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Dark matter

Taking the measure of the cosmos

München, 08/03/2017

Together, dark matter and dark energy account for about 95% of the cosmos. This has now been confirmed by the large-scale Dark Energy Survey, which has probed the structure of the Universe with a degree of accuracy never before attained.

A map of the distribution of dark matter covering approximately one-thirtieth of the heavens. In the areas shown in red, the density of dark matter is higher than average; in the blue areas the density is lower than the mean. Source: Chihway Chang, Kavli Institute for Cosmological Physics, University of Chicago, and the DES Collaboration.

The newly published data from the Dark Energy Survey (DES) have yielded a picture of the total amount and the distribution of dark matter in the cosmos that achieves a level of precision comparable to that recently accomplished by the European Space Agency’s Planck satellite. However, the two efforts differ in one important respect: The Planck Observatory mapped the microwave background radiation, which reflects the state of the Universe at an early stage in its evolution – some 400,000 years after the Big Bang. The data collected by the DES, on the other hand, show what the Universe looks like to observers on Earth at the present time. Like the Planck project, the DES was a collaborative effort involving approximately 400 researchers in more than 25 countries, and the research groups of professors Jochen Weller, Joseph Mohr and Dr Stella Seitz of the LMU Observatory made a significant contribution to the survey.

“The results are even more exciting than we expected,” says Scott Dodelson of the Fermi National Accelerator Laboratory (Fermilab) near Chicago, one of the leaders of the campaign of observations. “They will make it possible for us to trace the evolution of the Universe over the past 14 billion years.”

The new findings confirm the predictions made on the basis of the Planck project. In particular, they validate its conclusion that dark matter makes up about 26% of the total energy content of the Universe, while dark energy accounts for a further 70%.

Dark matter is invisible to even the most sensitive astronomical probes, because it neither emits nor absorbs light. It only manifests itself by virtue of its gravitational attraction, which can be measured with the aid of the gravitational lensing effect predicted by Einstein. This refers to the deflection of light rays emitted by a background object – such as a distant galaxy – by the gravitational effect exerted by a massive foreground object. In the new survey, the distribution of dark matter was determined with the help of the world’s most sensitive camera, and the level of detail achieved means that the resulting map is the most comprehensive measurement of the disposition of dark matter in the cosmos yet performed. The DES researchers measured a total of 26 million galaxies. The aim of this huge endeavor is to obtain a better understanding of the role of dark matter in the formation of galaxies. A study of localized concentrations of dark matter also provides insights into the nature of the enigmatic dark energy, which is thought to be the cause of the recently discovered increase in the rate of expansion of the Universe.