Astronomers Create 3D Map of Dark MatterRead the rest.
By Ker Than
Staff Writer
posted: 07 January 2007
01:37 pm ET
This story was updated at 3:02 pm EST.SEATTLE - Astronomers have mapped the positions of vast, invisible isles of dark matter in the sky, within which normal "bright" matter galaxies are embedded like glittering gems. The three-dimensional map [image] spans not only space, but also time, and stretches back to when the universe was only about half its present age.
Dark matter is a mysterious hypothetical substance [image] that is thought not to interact with light photons and is thus invisible to current detection instruments.
Scientists first invoked the concept in the 1930s to explain why fast-spinning galaxies with relatively little mass don't break apart. The unusual solution: They contain a large amount of invisible matter whose heft and gravity hold the galaxies together. Scientists have since estimated that only about one-sixth of the matter in the universe is visible, and that the rest is dark matter.
Some of the strongest evidence for dark matter's existence was announced last year, and even that was highly debated.
Even though dark matter can't be seen directly, some scientists say its presence and distribution in the universe can be observed indirectly by the way its gravity distorts the light of distant galaxies streaming toward us.
"We look at galaxies which are behind the dark matter that we're interested in," explained study team member Richard Massey of Caltech. "The light from these distant galaxies doesn't travel in a straight line because space itself is distorted and bent, and the light follows that distortion."
Using a dark matter lens
Massey likens this indirect technique, called "gravitational lensing," to peering at a page of text with a magnifying glass.
"The first thing you notice is that the text is bigger, but also, if you look around the edges of the magnifying glass, the text is slightly distorted," he said. "You can investigate the properties of the magnifying glass by observing these distortions and actually find out what shape the glass lens is."
The researchers took an analogous approach to create their dark matter map. By analyzing the distortions in galaxy shapes, they inferred properties of the dark matter "lens" itself, including its mass and position in the sky.
The new map, created using data collected by the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS), confirms what previous, smaller, maps have hinted at: Dark matter is distributed across the universe in thick clumps and fat filaments within which galaxies are anchored like set jewels.
The map relied on data collected from more than half a million galaxies and spans a swath of the night sky that is nearly nine times the diameter of the full Moon. The map is detailed online in the journal Nature and was presented here Sunday at the 209th meeting of the American Astronomical Society.
"We've seen the first glimpse of the cosmic web which acts as the basic framework for large scale structures," said study team member Richard Ellis of Caltech. "This has been a prediction of the numerical simulations for many, many years."
"The point of it is that it's huge," Massey told SPACE.com. "Now we can really see a representative chunk of the universe. It's new in that we see the big picture."
A map of space and time
The map also shows how the structure of dark matter has evolved through time. The researchers split the galaxies they observed into three groups, or "shells," depending on how far away they were from us. They determined distance by the amount of "redshift" observed in the galaxies' light. As the universe expands and stars and galaxies continue to race away from us, the wavelength of light they emit stretches and becomes slightly redder.
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