Dark Matter

       Dark matter            Not dark matter
There is perhaps no current problem of greater importance to astrophysics and cosmologythan that of "dark matter". The controversy, as the name implies, is centered around the notion that there may exist an enormous amount of matter in the Universe which cannot be detected from the light which it emits. This is "stuff" which cannot be seen directly. So what makes us think that it exists at all? Its presence is inferred indirectly from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations.

The basic principle is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous stuff. Hence we infer that there is dark matter in the Universe. There are many different pieces of evidence on different scales. And on the very largest scales, there may be enough to "close" the Universe, so that it will ultimately re-collapse in a Big Crunch.

Various means of weighing the universe lead us to believe in the presence of dark matter. There is evidence from different astronomical objects, in order of increasing size:

Dark matter has important consequences for the evolution of the Universe. According to standard cosmological theory, the Universe must conform to one of three possible types: open, flat, or closed. A parameter known as the "mass density" - that is, how much matter per unit volume is contained in the Universe - determines which of the three possibilities applies to the Universe. In the case of an open Universe, the mass density (denoted by the greek letter Omega) is less than unity, and the Universe is predicted to expand forever. If the Universe is closed, Omega is greater than unity, and the Universe will eventually stop its expansion and recollapse back upon itself. For the case where Omega is exactly equal to one, the Universe is delicately balanced between the two states, and is said to be "flat".

In the figure above we show graphically some of the measurements of the density of the universe which we have discussed above. What is plotted is the density of the universe, both visible matter and the inferred "dark matter", as a function of the "scale" at which the measurement was made, from the local neighbourhood up to the largest scales. On the smallest scales, probed by Oort, the visible matter and three times as much dark matter give Omega about 1/1000. As we go to larger and larger scales the inferred value of Omega increases, although the measurements become harder and progressively more uncertain. The next point to the right is the mass in galaxies, which moves to the position of the higher dot if we include the dark matter inferred from rotation curves. Then on larger scales we have the measurements from the motions of clusters of galaxies and the cosmic microwave background. The yellow band indicates the amount of matter that can reside in "normal" matter, or baryons, as inferred from Nucleosynthesis. If there is more matter in the universe than this, as the measurements appear to be telling us, then it must be made up of some strange particle which is not familiar to us here on earth.

There is also a somewhat philosophical idea that makes Omega=1 attractive. The point is that as the Universe expands the value of Omega changes. In fact the value 1 is unstable, and the Universe would prefer to evolve towards one of the two natural values: 0, if the expands forever further apart until the Universe is almost totally empty ; and infinity, if the matter recollapses to a state of higher and higher density. Then the observation that Omega is fairly close to 1 today, means that it must have been even closer to 1 in the past. It is unsatisfying to believe that we just happen to live at the time when Omega is just starting to depart from 1 by a small factor. It is much more appealing to consider that we do not live at a special epoch, so that Omega is still close to 1 today. But then we need to explain why Omega started out very close to 1 in the early universe. The theory of inflation provides just such a justification - it predicts that the early Universe was driven extremely close to flat, and that it is still very close to flat today. If this is so, then at least 90% of the mass of the Universe is dark. Dark matter (DM) candidates are usually split into two broad categories, with the second category being further sub-divided:

depending on their respective masses and speeds. CDM candidates have relatively large mass and travel at slow speeds (hence "cold"), while HDM candidates include minute-mass, rapidly moving (hence "hot") particles.

For more information

Emad Iskander, Douglas Scott, Joe Silk & Martin White
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