3.2 Primordial nucleosynthesis and light element abundances

Primordial Nucleosynthesis

Within the first few minutes after the Big Bang, the universe was hot and dense enough for nuclear fusion to occur. Protons and neutrons slammed together and fused into the lightest elements: hydrogen, helium, and trace amounts of lithium. This process is called primordial nucleosynthesis, and it set the chemical starting point for everything that came after.

The Big Bang model predicts specific ratios of these light elements, and observations match those predictions remarkably well. That agreement is one of the strongest pieces of evidence supporting the Big Bang theory.

Process of Primordial Nucleosynthesis

Primordial nucleosynthesis took place roughly 10 seconds to 20 minutes after the Big Bang. During this window, temperatures dropped from about $10^{10}$ K to $10^{9}$ K, cool enough for nuclei to stick together but still hot enough for fusion to proceed.

Here's how the fusion chain worked:

1. Protons and neutrons fused into deuterium (a hydrogen isotope with one proton and one neutron). This is the essential first step because nearly all heavier nuclei build from deuterium.
2. Deuterium nuclei fused further, producing helium-3 (two protons, one neutron) and tritium (one proton, two neutrons).
3. Helium-3 and tritium then fused to form helium-4 (two protons, two neutrons), which is by far the most abundant product after hydrogen.

Trace amounts of lithium-7 and beryllium-7 also formed, but the rapidly dropping temperature shut down fusion before anything heavier could be built. That's a key distinction: primordial nucleosynthesis only produced the lightest elements. Heavier elements like carbon and oxygen were forged much later inside stars through stellar nucleosynthesis.

Predicted Light Element Abundances

The Big Bang model makes precise, testable predictions about how much of each light element should exist. These predictions depend heavily on one parameter: the baryon-to-photon ratio (the density of ordinary matter in the early universe).

• Hydrogen: ~75% of all baryonic matter by mass
• Helium-4: ~25% of all baryonic matter by mass, giving a helium-to-hydrogen ratio of roughly 1:12 by mass
• Deuterium: ~1 part in $10^5$ relative to hydrogen
• Helium-3: ~1 part in $10^5$ relative to hydrogen
• Lithium-7: ~1 part in $10^{10}$ relative to hydrogen

The deuterium abundance is especially useful as a diagnostic tool. Unlike helium-4, deuterium is fragile and gets destroyed inside stars rather than created. So any deuterium we observe today must be primordial, making it a clean test of the model.

Observational Evidence and Implications

Evidence Supporting the Predictions

Several independent lines of observation confirm the predicted abundances:

• Cosmic microwave background (CMB): The CMB is the remnant radiation from when the universe cooled enough for atoms to form, about 380,000 years after the Big Bang. Its spectrum encodes information about the baryon density, which directly constrains the predicted element ratios. Measurements from satellites like WMAP and Planck are consistent with the nucleosynthesis predictions.
• Deuterium in distant quasar absorption systems: When light from distant quasars passes through primordial gas clouds, absorption lines reveal the deuterium content of that gas. Because these clouds have undergone very little stellar processing, they preserve near-primordial compositions. Measured deuterium abundances match the Big Bang prediction well.
• Helium in low-metallicity regions: Astronomers measure helium-4 abundances in metal-poor galaxies and H II regions (clouds of ionized hydrogen). These environments have experienced minimal enrichment from stellar nucleosynthesis, so their helium content reflects primordial levels. The observed helium-to-hydrogen ratio of roughly 1:12 by mass matches the prediction.
• The cosmological lithium problem: One notable tension exists. The observed lithium-7 abundance in old, metal-poor stars is about a factor of 3 lower than what the standard model predicts. This discrepancy remains an active area of research and could point to new physics or to poorly understood processes that deplete lithium in stellar atmospheres.

Implications for the Big Bang Theory

The close match between predicted and observed light element abundances is one of the three classic pillars of the Big Bang theory, alongside the expansion of the universe and the CMB.

This agreement tells us that the universe really was once hot enough and dense enough for nuclear fusion to occur everywhere, and that it expanded and cooled on a timescale consistent with the model. The fact that a single parameter (baryon density) simultaneously fits the abundances of multiple elements makes the case especially compelling. It would be very difficult for an alternative cosmological model to reproduce these results by coincidence.

Together with the CMB and the observed expansion of the universe (Hubble's law), primordial nucleosynthesis provides a coherent, well-tested framework for understanding the origin and early evolution of the universe.