THEORY
Expansion Rate of the Early Universe
- The constraint arises from the fact that the
early Universe is dominated by the energy contained
in relativistic particles (which includes photons, leptons,
and their anti-particles). This means that the
expansion (dynamics) of the early Universe is controled by the
photons, leptons (electrons, muons, tau's),
neutrinos, and their anti-particles.
The rate at which the Universe expands is independent of the
density of protons and neutrons (baryons) in the early
Universe (the baryons are simply along for the ride).
Nuclear Reaction Rates in the Early Universe
- The yield of Big Bang
nucleosynthesis, however, depends strongly upon the density of
protons and neutrons in the early Universe. Because the density
of protons and neutrons controls the rate at which particles
collide, the density plays
a crucial role in the rate of nuclear reactions.
The reaction rates between the protons and neutrons
(and therefore the yields of Big Bang nucleosynthesis)
can be changed by making different assumptions
about the baryon density in the Universe. The important point is
that this twiddling does not alter
the rate at which the Universe expands. This is a nice
state of affairs as
it then makes it very easy to think about how changes in the baryon
density of the
Universe change the outcome of Big Bang nucleosynthesis.
Detailed calculations lead to the following relationships between Big Bang
nucleosynthesis yields and
Omega.
