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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Monday, January 08, 2007

3-D Map of Dark Matter

3-dimensional distribution of dark matter in current universe (image courtsey: HubbleSite.org)

Dark matter is an invisible form of matter that accounts for most of the universe's mass, but that so far has eluded direct detection, or even a definitive explanation for its makeup [See our past posting on the evidence of the existence of dark matter].

Now, an international team of astronomers, using NASA's Hubble Space Telescope, has created a comprehensive 3-dimensional map that offers a first look at the weblike large-scale distribution of dark matter in the universe. The map provides the best evidence yet that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The map reveals a loose network of filaments that grew over time and intersect in massive structures at the locations of clusters of galaxies.

Researchers created the map using Hubble's largest survey of the universe, the Cosmic Evolution Survey ("COSMOS") with an international team of 70 astronomers led by Nick Scoville of California Institute of technology. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the universe using the Advanced Camera for Surveys' (ACS) Wide Field Camera onboard Hubble. The survey covers a sufficiently wide area of sky allowing for the large-scale filamentary structure of dark matter to be evident. To add 3-D distance information, the Hubble observations were combined with multicolor data from powerful ground-based telescopes.

Almost all current scientific knowledge of the universe is related to only baryonic matter or the normal form of matter that we are familiar with. Now that scientists have begun to map out where dark matter is and how they are distributed alongside the baryonic matter, the next challenge is to determine what it is, and specifically its relationship to normal matter. This 3-D information is thus vital to studying the evolution of the structures of the distribution of matter over cosmic time.

The research results were presented at the 209th meeting of the American Astronomical Society in Seattle, Washington and also appeared online in the journal Nature yesterday.

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