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Dark Matter

There is too much movement in the Universe to be explained by the gravity of the visible matter. To provide enough gravity, astronomers assume that we share the Universe with exotic particles that interact only through gravity. Detecting these particles is vital to advance this theory.
Scientists originally thought dark matter to be dust and gas.
As cosmology has progressed, theories have since changed.
X-ray satellites, such as the ROSAT satellite, weighed the gas in galaxy clusters and found it to be wanting.
There was simply not enough to provide the gravity to hold the clusters together.
Astronomers came to realize that there might be a whole world of exotic particles predicted by particle theorists that could provide the necessary dark matter.
Detecting them is not easy. Each individual particle of dark matter is separated by several metres of empty space.
Plus, dark matter hardly interacts with normal matter and it is perhaps no wonder that astronomers and physicists have spent decades wondering how to detect it.
In 2007, the Large Hadron Collider (LHC) will begin to collect data at CERN. In its cathedral-sized detectors, particle physicists and cosmologists believe that they may finally be able to create dark matter.
The LHC has colossal importance for cosmology. It will go where no particle accelerator has gone before in terms of energy. Using Einstein's relationship that mass and energy are equivalent, this means that the energy liberated in the LHC collisions will be able to make particles more massive than any previous manmade device.
At LHC's energy levels, roughly a million million electron volts, to use the strange units that physicists work in, a whole raft of never-before-glimpsed particles should become available for study. Astronomers believe these could be the dark matter.
Physicists know them as supersymmetric particles and use them to explain a totally different problem.
It's known as the hierarchy problem and stems from the problems associated with calculating how particles gain mass. In the standard model of particle physics, the mass of a particle is determined by how strongly it interacts with hypothetical particles called Higgs bosons.
The problem is: what gives the Higgs bosons mass?
Physicists figured they must interact with each other but, when they do this, the mass calculated for them was far too high.
The problem is: what gives the Higgs bosons mass?
Physicists figured they must interact with each other but, when they do this, the mass calculated for them was far too high.
In the theory of supersymmetry, every standard particle possesses a partner with different properties. These supersymmetric particles would also interact with the Higgs bosons and according to the mathematics of supersymmetry, would lower the Higgs' mass to acceptable limits.
Known as the neutralino, the supersymmetric partner of the neutrino, provides an almost perfect banditate for the dark matter.
The neutralino would have a mass somewhere above 100,000 million electron volts, which brings it into the range of the LHC.
What would happen is that the energy and momentum going into the reaction will not be matched by the energy and momentum coming out.
This would signify the production of ghostly particles that hardly interact with normal matter - perhaps the very dark matter that astronomers seek.
As these ghostly particles slip unseen out of the detector, they carry away energy and momentum, causing the imbalance.
If the LHC doesn't see the expected supersymmetric particles, it won't be the end of supersymmetry because some variants of the theory suggest that the supersymmetrical particles would be too heavy to be created in the LHC anyway.
However, it would mean the end of supersymmetry as a dark matter candidate.
If the supersymmetric particles are too heavy to be created in the LHC, they could not have been created by the big bang in sufficient numbers to account for the dark matter.
Despite the fact that, on average, the dark matter in the Universe is extremely dilute, every now and again, two dark matter particles will collide.
In fact, the number of dark matter particles supposed to exist around our Galaxy is so large that some will be colliding every second. That means that the dark matter could be annihilating itself into detectable radiation.
If the dark matter candidate is the neutralino of supersymmetry, those photons of radiation will be at gamma-ray energies. Astronomers know of approximately two hundred unidentified gamma-ray sources.
Called EGRET sources, they were detected by the Energetic Gamma Ray Experiment Telescope (EGRET) on NASA's Compton Gamma Ray Observatory.
Astronomers expect that the EGRET sources are probably the fading embers of exploding stars. Nevertheless, there is an outside chance that they could be pockets of self-destructing dark matter.
When NASA launches its Gamma-Ray Large Area Space Telescope (GLAST) in autumn 2007, many astronomers will be keeping an eye open for unexplained gamma rays, just in case the dark matter reveals itself that way.

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