How has quantum theory affected chemistry?
|This article deals with the elementary particles WIMP. For other meanings, see WIMP (disambiguation).|
WIMP (from English W.eakly I.nteracting M.assive Particles, Germanweakly interacting massive particles, Word game on Engl. wimp 'Weakling') are hypothetical particles of particle physics with a mass between a few tens and about a thousand GeV / c² (one GeV / c², one billion eV, divided by the square of the speed of light c, is about the mass of a hydrogen atom). WIMPs have been postulated to solve the cosmological problem of dark matter in space.
The existence of dark matter was postulated because the gravity of the visible matter present in the universe would by far not be sufficient to explain the agglomeration of matter in the early phase of the cosmos, which led to the formation of galaxies. The majority of the matter contained in the universe must therefore consist of "dark matter" that is not directly visible, although it is not clear what is meant by it.
The existence of dark matter can be explained by heavy (mass of about two gold atoms), only weakly interacting WIMPs that cross space in large numbers. They have no charge and therefore no electric or magnetic field, so that their interaction with matter is limited to gravitation and the very short-range weak interaction. Since WIMPs, if they actually exist, are also not subject to the strong interaction, they could therefore cross entire planets like neutrinos without being disturbed. The English word “wimp” roughly means “weakling” in German, which alludes to this inability to influence matter, and is also to be understood as a contrast to the MACHOs, which were also proposed as a hypothesis for dark matter.
The experimental detection of WIMPs is the subject of current research. Because of the extremely rare interaction with any matter, attempts are made to detect WIMPs indirectly through decays. In extremely rare cases it must happen that a WIMP collides directly with an atomic nucleus, which would also radiate as a result. To detect this radiation, special detectors are required that not only measure the radiation, but can also distinguish it from the interfering background radiation.
The currently most sensitive experiments use cryogenic detectors (detectors that operate at very low temperatures). This includes:
- the American experiment CDMS-II (Cryogenic Dark Matter Search) in the Soudan Underground Laboratory,
- the Franco-German experiment EDELWEISS (Experience pour DEtecter Les Wimps En Site Souterrain) in the Laboratoire Souterrain de Modane,
- XENON Dark Matter Project in the Gran Sasso underground laboratory
- the British-German experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) in the Laboratori nazionali del Gran Sasso and
- the Italian experiment DAMA (DArk MAtter), also in the Laboratori nazionali del Gran Sasso.
In 2007, DAMA delivered a result that was doubted by many: the experimenters claim to have observed a signal from WIMPs with a large detector made of sodium iodide (NaI). This result is difficult to reconcile with the results of the other experiments and the theoretical expectations.
An expanded concept provides for so-called Super-WIMPs, caused by the decay of WIMPs. They have an even weaker interaction than WIMPs, since they only interact with gravity, but not with the weak nuclear force.
The existence of super WIMPs would have an impact on the formation of galaxies. Super WIMPs would have moved very quickly in the early universe. Only after they came to rest could galaxies have formed. This would also have had less time to compress the matter in the center of the galaxies, which would also have had an effect on the density in the center of the dark matter halos. In this way it could possibly be shown whether these halos consist of WIMPs or Super-WIMPs.
Another possibility of detection could arise from the decay of WIMPs into Super-WIMPs themselves, since this could produce photons and electrons that would break up light atomic nuclei if they hit them. Evidence that the universe contains less lithium than expected could be explained by this.
Alternative dark matter theories
Another particle proposed to solve the dark matter problem is the axion. This also hypothetical particle could be produced in stars, among other things. By interacting with strong magnetic fields, it could transform itself into a photon whose energy corresponds to that of the axion. Axions from the sun were supposed to generate photons with frequencies in the range of X-rays. The CAST experiment at CERN is concerned with detecting this particle with a 9 Tesla magnet.
Assumptions that dark matter is massive but cold and non-radiating celestial bodies that are present in large numbers in galaxies (MACHOs) could not be confirmed by research.
According to Jonathan Feng from the University of California at Irvine and Jason Kumar from the University of Hawaii at Manoa, supersymmetry also allows alternative concepts without WIMPs, with several different types of particles. Many of these concepts also assume the existence of “dark forces”, hidden versions of the weak and electromagnetic force. The existence of dark electromagnetism would result in dark matter being able to emit and reflect hidden light. Even if such light and dark forces were hidden from us, they could still have an effect. Clouds of dark particles interpenetrating each other would be distorted, which could also affect objects like clusters of galaxies. Corresponding studies on the bullet cluster have shown that this effect can at least not be very strong. The dark forces, if they exist, are therefore only weak.
Another effect would result from the fact that dark forces would enable the particles of dark matter to exchange energy and momentum. Dark matter halos that would have been skewed initially would become spherical over time, which in turn would affect galaxies, especially dwarf galaxies. Dark matter in their environment moves particularly slowly. This means that particles stay together longer. Small effects therefore have more time to take effect. The observation that small galaxies are consistently rounder than large ones could be explained by this, which would be an indication of the existence of dark forces.
- Lightest supersymmetric particle
- ↑ 1,01,1Jonathan Feng, Mark Trodden: The Hidden Blueprint of the Cosmos. In: Spectrum of Science, January 2011. Spectrum of Science Verlagsgesellschaft mbH, Heidelberg.
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