Big Bang nucleosynthesis

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In physical cosmology, Big Bang nucleosynthesis (also known as primordial nucleosynthesis, and abbreviated as BBN)^[1] is a model for the production of light nuclei, deuterium, ³He, ⁴He, ⁷Li, between 0.01s and 200s in the lifetime of the universe.^[2] The model uses a combination of thermodynamic arguments and results from equations for the expansion of the universe to define a changing temperature and density, then analyzes the rates of nuclear reactions at these temperatures and densities to predict the nuclear abundance ratios. Refined models agree very well with observations with the exception of the abundance of ⁷Li. The model is one of the key concepts in standard cosmology.

Elements heavier than lithium are thought to have been created later in the life of the Universe by stellar nucleosynthesis, through the formation, evolution and death of stars.