Astronomers have known for years that something unknown apears 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. 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
universe as we see it contains only the stable relics and leftovers of
the big bang: unstable particles have decayed away with time, and the
perfect symmetries have been broken as the universe has cooled, but the
structure of space remembers all the particles and forces we can no
longer see around us.
Discovering what it is that makes up the heart of the Great Attractor –
will surely rank as one of the greatest discoveries in the history of
science.
Recent findings suggest these motions are the result of
gravitational forces from not one, but two things: the Great Attractor, and a
conglomerate of galaxies far beyond it.
The location of the Great Attractor was finally determined in 1986
and lies at a distance of 250 million light years from the Milky Way,
in the direction of the Hydra and Centaurus constellations. That region
of space is dominated by the Norma cluster, a massive cluster of
galaxies, and contains a preponderance of large, old galaxies, many of
which are colliding with their neighbors, and or radiating large
amounts of radio waves.
Major concentration of
galaxies lies beyond the Great Attractor, near the so-called
Shapley Supercluster, 500 million light-years away—the most massive
known super-cluster. Mapping X-ray luminous galaxy
clusters in the Great Attractor region has shown that the pull our galaxy is experiencing is most likely due to
both the nearby Great Attractor and these more distant structures.
In the 1987, a group of astronomers known as the “Seven Samurai,”
at Cal Tech uncovered this coordinated motion
of the Milky Way and our several million nearest galactic neighbors.
They found that galaxies are very unevenly distributed in space,
with galactic super-clusters separated by incredibly huge voids of
visible ordinary matter. The place towards which we all appear headed
was originally called the
New Supergalactic Center or the Very Massive Object until one of the
discoverers, Alan Dressler, decided they needed a more evocative name
and
came up with “The Great Attractor.”
The motion of local galaxies indicated there was
something massive out there that are pulling the Milky Way, the
Andromeda Galaxy, and other nearby galaxies towards it. For a while,
nobody could see what it was, because it lies behind the plane of our
Galaxy — that means the gas and dust in our Galaxy obscures the light
from the Great Attractor, and it is outshone by the stars and other
objects in our Galaxy.
The Great Attractor is a diffuse concentration
of matter some 400 million light-years in size located around 250
million light-years away within the so-called “Centaurus
Wall” of galaxies
, about seven degrees off the plane of the Milky Way. X-ray observations with the ROSAT satellite then revealed that Abell
3627 is at the center of the Great Attractor. It lies in the
so-called Zone of Avoidance, where the dust and stars of the Milky
Way’s disk obscures as much as a quarter of the Earth’s visible sky.
Posted by Casey Kazan. Image credit: Wally Pacholtz
Related Galaxy posts:
http://www.solstation.com/x-objects/great
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/990924a2.html
http://www.world-science.net/exclusives/060419_attractorfrm.html
0 comentários:
Enviar um comentário