Dark matter means dark light

Cosmology: Gravitational Lenses and the Dark Matter Mystery

When it comes to the composition of the universe, cosmologists have a very specific idea: In addition to comparatively small amounts of ordinary matter (stars, gas nebulae, planets), the cosmos should largely consist of dark matter. The enigmatic substance would have to be invisible and interact with the rest of the universe practically only through gravity. Particles that move slowly through space and therefore easily clump together to form centers of mass would come into question.

This "cold" dark matter goes well with many observations in cosmology and astrophysics. However, there are also findings that question the hypothesis or at least require a strong adaptation. One example of this is a study that has just been published in the journal “Science”. According to her, eleven examined galaxy clusters focus more light selectively than can be easily explained with the prevailing theory of cold dark matter.

The researchers led by Massimo Meneghetti from the Osservatorio di Astrofisica e Scienza dello Spazio in Bologna, Italy, are making use of the gravitational lens effect: What is meant is a prediction from Einstein's general theory of relativity, which turns galaxies into a kind of magnifying glass. Because of their great mass, the accumulations of matter create a huge bulge in space-time, which forces light from sources behind it onto curved paths - and sometimes gives distant observers the impression that the source exists in multiple versions.

In the eleven examined galaxy clusters, this effect is sometimes more pronounced than one would expect on the basis of cold dark matter, reports Meneghetti's team. Specifically, more of the dark matter seems to be gathering in the area of ​​influence of individual galaxies than simulations suggest. As a result, the individual galaxies appear to be ten times more efficient lenses than expected, the researchers have calculated.

From the group's point of view, this allows two conclusions: Either the simulations are faulty or the dark matter interacts with ordinary matter differently than is commonly assumed. Perhaps there is a third possibility: observations of gravitational lenses in galaxy clusters are often difficult to evaluate and sometimes lead to wrong conclusions - especially when, as in this case, one is only dealing with a rather limited database.