Big Bang nucleosynthesis in physical cosmology, refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. Big Bang nucleosynthesis is believed to be responsible for the formation of Hydrogen (H-1 or simply H), its isotope deuterium (H-2 or D), the helium isotopes He-3 and He-4, and the lithium isotope Li-7 (all of these nuclides are normally shown asNX where X = standard name of this element and N = the number of nucleons in the nucleus, but for this page they will simply be referred to as X-N). The Big Bang nucleosynthesis is also refered to as the primordial nucleosynthesis.
Characteristic of Big Bang nucleosynthesis.
There are two important characteristics of Big Bang nucleosynthesis (Big Bang nucleosynthesis):
The key parameter which allows one to calculate the effects of Big Bang nucleosynthesis 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, Big Bang nucleosynthesis will result in about 75% of H-1, about 25% helium-4, about 0.01% 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.
In this field it is customary to quote percentages by mass, so that 25% helium-4 means 25% of the mass is tied up in helium-4. If you recalculate the numbers on an atom-by-atom or mole-by-mole basis, the percentage of helium-4 will be less.
Sequence of Big Bang nucleosynthesis.
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. This fraction is in favour of protons, because the higher mass of the neutron results in a spontaneous decay of neutrons to protons with a half-life of about 15 minutes. One feature of Big Bang nucleosynthesis is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence Big Bang nucleosynthesis 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 have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. At the time at which nucleosynthesis occurs, the temperature is high enough for the mean energy per particle to be greater than the binding energy of deuterium; therefore any deuterium that is formed is immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium (at about T = 0.1 MeV), 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 Big Bang nucleosynthesis (such as tritium) decay.
History of Big Bang nucleosynthesis
The history of Big Bang nucleosynthesis began with the calculations of Ralph Alpher and George Gamow in the 1940s.
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.
Heavy elements of the Big Bang nucleosynthesis.
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: Helium-4.
Big Bang nucleosynthesis predicts about 25% helium-4, and this number is extremely insensitive to the initial 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.
Big Bang nucleosynthesis: Deuterium.
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 only marginally stable 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 Big Bang nucleosynthesis 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 accordance with the Big Bang theory.
During the 1970s, there were major efforts to find processes that could produce deuterium, which turned out to be a way of producing isotopes other than 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. If one assumes that all of the universe consists of protons and neutrons, the density of the universe is such that much of the currently observed deuterium would have been burned into helium-4.
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 non-baryonic matter (also known as Dark matter) makes up most of the matter mass of the universe. This explanation is also consistent with calculations that show that a universe made mostly of protons and neutrons would be far more clumpy than is observed.
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 very subject to nuclear processes, and that collisions between atomic nuclei are likely to result either 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 failed to produce deuterium, but did unexpectedly produce other light elements.
Status and Implications of the Big Bang nucleosynthesis.
The theory of Big Bang nucleosynthesis 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.
As noted above, in the standard picture of Big Bang nucleosynthesis, 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 (but initially unknown to us) value of the baryon-to-photon ratio. To test Big Bang nucleosynthesis theory against observations thus is 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?
The answer at present is a qualified yes: the Big Bang nucleosynthesis light element predictions can be reconciled with observations for a particular range of baryon-to-photon values, when theoretical and particularly observational uncertainties are taken into account. This agreement is by no means trivial or guaranteed, and represents an impressive success of modern cosmology: Big Bang nucleosynthesis extrapolates the contents and conditions of the present universe (about 14 billon years old) back to times of about one second, and the results are in agreement with observation.
Non-standard Big Bang nucleosynthesis.
In addition to the standard Big Bang nucleosynthesis scenario there are numerous non-standard Big Bang nucleosynthesis scenarios. These should not be confused with non-standard cosmology: a non-standard Big Bang nucleosynthesis 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 Big Bang nucleosynthesis. The first, which is largely of historical interest, is to resolve inconsistencies between Big Bang nucleosynthesis 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 Big Bang nucleosynthesis resulted in abundances that were more inconsistent with observations rather than less. The second, which is largely the focus of non-standard Big Bang nucleosynthesis in the early 21st century, is to use Big Bang nucleosynthesis to place limits on unknown or speculative physics. For example, standand Big Bang nucleosynthesis assumes that no exotic hypothetical particles were involved in Big Bang nucleosynthesis. One can insert a hypothetical particle (such as a massive neutrino) and see what has to happen before Big Bang nucleosynthesis predicts abundances which are very different from observations. This has been usefully done to put limits on the mass of a stable tau neutrino.
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