Editing Big Bang (section)

===Abundance of primordial elements===
{{Main|Big Bang nucleosynthesis}}
[[File:Universe-09-00183-g004.png|right|thumb|upright=1.6|Time evolution of light element abundances during Big Bang nucleosynthesis]]
Using Big Bang models, it is possible to calculate the expected concentration of the isotopes [[helium-4]] (<sup>4</sup>He), [[helium-3]] (<sup>3</sup>He), deuterium (<sup>2</sup>H), and [[Isotopes of lithium#Lithium-7|lithium-7]] (<sup>7</sup>Li) in the universe as ratios to the amount of ordinary hydrogen.<ref name="kolb_c4"/> The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by abundance) are about 0.25 for <sup>4</sup>He:H, about 10<sup>−3</sup> for <sup>2</sup>H:H, about 10<sup>−4</sup> for <sup>3</sup>He:H, and about 10<sup>−9</sup> for <sup>7</sup>Li:H.<ref name="kolb_c4">{{harvnb|Kolb|Turner|1988|loc=chpt. 4}}</ref>

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for <sup>4</sup>He, and off by a factor of two for <sup>7</sup>Li (this anomaly is known as the [[cosmological lithium problem]]); in the latter two cases, there are substantial [[Observational error#Random errors versus systematic errors|systematic uncertainties]]. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium.<ref>{{cite journal |last=Steigman |first=Gary |author-link=Gary Steigman |date=February 2006 |title=Primordial Nucleosynthesis: Successes And Challenges |journal=[[International Journal of Modern Physics E]] |volume=15 |issue=1 |pages=1–36 |arxiv=astro-ph/0511534 |bibcode=2006IJMPE..15....1S |doi=10.1142/S0218301306004028 |citeseerx=10.1.1.337.542 |s2cid=12188807}}</ref> Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe before [[star formation]], as determined by studying matter supposedly free of [[stellar nucleosynthesis]] products, should have more helium than deuterium or more deuterium than <sup>3</sup>He, and in constant ratios, too.<ref name="Ryden2003"/>{{rp|182–185}}