¡¥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.¡¦
NGC 1977 Orion Nebula
Astronomical observations of the present helium abundance permit a surprisingly strong constraint on conditions in the early universe at an age of only one minute. Helium has been found throughout our galaxy and in many nearby galaxies. The regions where helium is seen include the atmospheres of old stars and nebulae of ionized gas surrounding young stars. It is also a constituent of the energetic particles called cosmic rays that pervade interstellar space. Helium has even been detected in the luminous and very distant objects known as quasars. Other elements are found to be strongly variable in abundance in different sources, as we might expect from the widely differing evolutionary histories of these regions. Yet, in all cases, the evidence strongly suggests that there is everywhere one helium nucleus for every ten hydrogen nuclei, neither significantly more nor less. (Actually a slight variation is found. The more chemically evolved or more-metal-rich, gas clouds have a slightly higher helium abundance, because of the contribution from stellar nucleosynthesis. Ideally, one would like the primordial or pregalactic helium abundance, and this is found in the most primitive, or most-metal-poor galaxies.) When confronted with such strong evidence for a universal abundance of helium, the hypothesis of a primordial origin in the big hang becomes compelling.
The helium abundance attained in the big bang is relatively insensitive to small changes in the standard model. There is little difference between the amount of helium produced in an open model (one that will expand forever) and that produced in a closed model (one destined eventually to recollapse). At first sight, this result rather surprising. The nucleosysthetic era is primarily determined by the value of the temperature, which has to be about 1 billion degrees Kelvin in either cosmological model when nucleosynthesis occurred. Consequently, the principal difference between these two models is that, in an open universe, the density during the nucleosynthetic era is much lower than in a closed universe. However, because neutrons are consumed with great efficiency in combining with protons to form helium nuclei, lowering the density by one or two orders of magnitude merely lowers the reaction rate but does not significantly affect the final abundance of helium. The predicted helium abundance is reduced by only 1 or 2 per cent.