Largest Structures in the Universe Reveal a Mystery --"An Unseen Mass"
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October 22, 2013

Largest Structures in the Universe Reveal a Mystery --"An Unseen Mass"

 

Shapley_Supercluster


The reality of superclusters was not apparent until the 1980s, when new telescopes and sensors could produce three-dimensional maps of the Universe. Superclusters are typically seen as long and thin strands of clusters and galaxies, intracluster gases and, presumably, "dark matter" on a two dimensional surface, interspersed by large voids nearly empty of matter. Astronomers discovered that the galaxies in the Universe are arranged in sheets and walls surrounding large nearly-empty voids. Spiral galaxies line up like beads on a string, with their spin axes aligned with the filaments that outline voids While scanning the sky for the oldest cosmic light, ESA’s Planck satellite captured snapshots of some of the largest galaxy clusters and superclusters in the Universe. Several hundred galaxies and the huge amounts of gas that permeate them are shown in the image above of the core of the Shapley Supercluster, the largest cosmic structure in the local Universe, discovered in the 1930s by American astronomer Harlow Shapley, as a remarkable concentration of galaxies in the Centaurus constellation.

Boasting more than 8000 galaxies and with a total mass more than ten million billion times the mass of the Sun, it is the most massive structure within a distance of about a billion light-years from our Milky Way Galaxy.The hot gas pervading galaxy clusters shines brightly in X-rays, but it is also visible at microwave wavelengths, which Planck sees as a distinctive signature in the cosmic microwave background – the afterglow of the Big Bang.

Looking for this signature – called the Sunyaev–Zel’dovich effect – Planck has already spotted more than 1000 galaxy clusters, including several superclusters and pairs of interacting clusters.

The image above of the core of the Shapley Supercluster combines the gas detected with Planck at large scales between the members of the supercluster (shown in blue) with that detected in X-rays within the galaxy clusters of Shapley using the Rosat satellite (pink), as well as a view of its rich population of galaxies as observed at visible wavelengths in the Digitised Sky Survey.The largest pink blobs of X-rays identify the two galaxy clusters Abell 3558 on the right and Abell 3562 on the left, as well as a couple of smaller groups between them.

The image measures 3.2 x 1.8 square degrees and shows the central portion of the Shapley Supercluster. It was produced by reconstructing the Sunyaev–Zel’dovich effect from the Planck frequency maps, and was first published in a Planck Collaboration paper in March 2013. Planck released a cosmic map in March refining the Universe’s age to 13.82 billion years. Planck will run out of fuel on Oct.23 and will be in parking orbit around the Su

The Virgo Supercluster or Local Supercluster (center of image below) is the irregular supercluster that contains the Virgo Cluster in addition to the Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 110 million light-years. It is one of millions of superclusters in the observable Universe.

 

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It was discovered in the 1980's that not only the Local Group, but all matter out to a distance of at least 50 Mpc is experiencing a flow on the order of 600 km/sec in the direction of the Norma cluster (Abell 3627). Lynden-Bell dubbed the cause of this "The Great Attractor". While astronomers are confident of the velocity of the LS, which has been measured against the Cosmic Microwave Background (CMB), the nature of what is causing it remains one of the great mysteries of astronomy.

Astronomers have known for years that something seems 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 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 (see image below), 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.

          
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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.

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.

The Great Attractor is a diffuse concentration of matter some 400 million light-years in size 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.

Major concentration of galaxies lies beyond the Great Attractor, near the Shapley Supercluster, 500 million light-years away. 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 Daily Galaxy via ESO, Planck Institute, universe-review.ca, solstation.com, nasa.gov world-science.net

Comments

None of this is predicted by the Big Bang, despite what the article says. This version of the universe (supported by the data) shows a clumpy universe with bits of it moving in odd directions. It's not the result of an explosion (or an expansion). It's interesting, but it's not a product of the Big Bang. (I have no idea what it is the product of, if anything, I just don't know.)

It is interesting to note how The Great Attractor seems to be accepted by the astronomical community but the Dark Flow spoken of in the "Plank Data Sparks Controversy" article causes, well, controversy. :)

I really don't understand what is so mysterious about any of this (or many other discoveries that seem to constantly confound "men of science"). If I let go of the apple in my hand, it falls to the ground. If the Earth stopped moving in its orbit, it would fall into the Sun. Eventually most of the stars in the Milky Way will end up coalescing into the supermassive black hole at the middle of it. All the "Great Attractor" is (whether it is the Shapley Supercluster or something else) is a concentration of matter so immense that it is affecting other matter billions of light years away (organized matter, like galaxies and solar systems, and disorganized matter like interstellar gas and clusters of galaxies). The fact that when you look at the known universe as a whole it is lumpy and disorganized is just the way it looks at that scale. Just like quantum froth looks lumpy and disorganized at its scale. But at a different scale, the "dark flow" will become patently obvious and probably commonplace: it's just what happens as more and more matter interacts and coalesces in different parts of the *unknown* universe. At *that* scale. Just like the apple falling from my hand.


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