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.

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FIGURE: Superposed on an optical picture of a group of galaxies is an X-ray image taken by ROSAT. The image shows hot gas (which produces X-rays) highlighted in false red color . The presence of this confined gas indicates that the gravity in groups and clusters of galaxies is larger than that expected from the matter that we can observe in those galaxies.

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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.

Hot Dark Matter and Cold Dark Matter

Discussions of dark matter typically consider two extremes

• Hot Dark Matter
• Cold Dark Matter

Hot dark matter is composed of particles that have zero or near-zero mass (the neutrinos are a prime example). The Special Theory of Relativity requires that massless particles move at the speed of light and that nearly massless particles move at nearly the speed of light. Thus, such very low mass particles must move at very high velocities and thus form (by the kinetic theory of gases) very hot gases. On the other hand, cold dark matter is composed of objects sufficiently massive that they move at sub-relativistic velocities. They thus form much colder gases. The difference between cold dark matter and hot dark matter is significant in the formation of structure, because the high velocities of hot dark matter cause it to wipe out structure on small scales.

Tutorial on Current Status of Dark Matter

The following is a brief tutorial on this issue (Source):

1. If inflation is correct the density of the Universe should be exactly the closure density. Luminous stars and galaxies contribute only about 0.5% of the closure density, so 99% of the Universe is in the form of dark matter. We may speculate on what particles could make up this dark matter.
2. The known neutrinoes have problems as candidates for dark matter because they are relativistic (hot dark matter) and therefore they erase fluctuations on small scales. Thus, relativistic neutrinos could form large structures like superclusters, but would have trouble forming smaller structures like galaxies. These arguments might be at least partially invalidated if one of the types of neutrinos (the tau neutrino is the obvious candidate) is considerably more massive than the electron or muon neutrino.
3. On smaller scales such as galaxies and clusters of galaxies, dynamical estimates of the mass based on rotation curves or velocity dispersions of galaxies indicate that 90% (not 99%) of the total mass is not seen ("sub-luminous"). This implies that the mass density of the Universe is 10% of the closure density. In this case, the sub-luminous mass could 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.
4. Although inflation demands that the Universe have a density equal to its critical density (and inflation is necessary to solve certain problems of the standard big bang model like 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 (peculiar velocities).
5. 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. 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) and some cold dark matter 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 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.
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. We cannot currently test for various 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 particle physics, that would remain unnoticed in any accelerator experiments.