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Dark matter
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Dark matter

Dark matter is matter that can't be detected by its emitted radiation but whose presence can be inferred from gravitationalal effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe based on gravitational effects consistently suggest that there is far more matter than is directly observable. In addition the existence of dark matter resolves a number of inconsistencies in the big bang theory.

Most of the mass of the universe is believed to exist in this form. Determining the nature of dark matter is also known as the dark matter problem or the missing mass problem, and is one of the most important problems in modern cosmology.

The question of the existence of dark matter may seem irrelevant to our existence here on Earth. However, the fact of whether or not dark matter really exists could determine the ultimate fate of the present universe. We know the universe is now expanding because of the red shift that light from distant heavenly bodies exhibits. The amount of ordinary matter seen in the universe is not enough for gravity to stop this expansion, and so the expansion would continue forever in the absence of dark matter. In principle, enough dark matter in the universe could cause the universe's expansion to stop or even reverse (leading to an eventual Big Crunch). In practice, it is currently thought that the dynamics of the universe are dominated by another component, dark energy.

Table of contents
1 Evidence of existence
2 Discovery of dark matter
3 Alternative explanations
4 Related topics
5 External links

Evidence of existence

Much of the evidence for dark matter comes from the study of galaxy clusters. Many of these appear to be roughly static and fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater, often off by an order of magnitude or so, and assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this.

With gravitational theory and new computer analyses, astronomers have now been able to work out where the dark matter is. It is just what you would expect if dark matter and galaxies are clustered in exactly the same way. Galaxies themselves also show signs of being composed largely of dark matter - for instance the rotation curves in and indeed the very existence of our galaxy's disc indicates the presence of a large extended halo.

Knowing where the dark matter is, also reveals how much of it exists. About seven times as much as ordinary matter (that is only one quarter of what is necessary to slow down the universe's expansion to a halt).

Since it cannot be detected via optical means, the composition of dark matter remains speculative. (Although some experiments are underway attempting to directly detect dark matter passing through the Earth, they have not yet succeeded.) Large masses like galaxy-sized black holes can be ruled out on the basis of lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs." The possible amount of baryonic dark matter is, however, restricted by big bang nucleosynthesis. At present, though, the most common view is that dark matter is made of elementary particles other than the usual electrons, protons, and neutrons, such as neutrinos, axions, or hypothetical particles known as weakly interacting massive particles (or "WIMPs"). Other hypothetical candidates for dark matter are supersymmetric particles (sparticles). It is hypothesized that WIMPs are actually sparticles such as neutralinos.

See also strange matter.

Discovery of dark matter

Dark matter was first hypothesized to exist by the Swiss astrophysicist Fritz Zwicky. In 1933 Zwicky estimated the amount of mass in the galaxy (based on the number of stars and their brightness) and then found the rate at which our own Milky Way and other galaxies spin around their center. When he used a different method independent from brightness he found about 400 times more matter than he had based on number of stars and brightness. He then found the velocity to be more than twice the possible rate with the amount of mass from the brightness estimate. If the normal physical laws were applied the galaxies would be torn to shred by the high speeds because the gravity they exert would not be sufficient to hold it together. This is known as the "missing mass problem." Based on these conclusions, he stated that there must be some other form of matter existent in the galaxy which we have not detected, which provides enough of the mass and gravity to hold the galaxy together.

From there the search for this source of the sufficient gravity has commenced. At present, the density of the universe (excluding dark matter) is estimated to be about one hydrogen atom per cubic meter of empty space. This is not enough density for the universe to collapse on itself. However, dark matter is said to form 90-95 percent of all matter in the universe. This means that only 5-10 percent of all matter is observable.

Cosmologists (astronomers who study the history, origin, and future of the universe) believe there are two classes of dark matter: baryonic (the name given to all "normal matter" composed of baryons: protons, neutrons, and electrons) dark matter, called MACHOs (Massive Compact Halo Objects) and the mysterious "shadow matter" composed of unknown non-baryonic subatomic particles known as WIMPs (Weakly Interacting Massive Particles), neutrinos, and axions. It is ironic that Wimps and Machos are the exact opposites in our language.

Alternative explanations

An alternative to dark matter is to suppose that gravitational forces become stronger than the Newtonian approximation at great distance. For instance, this can be done by assuming a negative value of the cosmological constant (the value of which is believed to be positive based on recent observations) or by assuming modified Newtonian dynamics. Another approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential with the expression

where B and ρ are adjustable parameters. However, all such approaches run into difficulties explaining the different behavior of different galaxies and clusters, whereas one can easily describe such differences by assuming different quantities of dark matter.

For a deeper discussion of this subject, see Modified Newtonian dynamics.

Data from galaxy rotation curves indicate that around 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect. There are several types of dark matter postulated to exist.

Hot dark matter consists of particles that travel with relativistic velocities. The best candidate for hot dark matter is the neutrino. Neutrinos have negligible mass, do not partake in either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is why they are such good candidates for hot dark matter. However, current bounds on the neutrino mass indicate that ordinary neutrinos have only a small contribution to dark matter. To explain the small neutrino mass so-called sterile neutrinos can be added to the Standard Model. These sterile neutrinos are expected to be heavier than the ordinary neutrinos. They are therefore a good candidate for dark matter.

Hot dark matter cannot explain how individual galaxies formed from the big bang. The microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such as smooth initial clumping. To explain small scale structure in the universe it is necessary to invoke cold dark matter. Hot dark matter therefore is nowadays always discussed as part of a mixed dark matter theory.

Dark matter is not to be confused with white matter or gray matter (two types of tissue in the nervous system)

Related topics

External links