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Primordial Nucleosynthesis and the Role of Weak Reactions in Astrophysics

This report explores the processes of primordial nucleosynthesis in the early universe, focusing on weak reactions in thermal equilibrium that govern the relationship between neutrons and protons. Following baryogenesis, weak reactions dictate particle ratios via Boltzmann factors until neutron decay becomes predominant. As the universe expands, these reactions undergo "freeze-out," coinciding with neutrino decoupling at around kT=0.80MeV. Afterward, nucleosynthesis initiates at kT=0.05MeV, leading to the formation of deuterons and the development of other nuclei crucial for cosmic evolution.

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Primordial Nucleosynthesis and the Role of Weak Reactions in Astrophysics

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  1. Study Report Particle Astrophysics D.H. PERKINS Chapter 2.10 Che-Yu Chen 2010-3-06

  2. Primordial nucleosynthesis • Weak reactions in equilibrium • Neutron decay • The formation reaction begins

  3. Weak reactions in equilibrium • After baryogenesis, the neutrons and protons are in equilibrium by the weak reactions. • The ratio of neutrons and protons is determined by the Boltzmann factors.

  4. Neutron decay • After the rate of these weak reactions becomes smaller than the universe expand rate, the reactions are ‘’freeze-out’’. • Accompanied with neutrino decoupling. • kT=0.80MeV • Mn(0)/Mp(0)=0.20

  5. Neutron decay • Neutrons disappear by decay

  6. The formation reaction begins • At kT=0.05MeV, nucleosynthesis can begin with the formation of deuterons: • It’s at t ~ 300s(derived from )

  7. Other reactions There are no stable nuclei with A=5, 6 or 8 . The further formation has to await to the onset of helium burning in stars core.

  8. The helium mass fraction

  9. The primordial abundances

  10. Evolution of the universe in the Big Bang model

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