Re: What are some possibilities of the nature of 'dark matter?'

Stephen:

Dark Matter could possibly take several forms. At the moment there are two main possibilities for the form it takes. One idea is that it is in the form of "exotic" matter, called WIMPS or Weakly Interacting Massive Particles. The other is that it is old dead stars that no longer radiate or large gas giants that never quite became stars, and so do not radiate. These would form a large part of the universe we can not detect easily. This is called the MACHO model, or Massive Compact Halo Objects as they are thought to exist in a large cloud, or halo, around Galaxies.

If dark matter is in the form of WIMPS then they would exist in and around you, that is, there are 90 times more of these than normal matter. They would pervade everything. As I'll explain, you could not hold them in your hands. If it is in the form of MACHO's then they would exist only in space and being almost stars you could not hold them, though they would be more tangible than a WIMP.

It is known from particle accelerators that there exist large numbers of particles. Some have large energies, some have small energies. Typically the larger the energy a particle has the shorter it's life before it decays. These particles are all `normal' and can be fully understood using the Standard Model of particle physics . The idea of WIMPS is to have a particle that is massive, have a large energy, but does not interact with anyhting, or at least it does not do so easily. These strange creatures would only be felt by their gravitational interactions. Some eight years ago a number of experiments where set up to try and detect these particles. One I know of is the

UK dark matter experiment. As sar as I know this has yet to discover any WIMP's so it is unlikely at present that dark matter takes this form. If WIMPS exist they do not obey any of the known models in physics though some new work on "super symmetry" seems to imply they may exist. This is all very theoretical at the moment with no experimental evidence to back it up.

We know from models on Stellar Evolution
that many stars are small red dwarfs. these are hard to detect as they have low luminosity, or brightness. The same models imply that there should exist large numbers of so called
brown dwarfs, these are objects that are just too small to become a star. One such brown dwarf, called Gliese 229B, has been detected by the Hubble Space Telescope . These brown dwarfs are an example of what a MACHO object is.

Based on present evidence it looks like dark matter is probably in the form of low mass objects that obey normal physics rather than strange new particles that are not explained by current ideas in physics. So, dark matter is probably found in space and though too large to actually do this, you could lift it or feel it.

If you want to know more I would suggest reading "The New Physics" edited by P. Davies, this has several chapters on stellar evloution, dark matterm particle pysics and super symmetry. Thankfully it is light on equations and good on explanations. Some of the links above take you to other web sites that explain more about these subjects as well.

As ever, I am more than happy to explain anything further if my explanation is unclear or lacking enough detail. Please E-Mail me on
my private account if you want to know more.

Yours
Dave Barlow