Measuring the Invisible Cosmos

Joachim Stadel (University of Zurich) uses computer simulations to search for dark matter

In 2021, the European research satellite Euclid will be launched into space. With Euclid, scientists from Switzerland and 15 other countries want to gain a better understanding of dark matter and dark energy. These two 'things' fill large parts of the universe in the view of modern astronomy, but neither dark matter nor dark energy have been directly observed so far. Researchers at the University of Zurich have prepared a complex computer simulation of dark matter in preparation for the Euclid mission. This mission is of great interest not only to astronomers and astrophysicists but also to particle physicists since particle physics is on the quest for the discovery of the nature and structure of dark matter with its own experiments.

Extract from the computer simulation created by Dr. Joachim Stadel and other scientists from the University of Zurich: every yellow dot is a halo of dark matter in which a galaxy is embedded. The dark 'threads' are places in the universe with a high density of dark matter, while in the light spots ('voids') only few dark matter is present.
Image : B. Vogel

Joachim Stadel had come a long way when he joined the University of Zurich in 2002, he was already a senior researcher. Born in 1968, he grew up in Canada, "in the prairie", as he says, earned his doctorate in astronomy in Seattle (University of Washington) and later worked as a postdoc at the University of Victoria in the southwestern tip of Canada. For a while he was involved in a NASA computer project. 16 years ago, he moved to Zurich, where he explores the vastness of the universe on the Irchel campus at the Institute for Computational Sciences.

Joachim Stadel is a numerical cosmologist. This means, among other things, that he gets to the bottom of the open questions of the universe processing large amounts of data. That's what is happening also in the last project the Canadian-German double citizen is working on: In three years, the satellite Euclid will be launched into space to - hopefully - unveil dark matter and dark energy. Dark matter is the hitherto unobserved form of matter of which the universe must be filled in order to apply the laws of gravitation between the stars as we know then today. Dark energy, on the other hand, is needed to explain why the universe is expanding at an exponential rate, as we now know from the corresponding measurements.

Simulation with 25 billion galaxies

Dr. Joachim Stadel created a computer simulation of the universe together with a team of scientists at the chair of Computer Aided Astrophysics (Prof. Romain Teyssier). They provide the foundation for Euclid's ability to make accurate measurements and to be able afterward to interpret the results correctly. The simulation constructed during the last three years shows the distribution of dark matter. Particularly fascinating: The astrophysicists can visualize in this way the distribution of dark matter in space and time in a very plastic way, although today we infer the existence of dark matter only by its gravitational interaction.

The simulation includes not only our own galaxy, the Milky Way, but 50 billion other galaxies. This corresponds approximately to an eighth of the visible universe. The simulation leads to a net-like structure in which filaments mark those places where dark matter clumps together, and in which yellow dots represent the so-called halos - huge dark matter formations that contain in itself a galaxy in the interior (see illustration). Whether the dark matter is actually distributed in this way in the universe will be shown in 2021, after the six-years lasting Euclid mission. Euclid can localize dark matter because light from distant galaxies is distracted differently on its way to us by differences in mass density ('weak gravitational lensing').

Replica of the universe

"The starting point of our simulation was the Universe as it was 300,000 years after the Big Bang. The universe of that time can be thought as a plasma lake of protons and electrons, in which exist small density differences (fluctuations). Our simulation shows what emerged from this lake under the influence of gravitational force after the 13.8 billion years development of the universe, "says Stadel, summing up the basic idea of the simulation. "We have made only a few assumptions for our calculations. To get an even more meaningful result, we would need to know from particle physics more about what dark matter actually is, "says Stadel. With this note he also expresses how strongly particle physics and astrophysics mutually condition each other.

The simulation of the dark matter distribution in the Universe is the result of a complex accounting process: To complete the very complex arithmetic operations, the supercomputer 'Piz Daint' of the National Supercomputing Centre of Switzerland (CSCS) in Lugano was used for 80 hours. During this time between 4000 and 5000 computer nodes were used, which corresponds to a computing time of approximately 300,000 server hours. The resulting simulation is one of several contributions to the Euclid mission. Euclid is a project of the European Space Agency ESA. In addition to Switzerland, 13 other European countries are involved, as well as Canada and the USA.

Author: Benedikt Vogel

During a visit to the University of Zurich, Joachim Stadel explained to us what a Dark Matter Halo is and also allowed us to have a look at the new computer simulation. Look here for the video:

CHIPP Joachim Stadel & Dark Matter


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