Dark Matter

[Xerxes314]

Dark matter is anything that has mass but does not radiate. It can be any of a large number of things, but the vast majority of it is made up of a currently unknown type of matter. Here are a few kinds of dark matter:

1) Cold neutral hydrogen. It's cold, so it doesn't emit light or IR, and it's neutral, so it doesn't emit or scatter radio. The only way to see it is in the lyman alpha forest or the 21cm line. The space between galaxies is filled with this stuff. We know it can't be all the dark matter because you can work out the baryon density of the universe from Big Bang nucleosynthesis (the ratio of elements found in the Universe). Since hydrogen is baryonic matter, we know there can't be that much of it.

2) MaCHOs (Massive Compact Halo Objects). These are anything big and star-like that orbits way out on the fringes of galaxies. Things like brown dwarves and black holes. Brown dwarves are too cool to be detected far from Earth and black holes don't radiate at all. However, we can estimate the number of these things by looking for them when they cross our view of a star. The number of microlensing events that we see is small, so we know there are too few MaCHOs to account for dark matter.

3) Neutrinos. These particles are neutral, so they don't interact with light. They're very weakly interacting and hard to detect. We know that the universe is packed with a huge number of neutrinos. They make up about 1% of dark matter. We know they can't make up more, because neutrinos are hot dark matter (they retain the heat they started with in the Big Bang because they don't interact). When the universe is filled with too much hot dark matter, galaxies cannot form; so we know most dark matter must be cold.

4) Cold dark matter (CDM). This is the stuff we don't know much about. You can work out from WMAP data, galaxy rotation curves and gravitational lensing how much of this stuff there should be. It's roughly 10 times the amount of ordinary light matter in the universe. People have a lot of different ideas what it might be:

4a) WIMPs (Weakly Interacting Massive Particles). These would be something like the lightest supersymmetric particle (usually the neutralino). They would be very heavy and very weakly interacting compared to ordinary particles. Also, they're predicted by SuSy, which is fairly well regarded theoretically.

4b) Axions. These particles are predicted by certain models of the strong interaction. They would explain why we only see a small amount of CP violation (matter-antimatter asymmetry) in the universe. They would be very light and very very very weakly interacting. So weak that people think it's unlikely that they exist; it's a problem called fine tuning.

4c) Weirder stuff. Magnetic monopoles? Quark matter nuggets? Cosmic strings? Anything else you can imagine.

Xerxes