The first 3D map of dark matter
In a letter to Nature, an international team of researchers -- 20 scientists from 11 labs around the world -- reveals it has produced the first 3D picture of dark matter. This map has been created by combining hundreds of images taken by the Hubble Space Telescope for the Cosmic Evolution Survey (COSMOS). As the cosmologists were expecting, this map, which represents about half a million distant galaxies, shows that dark matter forms a kind of skeleton which can attract baryonic particles -- such as protons and neutrons -- to produce stars and galaxies. But this is not always the case, and the researchers are now working to find the origin of these discrepancies.
Here is how Nature describes these discrepancies.
The map has a few puzzles within it. Some areas show clumps of dark matter that aren't accompanied by the bright features associated with conventional, visible material (made of baryonic matter), and vice versa.
"On the large scale the general picture is as expected, but there are some small-scale discrepancies," says Richard Massey at the California Institute of Technology, Pasadena, and one of the team members who pieced together the map from hundreds of slightly overlapping images from the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS).
Below is an example showing a comparison of baryonic and non-baryonic large-scale structure. "The total projected mass from weak lensing, dominated by dark matter, is shown as contours" (Credit: All the authors, via Nature). Here is a link to a larger version.
Are these discrepancies for real?
The discrepancies could be a simple error resulting from the way the observations were made. But if they are real, says Massey, they will bring a huge shock. "Baryonic structures are expected to form only inside the dark-matter scaffold," he says. "There will need to be a lot of follow-up work before we really believe any individual discrepancies."
But what is dark matter anyway? You can read this page on Wikipedia for explanations. Nature writes it "makes up a quarter of the Universe." But in another article, "Dark matter mapped in 3D for first time," New Scientist gives a very different estimation (David Shiga, New Scientist, January 7, 2007).
Dark matter is an invisible substance that betrays its presence through the gravitational tug it exerts on ordinary matter. It is six times more abundant than ordinary matter and is thought to have seeded the first distinct structures in the universe, which began as a very uniform soup of matter.
I'll come back to this number later, but here is how New Scientist describes how this 3D map of dark matter was created.
The key to determining the dark matter distribution is an effect called gravitational lensing, by which light rays from a distant object such as a galaxy are bent by the gravity of an intervening concentration of matter. Although dark matter cannot be seen directly, its presence can be inferred by the way its gravity distorts the images of galaxies behind it.
The Hubble Space Telescope (HST) mapped out these distortions over a patch of sky equivalent to the width of four Full Moons in the largest survey it has ever performed. It devoted 10% of its time over two years to complete the survey.
For more information, this research work has been published by Nature under the name "Dark matter maps reveal cosmic scaffolding" (Advance online publication, January 7, 2006). Here is a link to the abstract, which starts by another estimation of the 'weight' of dark matter.
Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious 'dark matter' component, which does not interact via electromagnetism and thus neither emits nor reflects light.
So we have now three different estimations for the percentage of dark matter in the total matter: 1/4 in Nature, 6/7 in New Scientist and 5/6 as reported by the researchers. Strange, isn't?
Sources: Katharine Sanderson, Nature, January 7, 2007; and various otherwebsites
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