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: http://zebu.uoregon.edu/1999/ph123/lec08.html
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How did Galaxies Form
The exact mechanism for this is unclear and most of the mass is probably darkmatter. Gravitational coalescence of this dark material then built the galaxies we observe today. Below is an N-body supercomputer simulation of this process.
These eventually grow to form galaxies. Unfortunately, there is
a rather wide range of different galaxy types, shapes and densities
which means the formation process was not simple. Also most all galaxies
today are embedded in some larger scale structure. The formation
of these structures is unclear but here are a couple of possibilities:
Structure formation could have either occurred from fragmentation of
very large regions into smaller regions or from the gravitational
coalesence of sub-units into successively large structures. Both
formation scenarios lead to a highly clustered Universe with structure
on many different size scales.
Large Scale Structure (a 22 Meg mpeg).
Formation of
a single Galaxy Cluster
The Dark Matter Universe
How do we know its there?
What we really observe:
Mass of a galaxy grows with radius. Requires dark
matter halo.
A Galaxy can be a Lens:
Curved spacetime is the Lens:
Different shapes can be seen
you need a heavy particle that is not effected by radiation
pressure in order to eventually form galaxies
What are the candidates
Baryonic: (e.g. made of protons and neutrons)
How much dark matter is there?
There is a big difference between Theory and Observation with good arguments coming from both sides. The situation is unresolved and has been for 20 years. Whomever solves the "dark matter" problem will likely win the Nobel prize.
However, its important to realize that the "dark matter" problem exists only in the context of one known long range force (gravity). Suppose there is another long range force that we are ignorant of. If this is discovered by future physicists then they will look back at this "dark matter" cosmology much the same way we now view the early "geocentric" cosmologies.