13.4 Nucleosynthesis and the early universe

The early universe's composition was shaped by primordial nucleosynthesis, a process that occurred within minutes after the Big Bang. As the universe cooled, protons and neutrons fused to form light elements, setting the stage for cosmic chemistry.

This process resulted in a universe composed of roughly 75% hydrogen and 25% helium by mass. The observed abundance of these elements, along with trace amounts of lithium, provides crucial evidence supporting the Big Bang theory and our understanding of the early universe.

Primordial Nucleosynthesis

Process of primordial nucleosynthesis

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• Primordial nucleosynthesis occurred during first few minutes after Big Bang between 10 seconds and 20 minutes transformed early universe composition
• Universe cooled and expanded allowing nuclear reactions to take place as temperature dropped below 10^10 K
• Protons and neutrons combined formed light elements through series of fusion reactions
• Process limited by rapid expansion and cooling of universe halted further element formation

Formation of light elements

• Proton-neutron ratio stabilized at about 7:1 due to weak nuclear interactions reaching equilibrium
• Deuterium formation initiated when universe cooled to about 1 billion K through fusion of proton and neutron
• Helium-4 production followed two-step process: deuterium fused with proton to form helium-3, then helium-3 combined with neutron
• Trace amounts of lithium-7 synthesized through fusion of helium-4 with tritium
• Majority of neutrons incorporated into helium-4 nuclei resulting in ~25% helium abundance by mass

Observational Evidence and Cosmic Abundance

Evidence for primordial nucleosynthesis

• Observed abundance of light elements in universe matches theoretical predictions (hydrogen, helium, lithium)
• Cosmic Microwave Background radiation provides crucial information about early universe conditions temperature and density
• Spectroscopic observations of old, metal-poor stars reveal primordial element abundances uncontaminated by stellar nucleosynthesis
• Deuterium abundance in high-redshift quasar absorption systems consistent with primordial nucleosynthesis predictions confirms theory's validity

Role in elemental abundance

• Established initial composition of universe approximately 75% hydrogen, 25% helium by mass shaped cosmic chemistry
• Set stage for later stellar nucleosynthesis provided raw material for first generation of stars
• Explains observed abundance of deuterium which cannot be produced in significant quantities in stars
• Provides constraint on number of neutrino species helps determine fundamental particle physics parameters
• Helps determine baryon-to-photon ratio in early universe crucial for understanding matter-antimatter asymmetry
• Supports Big Bang theory and standard cosmological model provides strong evidence for hot, dense early universe