Big Bang nucleosynthesis is a process believed to have taken place in the early moments of the universe, shortly after the initial expansion took place according to the Big Bang theory. This process, originally proposed formally in 1939 by George Gamow and Ralph Alpher, is consistent with the known astronomical observations and is widely accepted by the scientific community. It explains the creation of light elements in the intense heat of the early moments following the Big Bang through a process of nuclear fusion. The process did not contain enough heat to create heavier elements, which were formed through different nucleosynthesis methods.
What Is Big Bang Nucleosynthesis?According to the theory, the density of the early universe shortly after the Big Bang generated enough heat to trigger a process of nuclear fusion that combined single protons (the simplest form of the hydrogen atom) together creating lighter elements: helium, lithium, and beryllium. It also merged them with neutrons, allowing for isotopes such as deuterium, which is a variation of hydrogen that contains a proton and a neutron in its atomic nucleus.
Because the early universe was the same everywhere, this process seems to have taken place pretty much uniformly throughout the universe, which explains why everywhere we look, the major distribution of elements is roughly the same.
The total process lasted about 17 minutes, beginning about 3 minutes into the period of space expansion. (Prior to that, the universe was pretty much too hot for even protons to form, instead resulting in a soup of free gluons.) About 20 minutes after the universe began to expand, the universe had cooled to the extent that this fusion process was no longer taking place.
In the original 1939 paper that laid the foundation for this, "The Origin of Chemical Elements" (also called the Alpher-Bethe-Gamow paper, which is an amusing story in itself), physicist George Gamow and his doctoral student Ralph Alpher proposed that they believed this process should be sufficient to produce all the elements we see in our universe. This turned out to be reaching a bit too far. The theory itself turns out to only account for lighter elements, up through beryllium. However, Gamow and Alpher did predict a temperature of the cosmic microwave background (CMB) radiation which was later experimentally confirmed.
Elements beyond beryllium had to be formed by other processes, most notably stellar nucleosynthesis, later in the evolutionary history of the universe.