Editing Big Bang nucleosynthesis (section)

===Neutron–proton ratio===
The neutron–proton ratio was set by Standard Model physics before the nucleosynthesis era, 
essentially within the first 1-second after the Big Bang. 
Neutrons can react with positrons or electron neutrinos to create protons and other products in one of the following reactions:
:<chem>n \ + e+ <=> \overline{\nu}_e + p  </chem>
:<chem>n \ + \nu_{e} <=> p + e- </chem>

At times much earlier than 1 sec, these reactions were fast and maintained the n/p ratio close to 1:1. As the temperature dropped, the equilibrium shifted in favour of protons due to their slightly lower mass, and the n/p ratio smoothly decreased.  
These reactions continued until the decreasing temperature and density caused the reactions to become too slow, which occurred at about T = 0.7 MeV (time around 1 second) and is called the freeze out temperature. At freeze out, the neutron–proton ratio was about 1/6. However, free neutrons are unstable with a mean life of 880 sec; some neutrons decayed in the next few minutes before fusing into any nucleus, so the ratio of total neutrons to protons after nucleosynthesis ends is about 1/7.  Almost all neutrons that fused instead of decaying ended up combined into helium-4, due to the fact that helium-4 has the highest [[Nuclear binding energy|binding energy]] per nucleon among light elements. This predicts that about 8% of all atoms should be helium-4, leading to a mass fraction of helium-4 of about 25%, which is in line with observations. Small traces of deuterium and helium-3 remained as there was insufficient time and density for them to react and form helium-4.<ref>{{cite journal
| author = Gary Steigman
| title = Primordial Nucleosynthesis in the Precision Cosmology Era
| arxiv = 0712.1100
| journal = [[Annual Review of Nuclear and Particle Science]]
| pages = 463–491
| year = 2007
| doi = 10.1146/annurev.nucl.56.080805.140437| doi-access=free
| bibcode = 2007ARNPS..57..463S
| volume=57| issue = 1
| s2cid = 118473571
}}</ref>