Hydrogen-Helium Abundance
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Hydrogen-Helium AbundanceHydrogen and helium account for nearly all the nuclear matter in today's universe. This is consistent with the standard or "big bang" model. The process of forming the hydrogen and helium and other trace constituents is often called "big bang nucleosynthesis". Schramm's figures for relative abundances indicate that helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%. Carroll & Ostlie give 23 to 24% helium. There is a window of uncertainty, but it is clear that hydrogen and helium make up 98% plus of the ordinary matter in the universe. This high percentage of helium argues strongly for the big bang model, since other models gave very small percentages of helium. Since there is no known process which significantly changes this H/He ratio, it is taken to be the ratio which existed at the time when the deuteron became stable in the expansion of the universe. This ratio is significant as a test of cosmological models since it will be affected by the time period from the time when the temperature dropped below that necessary to produce neutrons from protons to the time when the deuteron became stable, halting the decay of the free neutrons. Basically , the hydrogen-helium abundance helps us to model the expansion rate of the early universe. If it had been faster, there would be more neutrons and more helium. If it had been slower, more of the free neutrons would have decayed before the deuterium stability point and there would be less helium.
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Index References Trefil Schramm Big Bang ... Harwit Ch 12 Carroll & Ostlie Ch 29 | |||
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Hydrogen-Helium CalculationsThe critical temperatures determining the hydrogen-helium abundance can be calculated with the assumption that they are driven by the internal energy of the expanding universe.
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Index Reference Trefil | ||
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