Thursday, September 13, 2007

Dark Matter - could be warm

Dark matter may be made of fast, lightweight particles, according to a new computer simulation. That could explain the peculiarly pure chemical makeup of some stars in the Milky Way, and the enormous mass of black holes that live at the hearts of large galaxies.

Because 'dark matter' reveals itself only by its gravity, astronomers have few clues to its nature. The most popular model is cold dark matter: heavy subatomic particles that tend to move very slowly.

Another possibility is warm dark matter: lighter particles that move faster. The rapid motion of these particles smoothes out the small dense knots of matter that would could or should otherwise form in the cores of galaxies, and there are hints that such dense knots are indeed missing.
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Liang Gao and Tom Theuns of Durham University in the UK have built a computer simulation to compare the behaviour of cold and warm dark matter in the early universe. At first the two varieties behave alike, collapsing under gravity into a network of filaments that crisscross the universe.

But cold dark matter then coalesces into blobs or haloes, while warm dark matter does not. The random motion of its particles smoothes out these blobs, so warm dark matter filaments just keep collapsing and getting denser until there is a narrow tube of matter typically 10,000 light years long with the mass of 10 million Suns.

Ordinary gas is dragged in by the dark matter, and eventually the first stars form. They are made almost entirely of hydrogen and helium, the two main elements created in the big bang.

With cold dark matter, one large star forms in the middle of each large halo. These large stars burn fast, fusing hydrogen and helium into heavier elements. They soon exhaust their fuel and then explode to seed the universe with denser elements, which go into the next generation of stars. No pure hydrogen-helium stars survive.

But in the dense filaments formed by warm dark matter, the formation is likely to be more chaotic, with stars of different sizes forming from random-sized bits of filament.

Some would be small stars, which burn slowly, so a few pure stars formed in these filaments could still be shining today.

In the past few years, astronomers have indeed discovered small stars in the Milky Way that are very low in heavy elements. It is suggestive that maybe dark matter is warm. If astronomers see a star with absolutely no heavy elements, that will be good evidence.

These filaments may also be good at making big black holes. Although many of the isolated stars created by cold dark matter would give birth to black holes, they would only be a few times the mass of the Sun, which seems too small to seed the billion-solar-mass black holes that are known to lurk in many galaxies.

But each warm-dark-matter filament should eventually collapse along its length, say Gao and Theuns, forcing stars, gas clouds and small black holes close together in the perfect environment for growing much bigger black holes.

In the simulation, researchers assumed a dark matter particle with just 0.6% of the mass of an electron. That would fit the gravitino, a particle predicted by the speculative theory called - supersymmetry – although any particle as light would have a similar effect.
Did our galaxy's black hole eat its baby brother? @ SNS
Small 'Hobbit' Galaxies Made Almost Entirely of Dark Matter
Explanation of Dark Matter Might Lie in Origin of Stars @ Live Science

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