Editing Big Bang nucleosynthesis (section)

==Measurements and status of theory==
The theory of BBN gives a detailed mathematical description of the production of the light "elements" deuterium, helium-3, helium-4, and lithium-7. Specifically, the theory yields precise quantitative predictions for the mixture of these elements, that is, the primordial abundances at the end of the big-bang.

In order to test these predictions, it is necessary to reconstruct the primordial abundances as faithfully as possible, for instance by observing astronomical objects in which very little [[stellar nucleosynthesis]] has taken place (such as certain [[Dwarf galaxy|dwarf galaxies]]) or by observing objects that are very far away, and thus can be seen in a very early stage of their evolution (such as distant [[quasar]]s).

As noted above, in the standard picture of BBN, all of the light element abundances depend on the amount of ordinary matter ([[baryon]]s) relative to radiation ([[photons]]). Since the [[Cosmological Principle|universe is presumed to be homogeneous]], it has one unique value of the baryon-to-photon ratio. For a long time, this meant that to test BBN theory against observations one had to ask: can ''all'' of the light element observations be explained with a ''single value'' of the baryon-to-photon ratio? Or more precisely, allowing for the finite precision of both the predictions and the observations, one asks: is there some ''range'' of baryon-to-photon values which can account for all of the observations?{{According to whom|date=January 2019}}

More recently, the question has changed: Precision observations of the [[cosmic microwave background radiation]]<ref>David Toback (2009). [http://bigbang.physics.tamu.edu/ChapterText/Ch12text.pdf "Chapter 12: Cosmic Background Radiation"] {{webarchive|url=https://web.archive.org/web/20100706222531/http://bigbang.physics.tamu.edu/ChapterText/Ch12text.pdf |date=2010-07-06 }}</ref><ref>David Toback (2009). [http://bigbang.physics.tamu.edu/ChapterText/Ch13text.pdf "Unit 4: The Evolution Of The Universe"] {{webarchive|url=https://web.archive.org/web/20100706223121/http://bigbang.physics.tamu.edu/ChapterText/Ch13text.pdf |date=2010-07-06 }}</ref> with the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) and [[Planck (spacecraft)|Planck]] give an independent value for the baryon-to-photon ratio. 
The present measurement of helium-4 indicates good agreement, and yet better agreement for helium-3. But for lithium-7, there is a significant discrepancy between BBN and WMAP/Planck, and the abundance derived from [[Population II stars]]. The discrepancy, called the "[[cosmological lithium problem]]", is a factor of 2.4―4.3 below the theoretically predicted value.<ref>{{cite journal |title=A Bitter Pill: The Primordial Lithium Problem Worsens |author=R. H. Cyburt, B. D. Fields & K. A. Olive |year=2008 |arxiv=0808.2818 |doi=10.1088/1475-7516/2008/11/012 |volume=2008 |issue=11 |journal=Journal of Cosmology and Astroparticle Physics |page=012|bibcode = 2008JCAP...11..012C |s2cid=122212670 }}</ref> that have resulted in revised calculations of the standard BBN based on new nuclear data, and to various reevaluation proposals for primordial [[proton–proton chain|proton–proton nuclear reactions]], especially the abundances of {{chem|<sup>7</sup>Be + n → <sup>7</sup>Li + p}}, versus {{chem|<sup>7</sup>Be + <sup>2</sup>H → <sup>8</sup>Be + p}}.<ref>{{cite web | last = Weiss | first = Achim | title = Elements of the past: Big Bang Nucleosynthesis and observation | url = http://www.einstein-online.info/en/spotlights/BBN_obs/index.html | website = Einstein Online | access-date = 2007-02-24 | archive-url = https://web.archive.org/web/20070208212728/http://www.einstein-online.info/en/spotlights/BBN_obs/index.html | archive-date = 8 February 2007 | url-status = dead }}<br />For a recent calculation of BBN predictions, see
*{{cite journal | author=A. Coc | title=Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements | journal=Astrophysical Journal | volume=600 | issue=2 | year=2004 | pages=544–552 | arxiv=astro-ph/0309480 | doi=10.1086/380121 | bibcode=2004ApJ...600..544C| s2cid=16276658 |display-authors=etal}}
For the observational values, see the following articles:
* Helium-4: {{cite journal |author1=K. A. Olive  |author2=E. A. Skillman  |name-list-style=amp | title=A Realistic Determination of the Error on the Primordial Helium Abundance | journal=Astrophysical Journal | volume=617 | issue=1 | year=2004 | pages=29–49 | arxiv=astro-ph/0405588 | doi=10.1086/425170 | bibcode=2004ApJ...617...29O|s2cid=15187664 }}
* Helium-3: {{cite journal | author=T. M. Bania, R. T. Rood & D. S. Balser | title=The cosmological density of baryons from observations of 3He+ in the Milky Way | journal=Nature | volume=415 | date=2002 | pages=54–7 | doi=10.1038/415054a | pmid=11780112 | issue=6867 | bibcode=2002Natur.415...54B| s2cid=4303625 }}
* Deuterium: {{cite journal | author=J. M. O'Meara | title=The Deuterium to Hydrogen Abundance Ratio Towards a Fourth QSO: HS0105+1619 | journal=Astrophysical Journal | volume=552 | issue=2 | date=2001 | pages=718–730 | arxiv=astro-ph/0011179 | doi=10.1086/320579 | bibcode=2001ApJ...552..718O| s2cid=14164537 |display-authors=etal}}
* Lithium-7: {{cite journal |author1=C. Charbonnel  |author2=F. Primas|author2-link=Francesca Primas  |name-list-style=amp | title=The Lithium Content of the Galactic Halo Stars | journal=Astronomy & Astrophysics | volume=442 | issue=3 | date=2005 | pages=961–992 | arxiv=astro-ph/0505247 | doi=10.1051/0004-6361:20042491 | bibcode=2005A&A...442..961C|s2cid=119340132 }} {{cite journal | author=A. Korn | title=A probable stellar solution to the cosmological lithium discrepancy | journal=Nature | volume=442 | date=2006 | pages=657–9 | arxiv=astro-ph/0608201 | doi=10.1038/nature05011 | pmid=16900193 | issue=7103 | bibcode=2006Natur.442..657K | s2cid=3943644 | display-authors=etal | url=https://cds.cern.ch/record/977482 }}</ref>