Interesting article about The Great Attractor.
Sent: 6/16/2013 6:25:36 P.M. Eastern Daylight Time
Subj: The Daily Galaxy: News from Planet Earth & Beyond
_The Daily Galaxy: News from Planet Earth & Beyond_
_"The Great Attractor" --Is Something is Pulling Our Region of the
Universe Towards a Colossal Unseen Mass?_
Posted: 15 Jun 2013 03:51 PM PDT
A busy patch of space has been captured in the image below from the
NASA/ESA _Hubble Space Telescope_ (http://hubble.nasa.gov/)
. Scattered with many
nearby stars, the field also has numerous galaxies in the background.
Located on the border of Triangulum Australe (The Southern Triangle) and Norma
(The Carpenter’s Square), this field covers part of the Norma Cluster
(_Abell 3627_ (http://en.wikipedia.org/wiki/Norma_Cluster)
) as well as a dense
area of our own galaxy, the Milky Way.The Norma Cluster is the closest
massive galaxy cluster to the Milky Way, and lies about 220 million light-years
away. The enormous mass concentrated here, and the consequent gravitational
attraction, mean that this region of space is known to astronomers as the
_Great Attractor_ (http://en.wikipedia.org/wiki/Great_Attractor)
, and it
dominates our region of the Universe.
he largest galaxy visible in this image is ESO 137-002, a spiral galaxy
seen edge on. In this image from Hubble, we see large regions of dust across
the galaxy’s bulge. What we do not see here is the tail of glowing X-rays
that has been observed extending out of the galaxy — but which is invisible
to an optical telescope like Hubble.
Observing the Great Attractor is difficult at optical wavelengths. The
plane of the Milky Way — responsible for the numerous bright stars in this
image — both outshines (with stars) and obscures (with dust) many of the
objects behind it. There are some tricks for seeing through this — infrared or
radio observations, for instance — but the region behind the center of the
Milky Way, where the dust is thickest, remains an almost complete mystery to
Recent evidence from the _European Space Agency_
(European%20Space%20Agency)&t=h) 's _Atacama Desert_
(Atacama%20Desert)&t=h) telescopes in Chile appears
to contradict the "great attractor" theory. Astronomers have theorized for
years that something unknown appears to be pulling our Milky Way and tens
of thousands of other galaxies toward itself at a breakneck 22 million
kilometers (14 million miles) per hour. But they couldn’t pinpoint exactly
what, or where it is.
A huge volume of space that includes the Milky Way and super-clusters of
galaxies is flowing towards a mysterious, gigantic unseen mass named mass
astronomers have dubbed "The Great Attractor," some 250 million light years
from our Solar System.
The Milky Way and Andromeda galaxies are the dominant structures in a
galaxy cluster called the _Local Group_
which is, in turn, an outlying member of the _Virgo supercluster_ (ht
tp://en.wikipedia.org/wiki/Virgo_Supercluster) . Andromeda--about 2.2 million
light-years from the Milky Way--is speeding toward our galaxy at 200,000
miles per hour.
This motion can only be accounted for by gravitational attraction, even
though the mass that we can observe is not nearly great enough to exert that
kind of pull. The only thing that could explain the movement of Andromeda
is the gravitational pull of a lot of unseen mass--perhaps the equivalent of
10 Milky Way-size galaxies--lying between the two galaxies.
Meanwhile, our entire Local Group is hurtling toward the center of the
Virgo Cluster (image above) at one million miles per hour.
The Milky Way and its neighboring _Andromeda galaxy_
, along with some 30 smaller ones, form what is
known as the Local Group, which lies on the outskirts of a “super cluster”—
a grouping of thousands of galaxies—known as Virgo, which is also pulled
toward the Great Attractor. Based on the velocities at these scales, the
unseen mass inhabiting the voids between the galaxies and clusters of galaxies
amounts to perhaps 10 times more than the visible matter.
Even so, adding this invisible material to luminous matter brings the
average mass density of the universe still to within only 10-30 percent of the
critical density needed to "close" the universe. This phenomena suggests
that the universe be "open." Cosmologists continue to debate this question,
just as they are also trying to figure out the nature of the missing mass,
or "dark matter."
It is believed that this dark matter dictates the structure of the
Universe on the grandest of scales. Dark matter gravitationally attracts normal
matter, and it is this normal matter that astronomers see forming long thin
walls of super-galactic clusters.
Recent measurements with telescopes and space probes of the distribution
of mass in M31 -the largest galaxy in the neighborhood of the Milky Way- and
other galaxies led to the recognition that galaxies are filled with dark
matter and have shown that a mysterious force—a dark energy—fills the
vacuum of empty space, accelerating the universe's expansion.
Astronomers now recognize that the eventual fate of the universe is
inextricably tied to the presence of dark energy and dark matter.The current
standard model for cosmology describes a universe that is 70 percent dark
energy, 25 percent dark matter, and only 5 percent normal matter.
We don't know what dark energy is, or why it exists. On the other hand,
particle theory tells us that, at the microscopic level, even a perfect
vacuum bubbles with quantum particles that are a natural source of dark energy.
But a naïve calculation of the dark energy generated from the vacuum yields
a value 10120 times larger than the amount we observe. Some unknown
physical process is required to eliminate most, but not all, of the vacuum
energy, leaving enough left to drive the accelerating expansion of the universe.
A new theory of particle physics is required to explain this physical
process.The new "dark attractor" theories skirt the so-called _Copernican
that posits that
there is nothing special about us as observers of the universe suggesting
that the universe is not homogeneous. These alternative theories explain the
observed accelerated expansion of the universe without invoking dark
energy, and instead assume we are near the center of a void, beyond which a
denser "dark" attractor pulls outwards.
In a paper appearing in Physical Review Letters, Pengjie Zhang at the
Shanghai Astronomical Observatory and Albert Stebbins at Fermilab show that a
popular void model, and many others aiming to replace dark energy, don’t
stand up against telescope observation.
Galaxy surveys show the universe is homogeneous, at least on length scales
up to a gigaparsec. Zhang and Stebbins argue that if larger scale
inhomogeneities exist, they should be detectable as a temperature shift in the
cosmic microwave background—relic photons from about 400,000 years after the
big bang—that occurs because of electron-photon (inverse Compton) scattering.
Focusing on the “Hubble bubble” void model, they show that in such a
scenario, some regions of the universe would expand faster than others, causing
this temperature shift to be greater than what is expected. But telescopes
that study the microwave background, such as the Atacama telescope in
Chile or the South Pole telescope, don’t see such a large shift.
Though they can’t rule out more subtle violations of the Copernican
principle, Zhang and Stebbins’ test reinforces Carl Sagan's dictum that
"extraordinary claims require extraordinary evidence."
The Daily Galaxy via PhysRevLett.107.041301 and
Elliptical Galaxy Harbors Largest Known Black Hole in Universe (Weekend Feature)_
_Massive "Missing-Link" Galaxy
Discovered --10 Times Size of Milky Way_
_26 New Black Holes Found In Andromeda,
Our Sister Galaxy_
_500 Billion --A Universe of
Galaxies: Some Older than Milky Way_
_"Cosmic Flows" --Mapping the Movements of the
) _Never-Before-Seen Cluster of Hydrogen Clouds Between Andromeda and
Triangulum Galaxies --May Be Result of Dark Matter_
_Dwarf Galaxy Found with Only 1,000
Stars Bound by Dark Matter_
_The Milky Way's
Violent Core --"Was It the Site of an Ancient Collision of Black Holes?"_
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