A Primer on Dark Matter

There are many reasons to believe that the universe is full of "dark matter", matter that influences the evolution of the universe gravitationally, but is not seen directly in our present observations. The adjacent image exhibits one recent piece of evidence for undetected matter: the hot gas seen in the X-ray spectrum would have dispersed if it were held in place only the by gravity of the mass that is producing light in this image (the so-called "luminous mass").

The nature of this dark matter, and the associated "missing mass problem", is one of the fundamental cosmological issues of modern astrophysics. The following is a brief tutorial on this issue extracted from an informal email message:

1. If inflation is correct then, since luminous stars and galaxies only contribute 0.5% of the closure density, then 99% of the Universe is in the form of dark matter and this, no doubt, must be a particle. There are always candidates. Neutrinoes have never been a viable candidate for a fairly simple reason. Neutrinoes are relativistic (e.g. hot dark matter) and therefore they erase fluctuations on small scales (they free stream and fill the horizon in the early universe). Thus the only fluctuations that can still exist in a neutrino dominated Universe are on a very large scale. These will cool and form structure but only on largescales, you will never form galaxies in this manner.

2. On smaller scales, such as galaxies and clusters of galaxies, dynamical estimates of the mass, based on either rotation curves of galaxies or velocity dispersions of galaxies indicate that 90% (not 99% which is another order of magnitude) of the total mass is sub-lumnous. This isn't so bad as it implies the mass density of the Universe is 10% of the closure density. In this case, the sub-luminous mass could very well be normal (baryonic) and be locked up in stellar remnants (white dwarfs, neutron stars, black holes) or just in very dim stars called "Brown Dwarfs". There is recent evidence for possible observation of one of these very dim Brown Dwarfs. Some of this is being tested with the microlensing experiments currently underway in australia (you can actually find some of what I am talking about at http://zebu.uoregon.edu/cosmo.html) and there are positive detections but the selection function is unknown at present and so the lensing population is also unknown.

3. Although inflation demands that the Universe has a density equal to its critical density (and inflation is necessary to solve the horizon problem) there has never been any observational evidence to support this high of mass density. Most dynamical studies suggest values of 10-20% of closure density. These studies are based on large scale deviations from hubble expansion velocities (so called peculiar velocities).

4. Large scale structure (e.g. the distribution of galaxies) is very hard to understand, particularly in light of the relatively smooth microwave background as measured by the COBE satellite. There is way too much power on large scales. One way to accomodate this is to go to a mixed dark matter model in which you have some hot dark matter (for the large scale power) and some cold dark matter (wimps, axions, photinos, supersymmetric particles, etc) to act as a seed for galaxy formation. None of those models, however, fit the data using the critical density. The best models to date (you can see a diagram in the http document referenced above) suggest mixed dark matter and an overall cosmological mass density of 20-30% of closure. Hence, to retain inflation, with its inescapable prediction that the Universe must be flat, requires re-invoking Einstein's cosmological constant - meaning the universe has vacuum energy (negative pressure) and is currently accelerating. This makes our cosmology complicated but much data is pointing this way.

5. Finally, there have been speculative papers that if the dark matter is really something toally new and mysterious then maybe it communicates with itself over some long range force (either attractive or repulsive). An intriguing idea as that would mess up all the comoslogical dynamics - but given the really surprising nature of the galaxy distribution - something clearly very funny is going on.

6. Supernova 1987a neutrino time of flight studies as well as the Solar Neutrino experiment are consistent with the neutrino having a mass, but a very small mass, not one that can cosmologically dominate. As for seeing particles in other ways - well since the SSC won't be built we can not do an accelerator test for seeing supersymmetric particles which would only be created at very high energy (e.g. the early universe) - so there remain many viable potential particles that are consistent with the Standard MOdel of particles (e.g. 3 generations of neutrinoes, 6 quarks) and which would remain unnoticed in any accelerator experiments.

Here are links to two experimental searches for Dark Matter candidates:

• The MACHO (MAssive Compact Halo Objects) Dark Matter Search Project

• The Optical Gravitational Lensing Experiment (OGLE)

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