Big Bang nucleosynthesis

From Wikipedia, the free encyclopedia
  (Redirected from Big bang nucleosynthesis)
Jump to: navigation, search


Essentially all of the elements that are heavier than lithium were created much later, by stellar nucleosynthesis in evolving and exploding stars.




The key parameter which allows one to calculate the effects of BBN is the baryon/photon number ratio, which is a small number of order 6 x 10−10. This parameter corresponds to the baryon density and controls the rate at which nucleons collide and react; from this we can derive elemental abundances. Although the baryon per photon ratio is important in determining elemental 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 10−10) of lithium, and negligible heavier elements. (Traces of boron have been found in some old stars, giving rise to the question whether some boron, not really predicted by the theory, might have been produced in the Big Bang. The question is not presently resolved.[4]) That the observed abundances in the universe are generally consistent with these abundance numbers is considered strong evidence for the Big Bang theory.

In this field, for historical reasons it is customary to quote the helium-4 fraction by mass, symbol Y, so that 25% helium-4 means that helium-4 atoms account for 25% of the mass, but only about 8% of the nuclei would be helium-4 nuclei. Other (trace) nuclei are usually expressed as number ratios to hydrogen.

Important parameters[edit]


Neutron–proton ratio[edit]


n + e+ ↔ anti-νe + p
n + νe ↔ p + e

At times much earlier than 1 sec, the n/p ratio was 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 decreased. These reactions continue until expansion of the universe outpaces the reactions, which occurs at about T = 0.7 MeV 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 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.[5]

Baryon–photon ratio[edit]

The baryon–photon ratio, η, is the key parameter determining the abundance of light elements present in the early universe. Baryons can react with light elements in the following reactions:




The main nuclear reaction chains for Big Bang nucleosynthesis

Big Bang nucleosynthesis began a few seconds after the big bang, when the universe had cooled sufficiently to allow deuterium nuclei to survive disruption by high-energy photons. This time is essentially independent of dark matter content, since the universe was highly radiation dominated until much later, and this dominant component controls the temperature/time relation. The relative abundances of protons and neutrons follow from simple thermodynamical arguments, combined with the way that the mean temperature of the universe changes over time. If the reactions needed to reach the thermodynamically favoured equilibrium values are too slow compared to the temperature change brought about by the expansion, abundances would have remained at some specific non-equilibrium value. Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at this point. The answer is that there are about seven protons for every neutron at the beginning of nucleosynthesis. This fraction is in favour of protons, primarily because their lower mass - with respect to the neutron - favors their production. Free neutrons decay to protons with a half-life of about 10.2 minutes, but this time-scale is longer than the first three minutes of nucleogenesis. It was during this time that a substantial fraction of neutrons were combined with protons into deuterium and then into He-4. The sequence of these reaction chains is shown on the image.[6]



History of theory[edit]

The history of Big Bang nucleosynthesis began with the calculations of Ralph Alpher in the 1940s. Alpher published the Alpher–Bethe–Gamow paper that outlined the theory of light-element production in the early universe.


Heavy elements[edit]


Big Bang nucleosynthesis produced no elements heavier than lithium, due to a bottleneck: the absence of a stable nucleus with 8 or 5 nucleons. This deficit of larger atoms also limited the amounts of lithium-7 produced during BBN. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei, producing carbon (the triple-alpha process). However, this process is very slow, taking tens of thousands of years to convert a significant amount of helium to carbon in stars, and therefore it made a negligible contribution in the minutes following the Big Bang.

In proton-rich regions of the early universe, lithium-6, beryllium-9, and boron-11 could have been produced in the reactions 4He(D,γ)6Li, 7Li(T,n)9Be, 7Li(D,γ)9Be, 7Be(T,p)9Be, and 9Be(T,n)11B.[8][9] Carbon-12 can then be produced when boron-11 captures a neutron or deuteron, allowing for the formation of nitrogen and oxygen in the CNO cycle. The predicted abundance of CNO isotopes produced in Big Bang nucleosynthesis is expected to be on the order of 10−15 that of H, making them undetectable and negligible.[8] Indeed, none of these primordial isotopes of the elements from lithium to oxygen have yet been detected, although those of beryllium and boron may be able to be detected in the future. So far, the only stable nuclides known experimentally to have been made before or during Big Bang nucleosynthesis are protium, deuterium, helium-3, helium-4, and lithium-7.[10]


Main article: Helium-4




Main article: Deuterium






Measurements and status of theory[edit]




More recently, the question has changed: Precision observations of the cosmic microwave background radiation[12][13] with the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck 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?


Non-standard scenarios[edit]



See also[edit]


  5. ^ Gary Steigman (December 2007). "Primordial Nucleosynthesis in the Precision Cosmology Era". Annual Review of Nuclear and Particle Science. 57: 463–491. arXiv:0712.1100free to read. Bibcode:2007ARNPS..57..463S. doi:10.1146/annurev.nucl.56.080805.140437. 
  8. ^ a b
  9. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 14–15. ISBN 0-08-037941-9. 
  10. ^
  11. ^ Bludman, S. A. (December 1998). "Baryonic Mass Fraction in Rich Clusters and the Total Mass Density in the Cosmos". Astrophysical Journal. 508 (2): 535–538. arXiv:astro-ph/9706047free to read. Bibcode:1998ApJ...508..535B. doi:10.1086/306412. 
  12. ^ David Toback (2009). "Chapter 12: Cosmic Background Radiation"
  13. ^ David Toback (2009). "Unit 4: The Evolution Of The Universe"
    For a recent calculation of BBN predictions, see A. Coc; et al. (2004). "Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements". Astrophysical Journal. 600 (2): 544–552. arXiv:astro-ph/0309480free to read. Bibcode:2004ApJ...600..544C. doi:10.1086/380121. 
    For the observational values, see the following articles:

External links[edit]

For a general audience[edit]

Technical articles[edit]