply of ordinary matter.[12]

The microwave background provides a separate measurement. Its acoustic pattern constrains the baryon density hundreds of thousands of years after nucleosynthesis, within a cosmological model. Comparing this with BBN tests whether one thermal history can explain both epochs.[13]

Broad agreement between deuterium, helium, and the CMB is a major success. At higher precision, the result depends on adopted nuclear reaction rates and observational uncertainties; some rate treatments yield a deuterium–CMB discrepancy. “Perfect agreement” would hide the work still being done.[12]

Standard calculations predict more lithium-7 than is inferred from the surfaces of many old, metal-poor stars—roughly a factor of three more. This cosmological lithium problem is a real qualification to the otherwise successful light-element picture.[16]

A stellar surface is not an untouched sample of primordial gas. Mixing, diffusion, and nuclear destruction can change the lithium visible there. Researchers also test nuclear reaction inputs and possible early-universe physics, but no explanation has achieved an unambiguous, broadly accepted resolution.[16]

A proposed solution must preserve the successful parts of BBN. Changing an early reaction or adding a decaying particle can alter deuterium and helium as well as lithium, so reducing one mismatch is not enough.[16]

BBN depends on more than nuclear fusion. Weak interactions set the neutron fraction; gravity determines the expansion rate; electromagnetic effects influence the plasma and nuclear collisions. That combination lets light-element abundances test the consistency of several parts of physics at once.[17]

The relativistic energy density is often expressed using N_eff, the effective number of neutrino species. It is a way of describing radiation density, not a direct count of neutrino flavors. Standard neutrino decoupling itself gives a value slightly above three because the process is gradual.[18]

Additional radiation would speed expansion. Weak reactions would then lose equilibrium earlier, typically preserving more neutrons and increasing helium production. Measurements can constrain such changes, along with scenarios involving unstable particles or altered interactions, under the assumptions of each model.[17]

From nuclei to stars

For hundreds of thousands of years after BBN, the universe remained an ionized plasma. Neutral atoms became abundant near 380,000 years after the hot Big Bang, greatly reducing photon scattering. The CMB photons already existed in an older thermal radiation bath; they were not all created when atoms formed.[9]

The first stars followed much later, on the scale of hundreds of millions of years. Nuclear fusion inside stars began a new chapter of element production, building on the hydrogen and helium mixture inherited from the early universe.[19]

The Grand Beginning

1. The Singularity and Moment of Creation
2. Quantum Fluctuations and Inflation
3. Big Bang Nucleosynthesis · You are here
4. Matter vs. Antimatter
5. Cooling and the Formation of Fundamental Particles
6. The Cosmic Microwave Background (CMB)
7. Dark Matter
8. Dark Energy
9. Recombination and the First Atoms
10. The Dark Ages and First Structures
11. Reionization: Ending the Dark Ages