Editing Big Bang nucleosynthesis (section)

==Characteristics==
The Big Bang nucleosynthesis (BBN) model assumes a homogeneous plasma, at a temperature corresponding to 1 MeV, consisting of electrons annihilating with positrons to produce photons. In turn, the photons pair to produce electrons and positrons: <math>e^+e^- \leftrightarrow \gamma\gamma</math>.
These particles are in equilibrium. A similar number of neutrinos, also at 1 MeV, have just dropped out of equilibrium at this density.  Finally, there is a very low density of [[baryons]] (neutrons and protons). The BBN model follows the nuclear reactions of these baryons as the temperature and pressure drops due to expansion of the universe.<ref name=Dodelson-2003>{{Cite book |last=Dodelson |first=Scott |title=Modern cosmology |date=2003 |publisher=Academic Press |isbn=978-0-12-219141-1 |location=San Diego, Calif}}</ref>{{rp|62}}

The basic model makes two simplifying assumptions: 
# until the temperature drops below 0.1 MeV only neutrons and protons are stable and 
# only isotopes of hydrogen and of helium will be produced at the end.
These assumptions are based on the intense flux of high energy photons in the plasma. Above 0.1 MeV every nucleus created is blasted apart by a photon.
Thus the model first determines the ratio of neutrons to protons and uses this as an input to calculate the hydrogen, deuterium, tritium, and <sup>3</sup>He.<ref name=Dodelson-2003/>{{rp|63}}

The model follows nuclear reaction rates as the temperature and density drops.
The evolving density and temperature follow from the [[Friedmann-Robertson-Walker model]].
Around <math>kT\approx 1</math> MeV, the density of neutrinos drops, and reactions like
<math display="block">n+e^+ \leftrightarrow p + \overline\nu_e</math>
which maintained neutron and proton equilibrium, slow down.
The neutron-to-proton ratio decreases to around 1/7.<ref name=Schramm-1998/>{{rp|315}}

As the temperature and density continue to fall, reactions involving combinations of protons and neutrons shift towards heavier nuclei.<ref name=Schramm-1998/>{{rp|315}} These include
<math display="block">p+n \rightarrow \textrm{D} + \gamma, \ \textrm{D} + \textrm{D} \rightarrow n +\,^3\textrm{He}, \  \,^3\textrm{He} + \textrm{D} \rightarrow p + \,^4\textrm{He} </math>
Due to the higher binding energy of He, the free neutrons and the deuterium nuclei are largely consumed, leaving mostly protons and helium.<ref name=Dodelson-2003/>{{rp|68}}

The fusion of nuclei occurred between roughly 10 seconds to 20 minutes after the Big Bang; this corresponds to the temperature range when the universe was cool enough for deuterium to survive, but hot and dense enough for [[Nuclear fusion|fusion]] reactions to occur at a significant rate.<ref name=RPP>{{cite journal
 |first=C. |last=Patrignani |collaboration=Particle Data Group
 | title=Big-Bang nucleosynthesis 
 | journal=Chin. Phys. C
 | volume=40
 | date=2016
 | pages=100001
 | url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-bbang-nucleosynthesis.pdf |archive-url=https://web.archive.org/web/20161201063050/http://pdg.lbl.gov/2016/reviews/rpp2016-rev-bbang-nucleosynthesis.pdf |archive-date=2016-12-01 |url-status=live
 }}</ref>

The key parameter which allows one to calculate the effects of Big Bang nucleosynthesis is the baryon/photon number ratio, which is a small number of order 6 × 10<sup>−10</sup>. This parameter corresponds to the baryon density and controls the rate at which nucleons collide and react; from this it is possible to calculate element abundances after nucleosynthesis ends. Although the baryon per photon ratio is important in determining element abundances, the precise value makes little difference to the overall picture. Without major changes to the Big Bang theory itself, BBN will result in mass abundances of about 75% of hydrogen-1, about 25% [[helium-4]], about 0.01% of deuterium and [[helium-3]], trace amounts (on the order of 10<sup>−10</sup>) of lithium, and negligible heavier elements.  That the observed abundances in the universe are generally consistent with these abundance numbers is considered strong evidence for the Big Bang theory.<ref name=Dodelson-2003/>{{rp|69}}<ref name=Schramm-1998/>{{rp|313}}