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Star Talk

Mysteries of universe include dark matter, dark energy, WIMPs

Sunday, February 24, 2013
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Star Talk


This photo supplied by NASA-ESA shows a ring of what NASA says is dark matter, which measures 2.6 million light-years across, which was found in the cluster ZwCl0024+1652, located 5 billion light-years from Earth. An international team of astronomers using the Hubble Space Telescope has discovered the ghostly ring of dark matter that was formed long ago during a titanic collision between two massive galaxy clusters. It is the first time that a dark matter distribution has been found that differs substantially from the distribution of ordinary matter. Astronomers have long suspected the existence of the invisible substance of dark matter as the source of additional gravity that holds galaxy clusters together. Although astronomers don't know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the Universe.
This photo supplied by NASA-ESA shows a ring of what NASA says is dark matter, which measures 2.6 million light-years across, which was found in the cluster ZwCl0024+1652, located 5 billion light-years from Earth. An international team of astronomers using the Hubble Space Telescope has discovered the ghostly ring of dark matter that was formed long ago during a titanic collision between two massive galaxy clusters. It is the first time that a dark matter distribution has been found that differs substantially from the distribution of ordinary matter. Astronomers have long suspected the existence of the invisible substance of dark matter as the source of additional gravity that holds galaxy clusters together. Although astronomers don't know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the Universe.

We are in the dark when it comes to solving some of the modern mysteries of the universe. Reinforced by several observations that the Hubble telescope made over a decade ago, the solutions to fundamental puzzles about the universe remain elusive.

One such mystery started during the 1930s when California Institute of Technology astronomer Fritz Zwicky used the 100-inch Lick Observatory telescope — then the largest in the world — to study distant clusters of galaxies. He used these observations to determine how much material it would take to keep all the galaxies together in the cluster, then compared that amount to the total amount of material he could observe in the cluster.

What he found remains unanswered: There was not enough matter in the clusters of galaxies to hold them together. He concluded that most of the matter within the clusters was not from the stars in those galaxies but rather from nonluminous material between the galaxies. This missing mass has become known as “dark matter.”

At the time, Zwicky’s colleagues dismissed his results as preposterous, thinking there was probably a mistake in his analysis. This was reinforced by Zwicky’s eccentric personality that thought beyond contemporary views at the time.

Thirty years later, a young female astronomer would show that Zwicky was not “reaching in the dark.”

More darkness

During the 1960s, Vera Rubin, an astronomer at the Carnegie Institution used the 200-inch Mount Palomar observatory — by then the largest in the world — to measure how fast stars orbit within galaxies.

She found that stars in the outer parts of galaxies orbited faster than expected because of the gravity of their galaxy’s stars alone. Rubin’s team concluded that there must be substantial amounts of unseen matter surrounding these galaxies.

By the 1980s, the scientific evidence for the missing mass around galaxies and in clusters of them was substantial. In the 1990s, the Hubble telescope photographed clusters of galaxies and found that these clusters formed images of galaxies behind — and more distant from — the clusters.

Einstein predicted this “gravitational lensing,” and it is an alternate way of “weighing” clusters to determine the amount of dark matter there.

What could it be?

There are two possibilities for dark matter. Either it is ordinary matter in forms that current technology cannot detect, or it is unknown subatomic particles that have yet to be discovered. Studies have shown that while some of the dark matter might be ordinary matter, such matter falls far short of the amount needed.

At first, scientists thought that dark matter could be the elusive neutrino particle that interacts very weakly with ordinary matter and travels close to the speed of light. These lightweights easily evade detection but also can easily escape the gravitational pull of galaxies.

Therefore, it might be weakly interacting particles that are heavier (relative to subatomic particles) and slower than neutrinos. These are termed Weakly Interacting Massive Particles or WIMPs.

Although astronomers hope to observe WIMPs with underground detectors that are shielded by rock from other cosmic particles, it may be that the most energetic “atom smashers” on Earth can make dark matter. Scientists are optimistic that the Large Hadron Collider in Geneva, Switzerland, currently the world’s most powerful particle accelerator, will soon reach energies great enough to produce dark matter particles and solve this major scientific mystery.

More dark stuff

In the mid-1990s, controversial observations arose suggesting that the oldest stars were slightly older than the universe — an obvious inconsistency. By studying very distant exploding stars, those manning the Hubble telescope found that to resolve this discrepancy the expansion of the universe must be speeding up over time. This suggests that there is some mysterious repulsive force driving this acceleration.

Einstein first dealt with a similar idea when he found that gravity should make the universe collapse upon itself. This concept conflicted with the prevailing view at the time that the universe was static, so he added a repulsive force to counteract the force of gravity and keep the universe from changing. When Edwin Hubble discovered the expansion of the universe, Einstein called this his greatest blunder.

Whatever “anti-gravity” influence is causing the Hubble expansion to accelerate, it has gone by different names — quintessence, the cosmological constant and negative pressure — but in analogy with dark matter, the term “dark energy” is most popular, leaving us with another modern mystery of the universe.

Comet PanStarrs

If you like to see comets and know of a location with an unobstructed view along the west to the northwestern horizon, take a pair of binoculars and plan on being outside during twilight in two weeks. A comet discovered nearly two years ago will have gotten bright enough to be seen through binoculars and might even be bright enough to be seen in the twilight sky without any optical aids.

Called Comet PanStarrs because it was found by the Hawaiian automated sky survey of the same name, the comet is in an orbit that places it low in the sky shortly after sunset.

Start your comet search about a half hour after sundown (about 6:30 p.m.) on March 12 by scanning above the western horizon. In your binoculars, you should see the comet as a bright, extended, fuzzy haze with its tail pointing away from the sun.

Searching to the right of the comet that evening, you might also see the very thin crescent moon. By 7 p.m., the moon and the comet will have set. On the next evening, the crescent moon will be at a greater altitude while the comet will be slightly higher and slightly to the right of its position the previous evening.

This trend will continue with the comet moving slightly higher and farther north each evening. By the third week of March, Comet PanStarrs will be in the west-northwest just above the sunset glow. After that, it will have faded and be too faint to be seen using ordinary binoculars.

Richard Monda is an astronomer living in the Capital Region.

 
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