A rchive Date
[ 18-10-2004 ]
Category
[ Science ]
sub-Categoy
[ Physics ]
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[http://www.space.com/news/cosmic_shear_000512.html
Scientists Map Dark Matter, Prove Einstein Right
By Maia Weinstock
Staff Writer
posted: 10:48 am ET
12 May 2000
Eighty-four years after Albert Einstein introduced the world to his theory of general relativity, scientists are seeing that he was right all along about measuring what we now call dark matter.
Astronomers supported by the National Science Foundation have found the first evidence of an effect called cosmological shear, a phenomenon predicted by Einstein’s theory, in which light from distant cosmic objects bends due to gravitational forces. What’s more, the detection of cosmological shear has allowed astronomers to track down significant amounts of dark matter, non-luminous matter whose presence in the universe has been predicted, but scantly detected until now.
"This marks a totally new way of finding dark matter," said Max Tegmark, a physicist at the University of Pennsylvania. "It’s going to revolutionize our ability to map out where all the dark matter is."
The results on cosmological shear were published in this week's issue of the journal Nature.
Observable matter takes up no more than about 10 percent of the total amount of matter predicted to exist in the universe. The rest – dubbed dark matter because it can’t be "seen" or detected in the same way that gas, dust, and other observable matter can – remains largely a mystery to astronomers precisely because it’s so difficult to find. Now, researchers have a new tool for uncovering the elusive stuff.
On the left is an optical image of a small cluster of galaxies. The center image shows a 1-degree image of the same field. The right image represents a mass map of the same patch of sky. |
"The existence of dark matter has been known for a long time, and has even been mapped in various places over the past decade," explained astrophysicist David Wittman of Bell Labs, Lucent Technologies. "But the places that had been mapped were small areas of the sky. What’s new here is that we have studied a fair sample of the universe … and so were able to deduce some properties of the universe in general."
Cosmological shear helps astronomers "see" dark matter because it makes the light from distant galaxies appear distorted. A distant spherical-shaped galaxy, for example, will appear elliptical to astronomers back on Earth. This is because dark matter existing in the path between the galaxy and Earth exerts a gravitational pull on the light, causing it to bend.
Wittman and a team of researchers analyzed 145,000 distant galaxies in order to find evidence of cosmological shear, also known as "weak gravitational lensing." They used a special camera called a charged couple device or CCD and the 4-meter Blanco telescope at the Cerro Tololo Interamerican Observatory in Chile.
The recent observations have also shed new light on the eventual fate of the universe. Astrophysicists currently predict that the total amount of matter present in the universe will determine whether the universe will continue to expand, or whether it will eventually slow down, or even begin to contract. According to Wittman, the scientists’ observations of cosmological shear have suggested that the overall density of matter in the universe is "too low to stop the expansion of the universe."
At the same time, astronomers admit that their new method for finding dark matter has not yet been tested enough to allow experts to make a definitive generalization about the fate of the universe. "Since our approach is new, it’s not very precise yet," said Wittman. "Really strict tests of the theory will come in the next few years as astronomers measure the [weak] lensing more and more accurately."
"Ultimately, we’d like to be able to map the whole sky to see just how much dark matter is out there," added Tegmark. For now, the use of cosmological shear to uncover dark matter in one region of the universe at a time will have to suffice. But as Bell Labs astrophysicist Anthony Tyson joked, "The future looks bright for dark matter."]
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