Big-Bang-Nucleosynthesis
Big-Bang Nucleosynthesis (BBN) is the process by which the first nuclei of light elements were formed in the early universe. This occurred shortly after the Big Bang, when the universe was only a few minutes old. Here are the key points about BBN:
Timeframe
- BBN began when the universe was about 1 second old and lasted for approximately 20 minutes.
Conditions for Nucleosynthesis
- The early universe was extremely hot and dense, with temperatures around 10^9 to 10^10 Kelvin. At these temperatures, photons had enough energy to break apart any nuclei that might form.
- As the universe expanded, it cooled down to about 10^9 Kelvin, allowing for the formation of stable nuclei.
Elements Produced
- The primary elements synthesized were Hydrogen, Helium, Deuterium, and trace amounts of Lithium.
- By mass, approximately 75% of the universe became Hydrogen, 25% Helium, with trace amounts of Deuterium and Lithium-7.
Process
- The process began with the formation of Deuterium (D or ²H) when protons (Hydrogen nuclei) and neutrons fused together.
- Deuterium then combined with another proton or neutron to form Helium-3 (³He) or Helium-4 (⁴He). Helium-4 is particularly stable due to its tightly bound nucleus.
- Some Deuterium and Helium-3 continued to react to form Lithium-7 (⁷Li).
Historical Context and Confirmation
- The theory of BBN was developed in the 1940s by George Gamow, Ralph Alpher, and Robert Herman, who predicted the abundance of light elements based on the conditions in the early universe.
- Observational confirmation came from the cosmic microwave background radiation (CMB) measurements and the observed abundances of light elements in old stars and interstellar gas clouds, which matched the theoretical predictions.
Importance in Cosmology
- BBN provides one of the key tests for the Standard Model of Cosmology. The agreement between predicted and observed element abundances supports the Big Bang theory.
- It also sets constraints on the number of neutrino species, the expansion rate of the universe, and the baryonic matter density.
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