The study of Big Bang Nucleosynthesis is the study of the conditions that existed for seventeen minutes following the Big Bang that created the universe. The three minutes of Big Bang Nucleosynthesis is the amount of time that it took for the temperature created by the Big Bang lowered below the point that nuclear fission occurs. Nucleosynthesis is the process which elements heavier than hydrogen are cosmically synthesized, typically within stars. Big bang Nucleosynthesis is different from stellar nucleosynthesis insofar as the seventeen minutes of nuclear fission only allowed for light weight and less complex elements to be synthesized. The conditions of Big Bang Nucleosynthesis occurred throughout the entire existing universe during its initial period of expansion.
Big Bang Nucleosynthesis, or BBN, occurred after the initial three minutes following the Big Bang and lasted until the universe was about twenty minutes old. Those first three minutes before BBN are referred to baryogenesis, a relatively little understood period consisting of temperatures which were so high that protons and neutrons could not exist. Once conditions had cooled sufficiently, normal hydrogen isotopes synthesized into a number of light elements, including lithium, helium, and deuterium. One of these elements, deuterium, existed in substantial amounts only as a result of BBN, as this unstable element typically combines into something different in non-BBN conditions.
The matter that was created through Big Bang Nucleosynthesis is not the stuff of the universe as we know it today. Matter that makes up the physical matter we observe and manipulate is the result of stellar nucleosynthesis which is an longer process capable of creating the heavier elements. It was the primordial matter, referred to as light-element abundance, created by the Big Bang that was later converted by stellar processes into the matter that we know today.
The first stars did not come into existence until hundreds of millions of years after the Big Bang. The presence of these light elements created by BBN in the early universe are observed through the study of cosmic background radiation, and an excellent explanation of the theories and observations that prove the theories of BBN can be found at einstein-online.com
Big Bang Nucleosynthesis was postulated as a result of the Alpher-Bethe-Garnow paper published in the 1940’s. The calculations of Ralph Alpher and George Garnow provided the backbone of understanding what light elements were created at the beginning of the universe, although the paper was later found to be incorrect in regards to the details of the creation of heavier elements.