The search for dark matter
Ethereal yet weighty
Jul 16th 2009
From The Economist print edition

Two new ways to detect the elusive stuff of the universe

Science Photo Library


Embedded in darkness

MOST of reality appears to be missing. Physicists reckon that the missing matter must be there, but that it is dark. Finding the stuff is damned tricky because dark matter, by definition, cannot be seen. So some of those physicists have been busy trying to devise ways of glimpsing it indirectly—and two groups of them now think their methods are ready to test.

One reason to believe dark matter exists is that galaxies rotate at such speeds that they would fly apart without it, or so the argument goes. The fact that galaxies persist suggests they are held together by the gravitational pull of something invisible—in other words, dark matter. This stops stars being shed from their edges.

Claims to have spotted dark matter have so far rested mostly on evidence gathered by looking at collisions between clusters of galaxies. Some of these appear to have separated dark matter from its visible counterpart. Three months ago, however, a team of physicists reported subatomic evidence. They think they have seen an abundance of high-energy positrons, the antimatter versions of electrons, coming from space, and they speculate that this, too, is a sign of dark matter.

One possible explanation for these positrons is the mutual annihilation of the matter and antimatter forms of a type of dark matter called weakly interacting massive particles, also known as WIMPs. Calculations suggest that when WIMPs and antiWIMPs meet, one product will indeed be a lot of positrons.

Now a team led by Michael Kuhlen of the Institute for Advanced Study in Princeton, New Jersey, has devised an explanation that lends weight to this idea. For the past few years Dr Kuhlen and his colleagues have been working on a computer model of the halo of dark matter in which the Milky Way (the Earth’s home galaxy) is thought to be embedded. Their simulation starts about 50m years after the Big Bang, when the first galaxies formed, and calculates the interactions of a billion WIMPs over the intervening 13.7 billion years.


The researchers included in their model a hitherto-neglected effect proposed by a German physicist, Arnold Sommerfeld, in the 1930s. The Sommerfeld effect, a quantum-mechanical process that makes particles more likely to interact with one another in some circumstances and less likely in others, is mostly ignored in particle physics because it applies only in a limited range of conditions. However, one of those is when a particle is slow-moving and massive. As that describes WIMPs rather well, Dr Kuhlen and his colleagues included it in their model. After running their simulation on a supercomputer, they found that dark matter should annihilate much more readily than had previously been thought. They report their work in the current edition of Science.

If they are correct, it would explain the newly observed abundance of high-energy positrons. Moreover, they also point out that there should be a second signal from annihilating dark matter, in the form of gamma rays coming from the Milky Way’s halo. It should be relatively straightforward to detect these using satellite-based telescopes such as Fermi, which was launched last year.

Meanwhile Pierre Colin of the Max Planck Institute for Physics in Munich and his colleagues reckon that the shadow of the moon could be used to help work out whether the abundance of positrons actually does come from annihilating dark matter, using telescopes that look for Cherenkov radiation—flashes of light in the sky—that are created when cosmic rays, in the form of high-energy electrons and positrons, hit the upper atmosphere.

As Dr Colin described on July 9th to the International Cosmic Ray Conference held in Lodz, Poland, the moon blocks electrons and positrons as effectively as it blocks light. This creates, as it were, electron and positron shadows. But because these particles are electrically charged, they also interact with the Earth’s magnetic field. And since their charges are opposite, the interaction deflects the two sorts of particles in different directions.

The result is that the electron shadow and the positron shadow are separated. Normally, it is impossible to distinguish Cherenkov radiation generated by positrons from that created by electrons. However, when the moon is between the telescope and the source of the cosmic rays, Dr Colin and his colleagues reckon that it will be possible to separate the two by looking at the edges of the shadows. That would enable physicists to see whether the number of high-energy positrons matches the theory’s expectation and thus, appropriately, use shadows to cast light on the existence of dark matter.
http://www.economist.com/...d=14030304

interesting read.

SUPRMAN said:

Wow. All these dark matter and black hole talk really blows my mind. I'm still trying to get through BRIEF HISTORY OF TIME. It is really blowing my mind. It's somewhat of a difficult read but interesting.

"International Cosmic Ray Conference"

sounds fun !

But think about this, Dark Matter has to be see through. We can see distant galaxies, nebulas (why isn't the plural nebuli?) etc in spectrums of light which would have to come through dark matter. Even statistically speaking, the Dark Matter has to be see through as Dark Matter is expected to be more than twice all the visible matter we can detect. And how far away is the nearest dark matter from us?
[Edited 7/17/09 18:46pm]

"dark matter"

SUPRMAN said:

and which dimension?

XxAxX said: