http://www.facebook.com/ScienceReason ... The Mystery of Dark Matter (Chapter 1/4): Stars Are Moving Too Fast.
A
mystery exists! Galaxies do not seem to have enough mass for stars to
orbit at their observed speeds. Galaxies should be flying apart, but
they don't. Why not? Explore the surreal world of dark matter - one of
the universe's greatest mysteries.
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Shedding Light on Dark Matter
Over
the last few decades, physicists have discovered that around ninety
percent of every galaxy in the universe is made of an unseen substance
called dark matter. Damian Pope, PIs Senior Manager of Scientific
Outreach, comments, Its currently one of the hottest topics in physics.
The module provides teachers with tools to show how dark matter was
discovered, to explain why it remains a mystery, and to share the
passion of scientists who are trying to discover what its made of.
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http://www.perimeterinstitute.ca---
In
astronomy and cosmology, dark matter is a form of matter that is
undetectable by its emitted electromagnetic radiation, but whose
presence can be inferred from gravitational effects on visible matter
and background radiation. According to present observations of
structures larger than galaxies, as well as Big Bang cosmology, dark
matter accounts for the vast majority of the mass in the observable
universe.
Dark matter was postulated by Fritz Zwicky in 1934, to
account for evidence of "missing mass" in the orbital velocities of
galaxies in clusters. Subsequent to then, other observations have
indicated the presence of dark matter in the universe, including the
rotational speeds of galaxies, gravitational lensing of background
objects by galaxy clusters such as the Bullet Cluster, and the
temperature distribution of hot gas in galaxies and clusters of
galaxies.
Dark matter plays a central role in state-of-the-art
modeling of structure formation and galaxy evolution, and has measurable
effects on the anisotropies observed in the cosmic microwave
background. All these lines of evidence suggest that galaxies, clusters
of galaxies, and the universe as a whole contain far more matter than
that which interacts with electromagnetic radiation: the remainder is
frequently called the "dark matter component," even though there is a
small amount of baryonic dark matter. The largest part of dark matter,
which does not interact with electromagnetic radiation, is not only
"dark" but also, by definition, utterly transparent.
The vast
majority of the dark matter in the universe is believed to be
nonbaryonic, which means that it contains no atoms and that it does not
interact with ordinary matter via electromagnetic forces. The
nonbaryonic dark matter includes neutrinos, and possibly hypothetical
entities such as axions, or supersymmetric particles. Unlike baryonic
dark matter, nonbaryonic dark matter does not contribute to the
formation of the elements in the early universe ("big bang
nucleosynthesis") and so its presence is revealed only via its
gravitational attraction. In addition, if the particles of which it is
composed are supersymmetric, they can undergo annihilation interactions
with themselves resulting in observable by-products such as photons and
neutrinos ("indirect detection").
Nonbaryonic dark matter is
classified in terms of the mass of the particle(s) that is assumed to
make it up, and/or the typical velocity dispersion of those particles
(since more massive particles move more slowly). There are three
prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter
(HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some
combination of these is also possible. The most widely discussed models
for nonbaryonic dark matter are based on the Cold Dark Matter
hypothesis, and the corresponding particle is most commonly assumed to
be a neutralino. Hot dark matter might consist of (massive) neutrinos.
Cold dark matter would lead to a "bottom-up" formation of structure in
the universe while hot dark matter would result in a "top-down"
formation scenario.
As important as dark matter is believed to be
in the universe, direct evidence of its existence and a concrete
understanding of its nature have remained elusive. Though the theory of
dark matter remains the most widely accepted theory to explain the
anomalies in observed galactic rotation, some alternative theories such
as modified Newtonian dynamics and tensor-vector-scalar gravity have
been proposed. None of these alternatives, however, has garnered equally
widespread support in the scientific community.
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http://en.wikipedia.org/wiki/Dark_matter.
