The difficult search for dark matter

The difficult search for dark matter

The gravitational force of the dark matter distributed in the galaxy cluster abell 1689 (indicated in blue) bends the light of galaxies lying behind the cluster – from this we can deduce their properties

News from the search for traces of dark matter and dark energy: should the construct very useful for saving the general theory of relativity turn out to be untraceable after all?

The concept of dark matter has a lot going for it – for example, the very useful property for a theory that predictions made from it can be confirmed in reality. In the search for its components, however, successes are regularly followed by contradictory results. In 2008, for example, researchers published the first results of the pamela experiment, which has been collecting data on board the russian satellite resurs-DK1 since 2006. Pamela searches for evidence of dark matter in cosmic rays. The same was thought to have been found in the form of positrons of a certain energy range, which could actually only come from a hitherto unknown source – namely the annihilation of dark matter particles (see traces of dark matter – with question mark).

The fact that the antiprotons, which theoretically also occur in the process, were not registered is the smaller problem. The cruder argernis was that the dark matter tracked in this way must have properties that make it impossible for the same particles to be found in crude, earth-based experiments like the CDMS-II. Nevertheless, these detectors have recently shown evidence for wimps, the dark matter candidates (see close to a proof for dark matter).

In phys. Rev. D, researchers therefore put forward a theory in june according to which dark matter could be made up of two components at once. The one, more conventional, became noticeable in direct detector experiments. For him the neutralino comes into question, which is its own antiparticle. The second, mysterious part could only be detected by secondary particles, as in the pamela instrument. It had to come from the "hidden sector", an addition to the standard model of physics, which gives birth to various new particles. According to theory, one of these hidden sector particles decays into positrons without releasing any anti-protons in the process.

Possibly the anisotropy of the background radiation is much less pronounced

But perhaps the researchers are also on the trail of a phantom. In mid-june, two british astronomers published in the lesser-known monthly notices of the royal astronomical society their analysis of the experiment that had provided clues to dark matter in the first place. The WMAP experiment, also aboard a satellite, examines the cosmic background radiation for anisotropy. From the coarseness of the wave structures in the background radiation, which is linked to the structure of the early universe, science had first inferred the composition of the universe from 74 percent dark energy and 22 percent dark matter.

The british researchers have now come to the conclusion that WMAP is much less accurate than previously thought. If the two are right, the anisotropy of the background radiation is much less pronounced – and to explain it, dark energy and dark mass are probably no longer needed. Some theoretical physicists may even be happy about this – if the model of the universe only holds by the amption of exotic particles, then it is better that the model collapses and makes room for a better one.

Traces of light

Up to then however still some time could pass, because there are also again and again signs that there must be a force driving the universe apart (the dark energy) and a star cement (the dark mass) invisible for us. Already in march an international research team had calculated a mass distribution in the universe with data of the hubble space telescope. To do this, they used the "weak lensing" effect, which is based on the gravitational effect of (amed) dark matter. The unevenly distributed dark matter deflects light in different ways – with observations from different situations, we get a three-dimensional picture of how the universe is structured.

In a paper published in the current ie of the journal science another group of scientists is now also using data from hubble and various earth-based telescopes to study the opposite effect of "strong lensing," which is based on the gravitational deflection of light by entire clusters of galaxies. The idea: if one knows, how the light of distant galaxies had to reach us, one can fall back on otherwise invisible influences by comparison with the reality.

The researchers thus analyzed the observational results of 34 extremely distant galaxies whose light is deflected by abell 1689, one of the most powerful galaxy clusters in the universe. In this way, which at first seems to be a detour, the astronomers come a bit closer to the properties of dark energy: they were able to reduce the area in which the effect of dark energy on the universe had to lie by three per cent.

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