•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