Wednesday, November 30, 2011

The illusive dark matter is probably quite heavy

The stuff that makes up almost a quarter of our universe but has never been observed directly, probably consists of a particle that is relatively heavy, a new study suggests. Because we know dark matter has a gravitational pull on the matter we can see, we know it consists of particles that possess mass. However, it is unknown what the characteristics of these particles are, because we can not observe them, hence the name dark matter. A team of scientists from the famous Fermi lab set out to discover the mass of this unknown particle, by looking at radiation coming from so-called dwarf galaxies. They found out that the dark matter particle is likely to be 44 times heavier than a proton, the latter being a building block for atoms and thus for matter we can observe. With this discovery, scientists can calibrate their particle detectors to look for things that fit the size range found by Fermi lab, which might speed up the search to find the illusive dark matter particle. Additionally, a newly constructed dark matter map, ought to help in the process.

Radation coming from small galaxies, called dwarf galaxies, is often used to study dark matter. Because these little galaxies, which nevertheless contain billions of stars, have a relatively high amount of dark matter compared to visible matter, they are better suited for measurements than conventional large galaxies such as our own Milky Way.

Scientists measure gamma radiation, a form of electromagnetic radiation similar to visible rays of light, to determine the make-up of a galaxy. When particles interact, they often emit gamma rays. While dark matter is thought to emit no electromagnetic radiation, scientists hope to find a signal from somewhere in the radiation coming from a dwarf galaxy, that reveals the existence of dark matter. They assume that when particles of dark matter collide, they produce gamma rays. Therefore, if dark matter were to create its gravitational pull with a low-weight particle, more of them are needed to produce the effect on gravity, and therefore there must be more collisions than with fewer, but heavier particles. Gamma ray measurements from Fermi lab reveal that it must be the latter: a heavy particle, that can assert its gravitational effect by a relative low amount of particles.

We still do not know what the actual dark matter particle is, however. Scientists postulated a particle dubbed the Weak Interacting Mass Particle, which refers to the characteristics of dark matter that make it hard to detect, while the particles do possess mass. It is the prime candidate to make up dark matter, but physical proof is needed. In addition to finding them somewhere in the universe, researchers try to generate them on Earth. In the Large Hadron Collider at CERN, famous for its quest to find the Higgs Boson particle, proton collisions could generate WIMPs, but once again physical proof is yet to be found.

The results from Fermi lab are important in the quest to find what makes up dark matter. Because we have an estimate of its weight, it narrows our search to find the actual particle. In addition, the findings eliminate a bunch of particles from the list of possible candidates. However, not all scientists are entirely sure that particles that do not fall within the suggested weight range can be ruled out. It is clear that the search for dark matter is far from over.

Dark energy
Dark matter is frequently confused with dark energy, but the two are not the same. While dark matter consists of particles that assert a gravitational effect, dark energy has an opposite effect. It opposes gravity and accelerates the expansion of the universe. This form of energy has, like dark matter, not been detected before, but we know it must be there because of its effect on visible matter. Dark energy makes up almost three quarters of the universe, which means that along with dark matter, there is little left for the stuff we can actually observe. While we do not know much about it, this year's Nobel prize for physics was awarded to scientific work on dark energy, which is quite remarkable for something that we have not even really discovered yet.

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