Big Bang nucleosynthesis (or primordial nucleosynthesis) is nucleosynthesis (the creation of atomic nuclei) occurring immediately after the Big Bang, i.e., in the very early universe. In the study of cosmology, the effect of the extreme temperature immediately after the Big Bang along with what's known of the laws of physics is worked out, calculating the abundances of elements likely to have been produced. Today's abundances would be the result of this plus the effects of the later nucleosynthesis within stars and supernovae, offering a constraint on all these nucleosynthesis theories.
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, termed relic abundances or primordial abundances:
|Isotope||(rough) mass fraction|
|2H aka D||10-4 (Dp)|
The primordial radioactive isotopes would have long since disappeared, and their current abundance depends upon relatively recent nucleosynthesis. A more precise value of the 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.