Once the first atoms formed, what was the composition of the universe?

Question

BlackTom AI · Accepted Answer

Big Bang nucleosynthesis produced about 75% hydrogen and 25% helium by mass.

Relevant concepts: In the early universe, protons and neutrons interconverted until weak interactions froze out. The neutron-to-proton ratio (n/p) at the onset of nucleosynthesis (after some neutrons decayed) was approximately 1/7. Almost all available neutrons were bound into helium-4 (4He) because 4He is the most stable light nucleus; only trace amounts of deuterium, helium-3, and lithium-7 formed.

Step-by-step reasoning and calculation:
1. Assume n/p ≈ 1/7. Let n be the number of neutrons and p = 7n be the number of protons. Total nucleons = n + p = n + 7n = 8n.
2. Each helium-4 nucleus contains 2 protons and 2 neutrons. Since nearly every neutron ends up in 4He, the number of helium nuclei formed ≈ n/2 (because 2 neutrons per helium).
3. Mass (in nucleon units) bound in helium = 4 × (number of helium nuclei) = 4 × (n/2) = 2n.
4. Mass fraction of helium (Y_He) = mass in helium / total nucleon mass = (2n) / (8n) = 1/4 = 0.25 → 25% by mass.
5. Remaining mass is hydrogen (basically free protons): 100% − 25% = 75% hydrogen by mass.

Why the correct answer is correct: The n/p ≈ 1/7 argument directly yields a 25% mass fraction in helium because each helium-4 uses two neutrons, giving 2n nucleons in helium out of 8n total. This derivation matches detailed BBN calculations and observations of primordial abundances.

Why the other options are incorrect:
- "50% hydrogen, 50% helium" would require n/p ≈ 1/1, which is not supported by weak-interaction freeze-out and neutron decay.
- "25% hydrogen, 25% helium, 50% all other elements" is impossible: heavier elements are produced later in stars, not in primordial nucleosynthesis, and their primordial abundance is negligible.
- "All elements in equal proportions" contradicts nuclear physics and observations; light elements dominate.

Conclusion: The universe’s composition after the first atoms formed was about 75% hydrogen and 25% helium, which matches the provided answer.

Once the first atoms formed, what was the composition of the universe?Single choice

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