Density Fluctuations

Density Fluctuations:
The time of neutral atom construction is called recombination, this is also the first epoch
we can observe in the Universe. Before recombination, the Universe was too dense and
opaque. After recombination, photons are free to travel through all of space. Thus, the
limit to our observable Universe is back in time (outward in space) to the moment of
recombination.

The time of recombination is also where the linked behavior between photons and matter
decouples or breaks, and is also the last epoch where radiation traces the mass density.
Photon/matter collisions become rare and the evolution of the Universe is dominated by
the behavior of matter (i.e. gravity), so this time, and until today, is called the matter era.
Today, radiation in the form of photons have a very passive role in the evolution of the
Universe. They only serve to illuminate matter in the far reaches of the Galaxy and other
galaxies. Matter, on the other hand, is free to interact without being jousted by photons.
Matter becomes the organizational element of the Universe, and its controlling force is
gravity.
Notice that as the Universe ages it moves to more stable elements. High energy radiation
(photons) are unstable in their interactions with matter. But, as matter condenses out of
the cooling Universe, a more stable epoch is entered, one where the slow, gentle force of
gravity dominates over the nuclear forces of earlier times.
Much of the hydrogen that was created at recombination was used up in the formation of
galaxies, and converted into stars. There is very little reminant hydrogen between
galaxies, the so-called intergalactic medium, except in clusters of galaxies. Clusters of
galaxies frequently have a hot hydrogen gas surrounding the core, this is leftover gas from
the formation of the cluster galaxies that has been heated by the motions of the cluster
members.
Baryon Fraction:
The amount of hydrogen in the Universe today, either in stars and galaxies, or hot gas between galaxies, is called the
baryon fraction. The current measurements indicate that the baryon fraction is about 3% (0.03) the value of closure for
the Universe (the critical density). Remember the value from the abundance of light elements is 10% (0.10) the closure
value.

The most immediate result here is that the mass density of the Universe appears to be much less than the closure value,
i.e. we live in an open Universe. However, the inflation model demands that we live in a Universe with exactly the
critical density, Omega of 1. This can only be true if about 90% of the mass of the Universe is not in baryons.