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William Atkins
Wednesday, 16 May 2007 08:19
The Hubble photograph of the dark matter about galaxy cluster Cl 0024+17 appears at the European Space Agency’s website "ESA News".
Team research leader Myungkook James Jee, of The Johns Hopkins University (Maryland), says of Hubble’s discovery, “I think it’s the strongest evidence for the existence of dark matter to date.” [New Scientist]
Dark matter is matter out there in the universe with unknown composition, which does not emit or reflect electromagnetic radiation in sufficient quantities to be observed directly by humans on the Earth. Its presence must instead be indirectly inferred from gravitational effects felt on visible matter.
Dark matter, according to the foundations of the Big Bang theory (the theoretical beginning of our universe) and observations of large celestial structures (such as galaxies), should account for the vast majority of the mass of the known universe. Cosmologists contend that without dark matter, gravity would be insufficient to hold galaxy clusters together (causing them to spiral apart).
The collision of two galaxy clusters involve Cl 0024+17 (sometimes also referred by the name ZwCl0024+1652), which is about five billion light-years from the Earth and in the direction of the constellation Pisces). It is actually a pair of clusters that have previously collided—approximately one to two billion years ago.
One light-year is the distance that light travels in vacuum in one year—about 9.4 trillion kilometers (5.8 trillion miles).
When the pair of clusters collided in Cl 0024+17, the dark matter exploded outwards, forming a spherical shell. The shell is about 2.6 million light-years in width.
The discovery by Hubble, a joint effort by NASA and ESA, is the first record of dark matter located away from ordinary matter. This separation between ordinary matter and dark matter gives scientists a better way to learn more about it.
The results of Jee, H. Ford, also of Johns Hopkins, and other associates will appear in the June 1, 2007 issue of Astrophysical Journal.
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