．What exactly does the Big Bang theory explain? A theory should be able to explain phenomenon, otherwise it is a conjecture only.・
．The most important contribution of Big Bang theory is interpreting the abundance of different elements in the universe.・
Although the production of helium is similar for open and closed big bang models, it turns out that an important byproduct of the reaction, deuterium, is extremely sensitive to density. In both the open and closed models, large amounts of deuterium were produced but were then destroyed by collisions with protons. Because the density was lower during the nucleosynthetic era for an open model than for a closed model, the amount of collisional destruction of deuterium is reduced in the open model. Consequently, the resulting deuterium abundance can be greatly increased in an open model, relative to the amount predicted in a closed model.
Deuterium
is not a very abundant isotope. About 1 deuterium atom is found for every 30,000
hydrogen atoms. This low abundance makes deuterium sensitive to a small change
in the much larger abundance
of helium. Unlike most other heavy atoms, however, it cannot be made in
ordinary stars. At the high temperatures in the center of the sun, the relatively
fragile deuterium isotope would he entirely destroyed. Thus, all the deuterium
we observe in the galaxy was probably synthesized in the first minutes of the
big bang. Because essentially no deuterium is produced in the standard closed
model, we might conclude that the universe is open.
We see one possible loophole in this argument when we consider a nonstandard variation on the big hang. During the transition from a quark-dominated to a hadron-dominated universe, which occurred when the temperature was about 200 million electron volts, large bubbles of the new hadron phase may have formed before the transition was completed. The neutrons in these regions of ordinary hadronic matter tend to diffuse more rapidly than do the charged particles. Hence, there are pockets of neutron-poor and also neutron-rich material. Variations in the neutron abundance from place to place can modify the ensuing deuterium and helium synthesis, and a dense universe can mimic a low-density universe. Another possibility is that a local speedup in the expansion rate during the nucleosvnthetic era over an area comparable in size to the observable universe at that time would inhibit destruction of deuterium by collisions with protons, because there would not be enough time for many collisions to occur, large amounts of deuterium would accordingly survive. Even if this happened only in a small fraction of the universe, the deuterium produced, after mixing with the unperturbed regions, would suffice to explain what we observe. Any excess helium that might result would be suitably diluted by the hulk of the matter in the universe to yield its standard abundance.
These possibilities are not appealing. Indeed, they generally result in the production of excessive lithium, another trace element produced in very small amounts (only 1 part in a billion), which also is generally destroyed rather than produced in ordinary stars. The standard big bang succeeds remarkably well in simultaneously accounting for the abundance of helium, deuterium, the isotope of helium of mass 3, and the lithium isotope of mass 7. In fact it has been argued that one can use this success to constrain unknown parameters in the standard model, such as the number of neutrino species. The presence of additional neutrino types helps to speed up the expansion and to make more helium: for each additional species, about 1 per cent more helium is produced. With astronomers・ estimates of the primordial helium abundance ranging from 22 to 24 per cent, relative to amounts of hydrogen, little room remains to add neutrino species to the three species known to exist. Experiments underway at SLAC and Fermilab can directly measure the total number of neutrino species, because these contribute to decay channels for the Z boson, a newly discovered unstable massive particle weighing about 90 proton masses which helps mediate weak interactions. Current limits already come close to the value allowed by the standard big bang model.
It
is important to realize that big bang nucleosynthesis
only constrains the baryonic content (that is, for our purpose, the protons)
of the universe. The universe need not be open if weakly interacting non-haryonic
particles provide the requisite contribution to the density. However the baryonic
density is inferred to be about one-tenth of the critical value for closure
in order for the simple model to account for the origin of the light elements.
Elements heavier than helium are not produced in any significant abundance in the big bang, except for certain isotopes of lithium and boron .Nature has arranged matters so that there are no stable elements of atomic masses 5 or 8. Thus, the process of neutron capture, which is a step-by-step process, must break down at this hurdle. A helium nucleus cannot capture a proton or another helium nucleus and form a new stable nucleus. The only other way to overcome this hurdle is to use a different nuclear process. The principal alternative involves the capture of helium nuclei by two other helium nuclei. This process requires a physical condition (a high enough density for a sufficiently long time) that is not attained in the big bang theory. By the time helium had been produced, the density was too low and the universe was thinning out and cooling too rapidly to provide the time for any further nucleosynthesis to occur. Astronomers believe that elements heavier than helium have been created in the cores of evolved stars or in supernova explosions at relatively recent eras.