Editing Big Bang nucleosynthesis (section)

===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>