Editing Big Bang nucleosynthesis (section)

===Deuterium===
{{more citations needed|section|date=February 2023}}

Deuterium is in some ways the opposite of helium-4, in that while helium-4 is very stable and difficult to destroy, deuterium is only marginally stable and easy to destroy. The temperatures, time, and densities were sufficient to combine a substantial fraction of the deuterium nuclei to form helium-4 but insufficient to carry the process further using helium-4 in the next fusion step. BBN did not convert all of the deuterium in the universe to helium-4 due to the expansion that cooled the universe and reduced the density, and so cut that conversion short before it could proceed any further. One consequence of this is that, unlike helium-4, the amount of deuterium is very sensitive to initial conditions. The denser the initial universe was, the more deuterium would be converted to helium-4 before time ran out, and the less deuterium would remain.

There are no known post-Big Bang processes which can produce significant amounts of deuterium. Hence observations about deuterium abundance suggest that the universe is not infinitely old, which is in accordance with the Big Bang theory.{{citation needed|date=August 2024}}

During the 1970s, there were major efforts to find processes that could produce deuterium, but those revealed ways of producing isotopes other than deuterium. The problem was that while the concentration of deuterium in the universe is consistent with the Big Bang model as a whole, it is too high to be consistent with a model that presumes that most of the universe is composed of [[proton]]s and [[neutron]]s.  If one assumes that all of the universe consists of protons and neutrons, the density of the universe is such that much of the currently observed deuterium would have been burned into helium-4.{{citation needed|date=January 2015}} The standard explanation now used for the abundance of deuterium is that the universe does not consist mostly of baryons, but that non-baryonic matter (also known as [[dark matter]]) makes up most of the mass of the universe.{{citation needed|date=January 2015}}  This explanation is also consistent with calculations that show that a universe made mostly of protons and neutrons would be far more ''clumpy'' than is observed.<ref>{{cite book|last=Schramm|first=D. N.|author-link=David Schramm (astrophysicist)|title=The Big Bang and Other Explosions in Nuclear and Particle Astrophysics|url=https://archive.org/details/bigbangotherexpl0000schr|url-access=registration|year=1996|publisher=World Scientific|location=Singapore|isbn=978-981-02-2024-2|page=[https://archive.org/details/bigbangotherexpl0000schr/page/175 175]}}</ref>

It is very hard to come up with another process that would produce deuterium other than by nuclear fusion. Such a process would require that the temperature be hot enough to produce deuterium, but not hot enough to produce helium-4, and that this process should immediately cool to non-nuclear temperatures after no more than a few minutes. It would also be necessary for the deuterium to be swept away before it reoccurs.{{citation needed|date=March 2017}}

Producing deuterium by fission is also difficult. The problem here again is that deuterium is very unlikely due to nuclear processes, and that collisions between atomic nuclei are likely to result either in the fusion of the nuclei, or in the release of free neutrons or [[alpha particles]]. During the 1970s, [[cosmic ray spallation]] was proposed as a source of deuterium. That theory failed to account for the abundance of deuterium, but led to explanations of the source of other light elements.{{citation needed|date=August 2024}}