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Big Bang nucleosynthesis (or primordial nucleosynthesis) is nucleosynthesis (the creation of atomic nuclei) occurring in the minutes immediately after the Big Bang, i.e., the first nucleosynthesis in the very early universe. In the study of cosmology, the effect of the extreme temperature immediately after the Big Bang along with known of the laws of physics is worked out, calculating the abundances of elements likely to have been produced. Today's observed abundances must be the result of this plus the effects of the later nucleosynthesis within stars and supernovae, and represent a constraint on theories of all the universe's nucleosynthesis.
Theorized stable products of this very early nucleosynthesis are largely hydrogen-1 (i.e., with mass number 1) with some helium-4, along with far less deuterium, helium-3, and lithium-7. Following are the abundances theorized to be left over after the initial drop in temperature, which are termed relic abundances or primordial abundances:
| Isotope | (rough) mass fraction | |
| H | 0.75 (Xp) | |
| He | 0.25 (Yp) | |
| 2H aka D | 10-4 (Dp) | |
| 3He | 10-4 | |
| 3H | 10-6 | (radioactive) |
| 7Be | 10-10 | (radioactive) |
| 7Li | 10-10 |
The primordial radioactive isotopes have virtually disappeared, and any current abundance depends upon relatively recent nucleosynthesis. A more precise value of the primordial helium abundance is of current research interest in part because cosmological models relate the exact value to other quantities of interest. Recent determinations still vary, one being 0.2446.