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In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) refers to the production of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as a nucleus) during the early phases of the universe. Primordial nucleosynthesis is believed by most cosmologists to have taken place from 10 seconds to 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as the isotope helium-4 (4He), along with small amounts of the hydrogen isotope deuterium (2H or D), the helium isotope helium-3 (3He), and a very small amount of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: the heavy hydrogen isotope tritium (3H or T); and the beryllium isotope beryllium-7 (7Be); but these unstable isotopes later decayed into 3He and 7Li, as above. Essentially all of the elements that are heavier than lithium and beryllium were created much later, by stellar nucleosynthesis in evolving and exploding stars. ==Characteristics== There are two important characteristics of Big Bang nucleosynthesis (BBN): * The era began at temperatures of around 10 MeV (116 gigakelvin) and ended at temperatures below 100 keV (1.16 gigakelvin).〔Doglov, A. D. "Big Bang Nucleosynthesis." Nucl.Phys.Proc.Suppl. (2002): 137-43. ArXiv. 17 Jan. 2002. Web. 14 Jan. 2013.〕 The corresponding time interval was from a few tenths of a second to up to 103 seconds.〔Grupen, Claus. "Big Bang Nucleosynthesis." Astroparticle Physics. Berlin: Springer, 2005. 213-28. Print.〕 The temperature/time relation in this era can be given by the equation: : , :where t is time in seconds, T is temperature in MeV and g * is the effective number of particle species.〔J. Beringer et al. (Particle Data Group), "(Big-Bang cosmology )" Phys. Rev. D86, 010001 (2012): (21.43)〕 (g * includes contributions of 2 from photons, 7/2 from electron-positron pairs and 7/4 from each neutrino flavor. In the standard model g * is 10.75). This expression also shows how a different number of neutrino flavors will change the rate of cooling of the early universe. * It was widespread, encompassing the entire observable universe. 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.) 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「big bang nucleosynthesis」の詳細全文を読む スポンサード リンク
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