Primordial Nucleosynthesis and the abundances of the light elements

In the time period between about 100 seconds and 30 minutes after the Big Bang, but mostly with the first three minutes, the temperature and density of the universe were appropriate for the efficient synthesis of the light elements. Just as in the Sun, some of the hydrogen nuclei were underwent fusion to form heavier nuclei including helium nuclei. This was the era of primordial nucleosynthesis. The current abundances of the light elements reflect what occurred during the epoch of primordial nucleosynthesis and therefore place strong constraints on the state of the universe and the baryon density during that time.

Therefore, the current abundance of the elements must be explained by any model of what happened in the early universe. The Hot Big Bang does explain what we see today, and the abundances place a very firm constraint on the baryon density in the universe.

The Wilkinson Microwave Anisotropy Probe has determined the baryon density, given by the cosmological parameter Omegabaryon, to be 0.044 +/- 0.004. Not much of the universe is made up of normal stuff!

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