13.2 Big Bang Nucleosynthesis

The Big Bang Nucleosynthesis marks a crucial phase in the early universe. In just 20 minutes, protons and neutrons fused to form the first atomic nuclei. This process set the stage for all matter we see today.

The resulting element ratios provide strong evidence for the Big Bang theory. Hydrogen and helium-4 dominate, with trace amounts of other light elements. However, the lithium problem remains an ongoing mystery in cosmology.

Primordial Nucleosynthesis Basics

Foundation of Early Universe Chemistry

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• Primordial nucleosynthesis occurred during the first few minutes after the Big Bang
• Protons and neutrons combined to form the first atomic nuclei in the universe
• Process began when the universe cooled to about 10^9 Kelvin, allowing stable nuclei to form
• Lasted approximately 20 minutes, setting the stage for the formation of the first atoms

Critical Ratios and Particle Interactions

• Proton-neutron ratio determined the relative abundance of hydrogen and helium
• Initially 1:1 ratio of protons to neutrons existed at very high temperatures
• Ratio shifted to about 7:1 in favor of protons as the universe cooled
• Baryon-to-photon ratio influenced the rate and efficiency of nucleosynthesis
• Measured at approximately 10^-9, indicating a photon-dominated early universe

Nuclear Reactions in the Early Universe

• Nuclear reaction network describes the series of fusion reactions in primordial nucleosynthesis
• Begins with proton-neutron reactions forming deuterium
• Continues with reactions producing helium-3, helium-4, and trace amounts of lithium-7
• Network includes both forward and reverse reactions, maintaining equilibrium until freeze-out

Key Processes and Challenges

The Deuterium Bottleneck

• Deuterium bottleneck delayed the onset of efficient nucleosynthesis
• High-energy photons initially broke apart newly formed deuterium nuclei
• Universe had to cool sufficiently for deuterium to survive and participate in further reactions
• Bottleneck lasted until the universe was about 3 minutes old
• Once overcome, rapid production of heavier elements ensued

The Persistent Lithium Problem

• Lithium problem refers to the discrepancy between predicted and observed lithium-7 abundance
• Big Bang nucleosynthesis theory predicts about 3 times more lithium-7 than observed
• Possible explanations include systematic errors in observations, new physics, or destruction of lithium in early stars
• Remains an active area of research in cosmology and nuclear astrophysics
• Impacts our understanding of the early universe and fundamental physics

Primordial Element Abundances

Helium-4 Production and Significance

• Helium-4 abundance serves as a crucial test of Big Bang nucleosynthesis theory
• Comprises about 25% of the baryonic matter in the universe by mass
• Production primarily occurred during the first 3 minutes after the Big Bang
• Abundance relatively insensitive to the baryon-to-photon ratio
• Provides strong evidence for the hot Big Bang model

Distribution of Light Elements

• Primordial element abundances include hydrogen (75%), helium-4 (25%), and trace amounts of other light elements
• Deuterium abundance approximately 10^-5 relative to hydrogen
• Helium-3 produced at levels of about 10^-5 relative to hydrogen
• Lithium-7 abundance predicted to be about 10^-10 relative to hydrogen
• These abundances set the initial conditions for subsequent stellar nucleosynthesis and galactic chemical evolution