| Synthesis
of Light Elements |
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| • | 3He
and D abundances strongly |
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| constrain the
baryon density. 4He is set |
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| by different
physics, essentially |
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| thermodynamics,
fixed by neutron to |
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| proton
ratio. Not so for D, etc., which are |
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| set by
reaction sequence before T falls |
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| too much. In a high density universe, |
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| there is time
for everything to become |
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| 4He,
giving a low D abundance. Opposite |
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| case for low
density. |
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