How did Galaxies Form

After recombination there were obviously pockets of density enhancements around which material accumulated

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.

• Galaxy Formation (?)

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:

• Large Scale Structure (a 22 Meg mpeg).
• Formation of a single Galaxy Cluster

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.

However, no matter what form you are talking about, Dark Matter plays a central role.

The Dark Matter Universe

What is Dark Matter?

• Dark matter is material that gravitates but does not emit very much light.
• More specifically, it is material which has a high ratio of mass-to-luminosity

How do we know its there?

• Rotation Curves of Galaxies:
  What we expected:
  

  What we really observe:

  Mass of a galaxy grows with radius. Requires dark matter halo.

• Motions of galaxies in clusters of galaxies
• Gravitational Lenses
  A Regular Lens:
  

  A Galaxy can be a Lens:
  

  Curved spacetime is the Lens:
  

  Different shapes can be seen
  
  

  Courtesy of Ned Wright, UCLA

  

  So the evidence for Dark Matter is fairly overwhelming. Well how much of it is there and what is it?
  • Theory: if the Universe is closed then all the luminous matter seen in galaxies contributes only 0.5% of the mass required to eventually halt the expansion

  • Best argument: baryonic density fluctuations will not produce galaxies you need a heavy particle that is not effected by radiation pressure in order to eventually form galaxies

  • This means that galaxies are surrounded by dark matter halos which trapped baryonic gas (e.g. hydrogen) and turned it into stars. So a galaxy like this is surrounded by an invisible dark matter halo.

  What are the candidates

  Baryonic: (e.g. made of protons and neutrons)

  • Beer cans
  • Jupiter size objects (see microlensing project ) (also read the book for this stuff!).
  • very low mass stars
  • black holes and other stellar remnants

  Non-baryonic (exotic matter):
  • neutrinos with mass ratio of MWB photons to neutrinos is 4 to 1 therefore even a tiny neutrino mass has cosmological significance
  • WIMPS - weakly interacting massive particles created in the extremely early universe
  • Particles associated with symmetry breaking

  How much dark matter is there?

  • Theory 99% of the Universe is Dark Matter
  • Observations 90% of the Universe is Dark Matter and the Universe is open

    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.

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