Editing Big Bang nucleosynthesis (section)

==Sequence==
[[File:Main nuclear reaction chains for Big Bang nucleosynthesis.svg|right|thumb|upright=1.4|Main nuclear reaction chains for Big Bang nucleosynthesis]]
Big Bang nucleosynthesis began roughly 20 seconds after the big bang, when the universe had cooled sufficiently to allow deuterium nuclei to survive disruption by high-energy photons. (Note that the neutron–proton freeze-out time was earlier). This time is essentially independent of dark matter content, since the universe was highly radiation dominated until much later, and this dominant component controls the temperature/time relation. At this time there were about six protons for every neutron, but a small fraction of the neutrons decay before fusing in the next few hundred seconds, so at the end of nucleosynthesis there are about seven protons to every neutron, and almost all the neutrons are in Helium-4 nuclei.<ref>{{cite book | first=Carlos A. | last=Bertulani | author-link=Carlos Bertulani | title=Nuclei in the Cosmos | publisher=World Scientific | date=2013 | isbn=978-981-4417-66-2}}</ref>

One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are 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, and proceeds independently of what happened before.

As the universe expands, it cools. [[Free neutron]]s are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. Before nucleosynthesis began, the temperature was high enough for many photons to have energy greater than the binding energy of deuterium; therefore any deuterium that was formed was immediately destroyed (a situation known as the "deuterium bottleneck"). Hence, the formation of helium-4 was delayed until the universe became cool enough for deuterium to survive (at about T = 0.1 MeV); after which there was a sudden burst of element formation. However, very shortly thereafter, around twenty minutes after the Big Bang, the temperature and density became too low for any significant fusion to occur. At this point, the elemental abundances were nearly fixed, and the only changes were the result of the [[radioactive]] decay of the two major unstable products of BBN, [[tritium]] and [[beryllium-7]].<ref>{{cite web | last = Weiss | first = Achim | title = Equilibrium and change: The physics behind Big Bang Nucleosynthesis | url = http://www.einstein-online.info/en/spotlights/BBN_phys/index.html | website = Einstein Online | access-date = 2007-02-24 | archive-url = https://web.archive.org/web/20070208212219/http://www.einstein-online.info/en/spotlights/BBN_phys/index.html | archive-date = 8 February 2007 | url-status = dead }}</ref>

===Heavy elements===
{{Nucleosynthesis periodic table.svg}}
Big Bang nucleosynthesis produced very few nuclei of elements heavier than [[lithium]] due to a bottleneck: the absence of a stable nucleus with 8 or 5 [[nucleon]]s. This deficit of larger atoms also limited the amounts of lithium-7 produced during BBN. In [[stellar nucleosynthesis|stars]], the bottleneck is passed by triple collisions of helium-4 nuclei, producing [[carbon]] (the [[triple-alpha process]]). However, this process is very slow and requires much higher densities, taking tens of thousands of years to convert a significant amount of helium to carbon in stars, and therefore it made a negligible contribution in the minutes following the Big Bang.

The predicted abundance of CNO isotopes produced in Big Bang nucleosynthesis is expected to be on the order of 10<sup>−15</sup> that of H, making them essentially undetectable and negligible.<ref name=iop>{{Cite journal |doi = 10.1088/1742-6596/665/1/012001|title = Primordial Nucleosynthesis|journal = Journal of Physics: Conference Series|volume = 665|pages = 012001|year = 2017|last1 = Coc|first1 = A|arxiv = 1609.06048| s2cid=250691040 }}</ref> Indeed, none of these primordial isotopes of the elements from beryllium to oxygen have yet been detected, although those of beryllium and boron may be able to be detected in the future. So far, the only stable nuclides known experimentally to have been made during Big Bang nucleosynthesis are protium, deuterium, helium-3, helium-4, and lithium-7.<ref>{{Cite arXiv |eprint = 1403.4156v1|last1 = Coc|first1 = Alain|title = Revised Big Bang Nucleosynthesis with long-lived negatively charged massive particles: Impact of new 6Li limits, primordial 9Be nucleosynthesis, and updated recombination rates|last2 = Vangioni|first2 = Elisabeth|class = astro-ph.CO|year = 2014}}</ref>

===Helium-4===
Big Bang nucleosynthesis predicts a primordial abundance of about 25% helium-4 by mass, irrespective of the initial conditions of the universe. As long as the universe was hot enough for protons and neutrons to transform into each other easily, their ratio, determined solely by their relative masses, was about 1 neutron to 7 protons (allowing for some decay of neutrons into protons). Once it was cool enough, the neutrons quickly bound with an equal number of protons to form first deuterium, then helium-4. Helium-4 is very stable and is nearly the end of this chain if it runs for only a short time, since helium neither decays nor combines easily to form heavier nuclei (since there are no stable nuclei with mass numbers of 5 or 8, helium does not combine easily with either protons, or with itself). Once temperatures are lowered, out of every 16 nucleons (2 neutrons and 14 protons), 4 of these (25% of the total particles and total mass) combine quickly into one helium-4 nucleus. This produces one helium for every 12 hydrogens, resulting in a universe that is a little over 8% helium by number of atoms, and 25% helium by mass.

"One analogy is to think of helium-4 as ash, and the amount of ash that one forms when one completely burns a piece of wood is insensitive to how one burns it."<ref name="karki_2011">{{cite journal |last=Karki |first=Ravi |date=May 2010 |title=The Foreground of Big Bang Nucleosynthesis |url=https://www.nepjol.info/index.php/HP/article/download/5186/4314 |url-status=live |format=PDF |journal=The Himalayan Physics |volume=1 |issue=1 |pages=79–82 |doi=10.3126/hj.v1i0.5186 |archive-url=https://web.archive.org/web/20180921114731/https://www.nepjol.info/index.php/HP/article/download/5186/4314 |archive-date=21 September 2018 |access-date=21 September 2018|doi-access=free }}</ref> The resort to the BBN theory of the helium-4 abundance is necessary as there is far more helium-4 in the universe than can be explained by [[stellar nucleosynthesis]]. In addition, it provides an important test for the Big Bang theory. If the observed helium abundance is significantly different from 25%, then this would pose a serious challenge to the theory. This would particularly be the case if the early helium-4 abundance was much smaller than 25% because it is hard to destroy helium-4. For a few years during the mid-1990s, observations suggested that this might be the case, causing astrophysicists to talk about a Big Bang nucleosynthetic crisis, but further observations were consistent with the Big Bang theory.<ref>{{cite journal
| author = Bludman, S. A.
| title = Baryonic Mass Fraction in Rich Clusters and the Total Mass Density in the Cosmos
| arxiv = astro-ph/9706047
| journal = [[Astrophysical Journal]]
| volume = 508
| issue = 2
| pages = 535–538
| date = December 1998
| doi = 10.1086/306412
| bibcode = 1998ApJ...508..535B| s2cid = 16714636
}}</ref>

===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}}

===Lithium===

Lithium-7 and lithium-6 produced in the Big Bang are on the order of: lithium-7 to be 10<sup>−9</sup> of all primordial nuclides; and lithium-6 around 10<sup>−13</sup>.<ref name="Primodal-lithium-problem-2012-arx">{{cite journal|doi=10.1146/annurev-nucl-102010-130445|doi-access=free|title=The Primordial Lithium Problem|year=2011|last1=Fields|first1=Brian D.|journal=[[Annual Review of Nuclear and Particle Science]]|volume=61|issue=1|pages=47–68|arxiv=1203.3551|bibcode=2011ARNPS..61...47F}}</ref>