Big Bang Nucleosynthesis
T, r both high enough at start to fuse protons into heavier elements
T, r both dropping quickly so only
have time enough to fuse
a certain amount.
Simple
models of expansion predict 25% abundance He
25%
is the amount of He observed
Abundance
of 2H, 3He, 7Li depends on rnormal matter
Suggests
rnormal matter is
only 5% of rcritical
But we
need to also consider dark matter and its gravity