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In physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis, abbreviated BBN) refers to the production of nuclei other than those of H-1 (i.e. the normal, light isotope of hydrogen, whose nuclei consist of a single proton each) during the early phases of the universe. Primordial nucleosynthesis took place just a few minutes after the Big Bang and is believed to be responsible for the formation of a heavier isotope of hydrogen known as deuterium (H-2 or D), the helium isotopes He-3 and He-4, and the lithium isotopes Li-6 and Li-7. In addition to these stable nuclei some unstable, or radioactive, isotopes were also produced during primordial nucleosynthesis: tritium or H-3; beryllium-7 (Be-7), and beryllium-8 (Be-8). These unstable isotopes either decayed or fused with other nuclei to make one of the stable isotopes.

Observational Tests and Status of Big Bang nucleosynthesis

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.

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 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 quasars).

As noted above, in the standard picture of BBN, all of the light element abundances depend on the amount of ordinary matter (baryons) relative to radiation (photons). Since the universe is 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?

More recently, the question has changed: Precision observations of the cosmic microwave background radiation^[1][2] with the Wilkinson Microwave Anisotropy Probe (WMAP) give an independent value for the baryon-to-photon ratio. Using this value, are the BBN predictions for the abundances of light elements in agreement with the observations?

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, and the abundance derived from Population II stars. The discrepancy is a factor of 2.4―4.3. and is considered a problem for the original models^[3], that have resulted in revised calculations of the standard BBN based on new nuclear data, and to various reevaluation proposals for primordial proton-proton nuclear reactions, especially the intensities of ⁷Be(n,p)⁷Li versus ⁷Be(d,p)⁸Be^[4].

Non-standard Big Bang nucleosynthesis

In addition to the standard BBN scenario there are numerous non-standard BBN scenarios. These should not be confused with non-standard cosmology: a non-standard BBN scenario assumes that the Big Bang occurred, but inserts additional physics in order to see how this affects elemental abundances. These pieces of additional physics include relaxing or removing the assumption of homogeneity, or inserting new particles such as massive neutrinos.

There have been, and continue to be, various reasons for researching non-standard BBN. The first, which is largely of historical interest, is to resolve inconsistencies between BBN predictions and observations. This has proved to be of limited usefulness in that the inconsistencies were resolved by better observations, and in most cases trying to change BBN resulted in abundances that were more inconsistent with observations rather than less. The second, which is largely the focus of non-standard BBN in the early 21st century, is to use BBN to place limits on unknown or speculative physics. For example, standard BBN assumes that no exotic hypothetical particles were involved in BBN. One can insert a hypothetical particle (such as a massive neutrino) and see what has to happen before BBN predicts abundances which are very different from observations. This has been usefully done to put limits on the mass of a stable tau neutrino.

See also

Astronomy portal

• Nucleosynthesis
• Stellar nucleosynthesis
• Ultimate fate of the Universe

References

External links

For a general audience

• Weiss, Achim. "Big Bang Nucleosynthesis: Cooking up the first light elements". Einstein Online. http://www.einstein-online.info/en/spotlights/BBN/index.html. Retrieved 2007-02-24. 
• White, Martin: Overview of BBN
• Wright, Ned: BBN (cosmology tutorial)
• Big Bang nucleosynthesis on arxiv.org
• Burles, Scott; Nollett, Kenneth M.; Turner, Michael S. (1999-03-19). "Big-Bang Nucleosynthesis: Linking Inner Space and Outer Space". arXiv, University of Chicago. http://arxiv.org/abs/astro-ph/9903300. Retrieved 2008-10-15. 

Technical articles

• Burles, Scott, and Kenneth M. Nollett, Michael S. Turner (2001). "What Is The BBN Prediction for the Baryon Density and How Reliable Is It?". Phys.Rev. D 63: 063512. doi:10.1103/PhysRevD.63.063512. arΧiv:astro-ph/0008495.  Report-no: FERMILAB-Pub-00-239-A
• Jedamzik, Karsten, "Non-Standard Big Bang Nucleosynthesis Scenarios". Max-Planck-Institut für Astrophysik, Garching.
• Steigman, Gary, Primordial Nucleosynthesis: Successes And Challenges arΧiv:astro-ph/0511534; Forensic Cosmology: Probing Baryons and Neutrinos With BBN and the CBR arΧiv:hep-ph/0309347; and Big Bang Nucleosynthesis: Probing the First 20 Minutes arΧiv:astro-ph/0307244
• R. A. Alpher, H. A. Bethe, G. Gamow, The Origin of Chemical Elements, Physical Review 73 (1948), 803. The so-called αβγ paper, in which Alpher and Gamow suggested that the light elements were created by hydrogen ions capturing neutrons in the hot, dense early universe. Bethe's name was added for symmetry
• G. Gamow, The Origin of Elements and the Separation of Galaxies, Physical Review 74 (1948), 505. These two 1948 papers of Gamow laid the foundation for our present understanding of big-bang nucleosynthesis
• G. Gamow, Nature 162 (1948), 680
• R. A. Alpher, "A Neutron-Capture Theory of the Formation and Relative Abundance of the Elements," Physical Review 74 (1948), 1737
• R. A. Alpher and R. Herman, "On the Relative Abundance of the Elements," Physical Review 74 (1948), 1577. This paper contains the first estimate of the present temperature of the universe
• R. A. Alpher, R. Herman, and G. Gamow Nature 162 (1948), 774
• Java Big Bang element abundance calculator