Quote:
Originally Posted by Cougar
You wonder about the other elements of the periodic table and why their ratios aren't predicted in the BBT like hydrogen and helium. Well, as RobA mentioned twice, no element heavier than lithium could have been fused during the first 3 minutes. There was not enough time because the universe was rapidly expanding, and therefore cooling very quickly. Fusion requires very high temperatures - particles moving very rapidly - to slam positively charged protons together and make them stick. (This is all worked out very rigorously using quantum theory and observations from high-energy particle accelerators.) So there was a very small window of time when the temperature was right for hydrogen to fuse, resulting in a certain amount of helium and very small quantities of deuterium and lithium. Then the window closed. The temperature cooled beyond a certain level, and no more fusion could occur. End of story.
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I'll add to that. We observe the "local" universe and measure its contents, in matter and energy. We then apply the FRW solutions of GR (what falls out on the adopted constraints of isotropy and homogeneity) to tell us how the matter/energy density evolve as a function of time. We run it backward and can predict
that the proton/neutron density, at the time in which nucleosynthesis was converting some of the protons and all of the neutrons into deuterons, helium (3He and 4He) plus Lithium,
was no where near high enough to allow the triple-alpha process (3 x 4He --> 12 C) to create carbon to occur. Why? Because that process is virtually a 3-body collision that involves unstable 8Be and an excited state of 12C. It therefore occurs only within the very high density cores of stars that are late in life, but could not occur at any significant level in the early universe. When you couple that with the fact that nucleon masses of 5 and 8 are unstable, the early expanding, cooling universe presented a bottle neck in the production of the elements. Setting aside the elements Be, B (which also likely have significant or predominant contributions from cosmic ray spallation) stars created the rest of the periodic table.
I think somebody has already
posted this link, but I'll do it again. We can make detailed computations of H:2H:4He:3He:6,7Li ratios from the first several minutes of the big bang, and these depend on the present Hubble parameter, and measurements of the present baryonic (protons + neutrons) matter density in the universe, as averaged over sufficiently large volumes of space. Both of these observable parameters have other, independent consequences on other observables (such as the fluctuation power spectrum of the cosmic background radiation, and the collapse of structure much later to form galaxies and galaxy clusters, and ...).
This is a
very brief overview of the current evidence for the Big Bang.