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Dark matter is matter that can't be detected by its emitted
radiation but whose presence can be inferred from gravitational 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.
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 10-5 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 Astrophysical 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.
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
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