Dark Matter
https://en.wikipedia.org/wiki/Dark_matter
Dark matter is a kind of matter hypothesized in astronomy
and cosmology to account for gravitational effects that
appear to be the result of invisible mass. Dark matter
cannot be seen directly with telescopes; evidently it
neither emits nor absorbs light or other electromagnetic
radiation at any significant level. It is otherwise
hypothesized to simply be matter that is not reactant to
light. Instead, the existence and properties of dark matter
are inferred from its gravitational effects on visible
matter, radiation, and the large-scale structure of the
universe.
What is Dark Matter? A Mystery of the Universe
http://www.youtube.com/watch?v=XCwWxrx1SIU
Explanation: The matter in galaxy cluster 1E 0657-56, fondly
known as the "bullet cluster", is shown in this composite
image. A mere 3.4 billion light-years away, the bullet
cluster's individual galaxies are seen in the optical image
data, but their total mass adds up to far less than the mass
of the cluster's two clouds of hot x-ray emitting gas shown
in red. Representing even more mass than the optical
galaxies and x-ray gas combined, the blue hues show the
distribution of dark matter in the cluster. Otherwise
invisible to telescopic views, the dark matter was mapped by
observations of gravitational lensing of background
galaxies. In a text book example of a shock front, the
bullet-shaped cloud of gas at the right was distorted during
the titanic collision between two galaxy clusters that
created the larger bullet cluster itself. But the dark
matter present has not interacted with the cluster gas
except by gravity. The clear separation of dark matter and
gas clouds is considered direct evidence that dark matter
exists.
Dark Matter Core Defies Explanation in Hubble Image
MARCH 2, 2012: Astronomers observed what appeared to be a
clump of dark matter left behind during a bizarre wreck
between massive clusters of galaxies. The dark matter
collected into a "dark core" containing far fewer galaxies
than would be expected if the dark matter and galaxies hung
together. Most of the galaxies apparently have sailed far
away from the collision. This result could present a
challenge to basic theories of dark matter, which predict
that galaxies should be anchored to the invisible substance,
even during the shock of a collision.
The initial observations, made in 2007, were so unusual that
astronomers shrugged them off as unreal, due to poor data.
However, new results obtained in 2008 from NASA's Hubble
Space Telescope confirm that dark matter and galaxies parted
ways in the gigantic merging galaxy cluster called Abell
520, located 2.4 billion light-years away. Now, astronomers
are left with the challenge of trying to explain dark
matter's seemingly oddball behavior in this cluster.
Dark Matter Core Defies Explanation in Hubble Image
http://hubblesite.org/newscenter/archive/releases/exotic/dark%20matter/2012/10/full/
Newton's law of universal gravitation
https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation
Newton's law of universal gravitation states that any two
bodies in the universe attract each other with a force that
is directly proportional to the product of their masses and
inversely proportional to the square of the distance between
them.
Einstein's Law of Gravity
https://en.wikipedia.org/wiki/General_relativity
General relativity, or the general theory of relativity, is
the geometric theory of gravitation published by Albert
Einstein in 1916 and the current description of gravitation
in modern physics. General relativity generalizes special
relativity and Newton's law of universal gravitation,
providing a unified description of gravity as a geometric
property of space and time, or spacetime. In particular, the
curvature of spacetime is directly related to the energy and
momentum of whatever matter and radiation are present. The
relation is specified by the Einstein field equations, a
system of partial differential equations.
Some predictions of general relativity differ significantly
from those of classical physics, especially concerning the
passage of time, the geometry of space, the motion of bodies
in free fall, and the propagation of light. Examples of such
differences include gravitational time dilation,
gravitational lensing, the gravitational redshift of light,
and the gravitational time delay. The predictions of general
relativity have been confirmed in all observations and
experiments to date. Although general relativity is not the
only relativistic theory of gravity, it is the simplest
theory that is consistent with experimental data.
The NASA/ESA Hubble Space Telescope provides new evidence for
dark matter around small galaxies
http://www.spacetelescope.org/news/heic0903/
Peering into the tumultuous heart of the nearby Perseus
galaxy cluster, Hubble discovered a large population of
small galaxies that have remained intact while larger
galaxies around them are being ripped apart by the
gravitational tug of other galaxies.
The Hubble images provide further evidence that the
undisturbed galaxies are enshrouded by a "cushion" of dark
matter that protects them from their rough-and-tumble
neighbourhood.
Hubble Studies Dark Matter Filament in 3-D
http://www.universetoday.com/98022/hubble-studies-dark-matter-filament-in-3-d/
Astronomy Picture of the Day Archive
http://apod.nasa.gov/apod/archivepix.html
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