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Current Science at NOAO

Evidence of Dark Matter Discovered

Stan Hart, NOAO RET
Based on a Solicited Contribution from David Wittman

June, 2000  |  When you look up into the sky on a dark night, the thousands of points of light you see we call stars. The reason you can see them is because they are hot enough to give off energy in the form of light. The nearest star to the earth is the sun. But even though half-a-million earths could fit inside the sun, it is puny compared to some stars. Some of the fuzzy stars you see are not stars at all but are galaxies, which are giant swirling bodies that contain billions of stars. Other fuzzy looking stars you see may be clusters of stars that can contain millions of stars. Add it all up and on a clear night you can see objects that account for billions and billions of stars. But there are many more objects just like those that are so far away and so faint we cannot see them at all.

Astronomers believe that all of the visible objects in the universe constitute only a fraction of the matter needed for our universe to function as it does. Approximately 90% of the matter that is predicted to exist can't be observed directly because it doesn't give off energy. Physicists call this matter dark matter because it is dark to all types of energy detectors. Until recently, there was no direct evidence for the existence of dark matter. Its presence was inferred from its effects on the visible matter nearby but scientists had not been able to quantify its distribution.

Scientists David Wittman, Tony Tyson, and David Kirkman (Lucent Technologies); Ian Dell'Antonio (NOAO); and Gary Bernstein (Michigan) have taken surveys of portions of the sky and have been able to quantify the existence of dark matter over large portions of the sky. As light from a distant galaxy travels near or through clumps of dark matter, gravitational effects bend it just as though it had passed through a lens. Scientists call this effect gravitational lensing. The degree of bending is proportional to the amount of mater. As a result, the image of the galaxy is stretched. This stretching is referred to as cosmic shear. The amount of cosmic shear can be measured, and the amount of dark matter between the earth and the imaged object inferred from this measurement. Because the scientists imaged large portions of the sky that included thousands of distant galaxies, they were able to make accurate assumptions about the amount of dark matter in the universe.

By using the National Science Foundation's Blanco 4-meter telescope in Cerro Tololo, Chile and the Mayall 4-meter telescope on Kitt Peak, Arizona, scientists were able to map and measure the distribution of dark matter over large swaths of the sky. These measurements agree with a model of an open universe as predicted by Einstein's equations and rule out a standard cold-dark-matter universe. Three other groups subsequently submitted papers in agreement with this measurement.

In order to maximize the scientific use of the images, the group is also searching for supernovae, variable stars as well as asteroids and Kuiper Belt objects, and other transient objects in the same data used for the dark sky survey. The data and images will be released to the general scientific community six months after the completion of each 40' patch.

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