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Plasma accounts for the overwhelming majority of matter in the universe, yet its behaviour is not fully understood. Researchers in the COMPLEX group aim to include the plasma-physical effects in hydrodynamical simulations of galaxy clusters, which could lead to new insights into how these clusters form and evolve, as Dr Klaus Dolag explains. The majority of the visible matter in

COMPLEX Research Group

the universe, specifically plasma, is shaped by highly complex physical processes. This plasma is extremely hot and has a very low density in comparison to materials in our own atmosphere. “The typical density of materials in these cosmic structures is something like 100 atoms per cubic centimetre (cm3) while on Earth there are something like 1020 atoms per cm3 in the air. So it’s a big difference,” outlines Dr Klaus Dolag, Head of the Computational Centre for Particle and Astrophysics of the Excellence Cluster ORIGINS at the Ludwig-Maximilians-Universität (LMU) in Munich. This is a major issue in terms of our theoretical understanding, as researchers don’t know exactly how this plasma actually behaves on the micro-physical scale, a topic that Dr Dolag is addressing in COMPLEX, an ERC advanced research group based at LMU. “Current simulations of galaxy clusters are typically based on certain, highly simplifying assumptions. The next step that we want to take within COMPLEX is to include plasma physics properties in the hydrodynamical simulations, and to see what changes. For example, what is the effect of viscosity? What is the effect of conductivity?” he asks.

This is part of the wider aim of improving simulations of galaxy clusters and gaining fresh insights into the highly complex relationship between gravitational collapse and processes which lead to the formation of galaxies, which has been a central theme of Dr Dolag’s research career. Cosmic objects may collapse under the force of gravity, while at the same time the universe is expanding, which in a way act as opposing forces. “There’s a kind of battle between the expansion of the universe and gravitational collapse. Galaxy clusters capture both these effects,” says Dr Dolag. These objects encode a lot of information about cosmology and the evolution of matter in the universe, so improved simulations could lead to new insights into some major outstanding questions, such as the nature of dark matter. “These objects are very important if you want to learn more about what kind of universe we live in. Researchers are taking measurements, and they are trying to draw inferences from these objects about the cosmological background,” continues Dr Dolag. “When we simulate these objects, we want to reproduce their observed properties.”

PhD student Ludwig Böss Works on MHD Simulations of galaxy clusters and develops the FokkerPlanck solver to directly model spectral cosmic ray electrons and protons within cosmological, hydrodynamical simulations.

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PhD student Frederick Groth Works on the implementation of new numerical methods for the hydro-dynamical solver in cosmological simulations to improve the treatment of turbulence in galaxy and galaxy cluster formation simulations.

PostDoc Dr. Ildar Khabibullin Expert in high energy astrophysics with rich experience in working on galaxy clusters, the interstellar and intergalactic medium as well as the Galactic center, X-ray binaries and supernova remnants.

Putting plasma in the cosmological picture

Researchers first draw on knowledge about the initial state of the universe when simulating galaxy clusters. The cosmic microwave background has been observed to a high level of detail, so the initial conditions are fairly well understood, while sophisticated modern telescopes provide new images of the universe at different stages of its evolution, which can then be confronted with simulations. “The recently deployed James Webb Space Telescope (JWST) for example is used to observe very tiny parts of the universe, but at great depth. Images from the JWST challenge us; what are the physical processes by which galaxies formed at such an early stage? We can see interesting galaxies with certain properties,” outlines Dr Dolag. The Euclid telescope, which has recently been launched, will essentially map the entire sky, complementing the images from the JWST. “The Euclid telescope will measure the distribution of matter in the universe very precisely,” continues Dr Dolag. A major challenge facing cosmologists is that the timescales associated with the evolution of cosmic structures are very long, and researchers only have access to static

PhD student Tirso Marin-Gilabert Works on MHD Simulations of turbulence within galaxy clusters and develops the treatment of viscosity within cosmological, hydro-dynamical simulations.

PhD student Lucas Valenzuela Works on kinematics of galaxies in cosmological simulations including tracer populations like globular clusters and planetary nebulae. For COMPLEX he further develops the web portal for sharing the outcome of hydro-dynamical, cosmological simulations.