Page 11, Cosmology: [Previous Page] [Next Page] [Class Home Page] Text Pages 425-427

It is expected that the universe is perfectly FLAT. That is, it contains precisely the critical density to keep it from collapsing or from expanding for ever. As we have seen, a significant amount of matter remains undetected in light. What is this dark matter? There are two forms:

1) Baryonic
This is the kind of matter you are familiar with. It is made up of neutron, protons, electrons. We are a sort of dark matter, so are cold rocks floating out in space, since we all emit very little detectable light. Other examples: planets, brown dwarfs and compact objects (like black holes), However, we have a pretty good idea of how much baryonic matter there is in the universe (from the fusion reactions which occurred during the big bang). Baryons account for only 5% of the critical mass in the universe, though we only 'see' about 10% of that. So there is significant "dark baryonic matter". One form of baryonic dark matter is commonly referred to as MACHOs (massive compact halo objects). Astronomical searches in our halo have found MACHOs, but there are not enough to account for all the dark baryonic matter.

2) Nonbaryonic matter
This you are not familiar with. In fact, no one is. All we really know is it exerts its gravitational influence on normal (baryonic) matter, but otherwise it doesn't interact (or very very little) with normal matter.

• Axions & photinos: just two rather exotic particles, predicted by theory, which have never been detected in the lab, but would have mass.
• WIMPs: weakly interacting (that is they interact via the Weak Force, as well as via gravity) massive particles, have also been proposed, but again no observations exist to confirm.
• Neutrinos: now that we know they oscillate, this tells us they have mass! And there are a LOT of neutrinos in the universe. However, even the greatest possible mass consistent with current experiments indicate the neutrino contribution to dark matter is negligible.

Formation of Galaxies and Dark Matter
At the time of recombination, we see a fairly smooth distribution of baryonic matter. Dark matter has acted upon that smooth distribution to make it clump and form the galaxies and clusters of galaxies we see today in a relatively short time period.

So, dark matter is actually needed in order to explain the formation of galaxies and galaxy clusters so quickly in our universe. For this reason, Cold Dark Matter, material that moves slowly and can help to clump up ordinary matter, is preferred over forms of dark matter called Hot Dark Matter, which moves very quickly and would not help to create the galaxy structures we see today.

How much dark matter is needed?
Interestingly enough, we do not need to have all the energy in matter (dark or light, baryonic or nonbaryonic) to keep the universe flat. We only need about 30% of the critical density in matter. There is another source of 'energy' which can keep the universe from expanding for ever, and which accounts for the other 70% of the critical density. More on that soon.