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
How do we know its there?
- Motions of galaxies in clusters
- Gravitational Lenses
- How a Lens basically works
- 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)
Non-baryonic (exotic matter):
- Beer cans
- Jupiter size objects (see microlensing project
- very low mass stars
- black holes and other stellar remnants
- neutrinoes with mass --> ratio of MWB photons to neutrinoes
is 4 to 1 therefore even a tiny neutrino mass has cosmological significance
- WIMPS - weakly interacting massive particles created in the extremely
- 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"
The above is currently a big huge debate with all sides yelling and
screaming. Here is what
Dr. DarkMatter has to say:
The Electronic Universe Project
- neutrinoes have a small mass (3-6 eV) and contributes 1/2
the mass of the Universe
- dim galaxies, stellar remnants, and low mass stars make up the
- The total mass density of the Universe is 10--20% of that required
to close it
- Deal with it