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Big Bang nucleosynthesis
In cosmology, Big Bang nucleosynthesis refers to the process of element production during the early phases of the universe, shortly after the Big Bang. It is believed to be responsible for the formation of hydrogen (H-1 or H), its isotope deuterium (H-2 or D), the helium isotopes He-3 and He-4, and the lithium isotope Li-7.
Characteristic of Big Bang nucleosynthesis
There are two important characteristics of Big Bang nucleosynthesis (BBN):
- It only lasted for about three minutes; after that, the temperature and density of the universe fell below that which is required for nuclear fusion. The brevity of BBN is important because it prevented elements heavier than beryllium from forming while at the same time allowing unburned light elements, such as deuterium, to exist.
- It was widespread, encompassing the entire universe.
The key parameter which allows one to calculate the effects of BBN is the number of photons per baryon. This parameter corresponds to the temperature and density of the early universe and allows one to determine the conditions under which nuclear fusion occurs. 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 25% helium-4; about 1% of deuterium; trace amounts of lithium and beryllium; and no other heavy elements. That the observed abundances in the universe are consistent with these numbers is considered strong evidence for the Big Bang theory.
Sequence of BBN
Big Bang nucleosynthesis begins about one minute after the Big Bang, when the universe has cooled enough to form stable protons and neutrons, after baryogenesis. From simple thermodynamical arguments, one can calculate the fraction of protons and neutrons based on the temperature at this point. Because neutrons are heavier than protons, fewer neutrons will form. One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies is very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant.
As the universe expands it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons are strongly motivated to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium, which is relatively unstable. Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium, when there is a sudden burst of element formation. Shortly thereafter, at three minutes after the Big Bang, the universe becomes too cool for any nuclear fusion to occur. At this point, the elemental abundances are fixed, and only change as some of the radioactive products of BBN (such as tritium) decay.
History of Big Bang nucleosynthesis
During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate. This puzzle was resolved in large part by postulating the existence of dark matter.
(the helium crisis in the mid-1990s)
Big Bang nucleosyntheis produces no elements heavier than beryllium. There is no stable nucleus with 8 nucleons, so there was a bottleneck in the nucleosynthesis that stopped the process there. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei (the triple-alpha process). However, the triple alpha process takes tens of thousands of years to convert a significant amount of helium to carbon, and therefore was unable to convert any significant amount of helium in the minutes after the Big Bang.
Big Bang nucleosynthesis predicts about 25% helium-4, and this number is extremely insensitive to the conditions of the universe. The reason for this is that helium-4 is very stable and so almost all of the neutrons will combine with protons to form helium-4. In addition, two helium-4 atoms cannot combine to form a stable atom, so once helium-4 is formed, it stays helium-4. One analogy is to think of helium-4 as ash, and the amount of ash that one forms when one completely burns a piece of wood is insensitive to how one burns it.
The helium-4 abundance is important because there is far more helium-4 in the universe than can be explained by stellar nucleosynthesis. In addition, it provides an important test for the Big Bang theory. If the observed helium abundance is much different from 25%, then this would pose a serious challenge to the theory. This would particularly be the case if the early helium-4 abundance was much smaller than 25% because it is hard to destroy helium-4. For a few years during the mid-1990s, observations suggested that this might be the case, causing astrophysicists to talk about a Big Bang nucleosynthetic crisis, but further observations were consistent with the Big Bang theory.
Deuterium is in some ways the opposite of helium-4 in that while helium-4 is very stable and very difficult to destroy, deuterium is unstable and easy to destroy. Because helium-4 is very stable, there is a strong tendency on the part of two deuterium nuclei to combine to form helium-4. The only reason BBN does not convert all of the deuterium in the universe to helium-4 is that the expansion of the universe cooled the universe and cut this conversion short before it could be completed. One consequence of this is that unlike helium-4, the amount of deuterium is very sensitive to initial conditions. The denser the universe is, the more deuterium gets converted to helium-4 before time runs out, and the less deuterium remains.
There are no known post-Big Bang processes which would produce significant amounts of deuterium. Hence observations about deuterium abundance suggest that the universe is not infinitely old, in accorance with the Big Bang theory.
During the 1970s, there were major efforts to find processes that could produce deuterium. The problem was that while the concentration of deuterium in the universe is consistent with the Big Bang model as a whole, it is too high to be consistent with a model that presumes that most of the universe consists of protons and neutrons.
This inconsistency between observations of deuterium and observations of the expansion rate of the universe, led to a large effort to find processes that could produce deuterium. After a decade of effort, the consensus was that these processes are unlikely, and the standard explanation now used for the abundance of deuterium is that the universe does not consist mostly of baryons and that dark matter makes up most of the mass of the universe.
It is very hard to come up with another process that would produce deuterium via nuclear fusion. What this process would require is that the temperature be hot enough to produce deuterium, but not hot enough to produce helium-4, and that this process immediately cools down to non-nuclear temperatures after no more than a few minutes. Also, it is necessary for the deuterium to be swept away before it reoccurs.
Producing deuterium by fission is also difficult. The problem here again is that deuterium is unstable, and that collisions between atomic nuclei are likely to either result in the absorption of the nuclei, or in the release of free neutrons or alpha particles. During the 1970s, attempts were made to use cosmic ray spallation to produce deuterium. These attempts proved unsuccessful at producing deuterium, but they were unexpectedly successful at producing other light elements.
In addition to the standard BBN scenario there are numerous non-standard BBN scenarios. These should not be confused with non-standard cosmology in the at non-standard BBN scenario assumes that the big bang occurs, but insert additional physics in order to see how this affects elemental abundances. These pieces of addition physics include relaxing or removing the assumption of homogenity or inserting new particles such as massive neutrinos.
There are have been motivations 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, standand BBN assumes that no exotic hypothetical particles were involved in BBN. One can insert a hypothetical particle (say 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.
- Burles, Scott, and Kenneth M. Nollett, Michael S. Turner, "What Is The BBN Prediction for the Baryon Density and How Reliable Is It?". FERMILAB-Pub-00-239-A, Phys.Rev. D63 (2001) 063512 (arXiv.org)
- Jedamzik, Karsten, "A Brief Summary of Non-Standard Big Bang Nucleosynthesis Scenarios". Max-Planck-Institut für Astrophysik, Garching.
- Steigman, Gary, Forensic Cosmology: Probing Baryons and Neutrinos With BBN and the CBR and Big Bang Nucleosynthesis: Probing the First 20 Minutes
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