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There are two important characteristics of Big Bang nucleosynthesis (BBN):
The key parameter which allows one to calculate the effects of BBN is the number of photons per baryon. This parameter corresponds to the temperature and density of the early universe and allows one to determine the conditions under which nuclear fusion occurs. From this we can derive elemental abundances. Although the baryon per photon ratio is important in determining elemental abundances, the precise value makes little difference to the overall picture. Without major changes to the Big Bang theory itself, BBN will result in 25% helium-4; about 1% of deuterium; trace amounts of lithium and beryllium; and no other heavy elements. That the observed abundances in the universe are consistent with these numbers is considered strong evidence for the Big Bang theory.
Big Bang nucleosynthesis begins about one minute after the Big Bang, when the universe has cooled enough to form stable protons and neutrons. From simple thermodynamical arguments, one can calculate the fraction of protons and neutrons based on the temperature at this point. Because neutrons are heavier than protons, fewer neutrons will form. One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies is very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant.
As the universe expands it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons are strongly motivated to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium, which is relatively unstable. Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium, when there is a sudden burst of element formation. Shortly thereafter, at three minutes after the Big Bang, the universe becomes too cool for any nuclear fusion to occur. At this point, the elemental abundances are fixed, and only change as some of the radioactive products of BBN (such as tritium) decay.
The history of Big Bang nucleosynthesis began with the calculations of Ralph Alpher and George Gamow in the 1940's.
(More stuff)
During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate. This puzzle was resolved in large part by postulating the existence of dark matterDark matter is matter that can't be detected by its emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe based on gravitational effe.
(the helium crisis in the mid-1990s)
Big Bang nucleosyntheis produces no elements heavier than beryllium. There is no stable nucleus with 8 nucleonsNucleon is the common name used in nuclear chemistry to refer to a neutron or a proton, the components of an atom's nucleus. The total number of nucleons in an atom is the mass number on the atom, as nucleons each have a mass of one amu. See also List of, so there was a bottleneck in the nucleosynthesis that stopped the process there. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei (the triple-alpha processThe triple alpha process is the process by which three helium nuclei ( alpha particles) are transformed into carbon. This nuclear fusion reaction can only occur rapidly at temperatures above 100,000,000 degrees and in stellar interior having a high helium). However, the triple alpha process takes tens of thousands of years to convert a significant amount of helium to carbon, and therefore was unable to convert any significant amount of helium in the minutes after the Big Bang.
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