Putting plasma in the cosmological picture
The majority of the visible matter in 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.
COMPLEX Research Group
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.”
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
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.
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.
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.
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.© Magneticum Box2b, K. Dolag
images. While it’s possible to observe a cosmic object at a certain point in a certain state, this represents just a snapshot, so researchers tend to bring together many observations to try and build a fuller picture. “We can observe many objects, hope that we see similar thing in different stages, then try to build a story or narrative out of that. Or we can try to learn how an object formed by observing different objects, which we believe to be fundamentally the same in principle, but which are thought to be in different stages of their evolution,” explains Dr Dolag. Previously researchers would try to learn about the physical processes affecting an object by comparing a statistical sample of observations to the results of simulations, but Dr Dolag says it’s now possible to go further. “We can now draw comparisons object by object. When we have a galaxy cluster in our simulation, representing an observed cluster, we can then do a much more detailed comparison,” he says.
Cosmic ray electrons and magnetic fields are fundamental parts of the plasma filling cosmic structures, and their interplay leads to very specific emission in radio wavebands, often tracing the current dynamics of the underlying structures. Our knowledge of these structures has grown dramatically over recent years thanks to the new generation of highly sophisticated radio telescopes, including the Low Frequency Array (LOFAR), which offers improved wavelength coverage and sensitivity. However, these observations still only show the tip of the iceberg when it comes to the complexity of cosmic structures, says Dr Dolag. “We can see clusters and galaxies, and we can infer that galaxies are typically connected in a diluted, kind of weblike structure, influencing their evolution but still to be detected in observations. But a lot of other effects may also influence our measurements,” he explains. The project will make an important contribution in this respect, developing new models which will help uncover how physical processes
shaped the visible matter in the universe; one important aspect of this work is improving numerical treatment methods. “That’s a computational part of our work. We need very precise methods to describe all the processes that we see. Therefore we have also developed improved hydrodynamic methods in COMPLEX,” says Dr Dolag.
Viscosity of plasma
T hese hydrodynamic methods are being applied to several different kinds of cosmological objects, while researchers are also working to include additional physical processes in models, building on Dr Dolag’s earlier work on the Magneticum pathfinder, a detailed hydrodynamical simulation of cosmic evolution. In one recently published paper, Dr Dolag and his colleagues have described their work in implementing a novel treatment of viscosity, which can be considered as a measure of a fluid’s resistance to flow. “Instead of assuming that plasma is a fluid with essentially zero
viscosity, we can assign a specific viscosity and see what differences emerge,” he explains. This can then affect the structure of a galaxy cluster and the way galaxies within clusters evolve, a good example of the type of issue that Dr Dolag and his colleagues are investigating in COMPLEX. “In the formation of a cosmological structure different things fold in together. We can
COMPLEX
COsmological Magnetic fields and PLasma physics in EXtended structures
Project Objectives
The COMPLEX group will develop a numerical framework to perform, for the first time, simulations of galaxy clusters with high enough spatial resolution to resolve the important scales on which turbulence acts. It will develop the novel and detailed sub-grid models necessary to describe the evolution of magnetic fields, cosmic rays and associated transport processes.
Project Funding
This project has received funding from the European Research Council ERC Advanced Grant
Contact Details
of our local environment, although Dr Dolag says there are some restrictions due to computational power limitations, with the typical simulations nowadays something like 500 megaparsec (1 Megaparsec = 1 million parsecs or 3.26 million light years) in size.
“We can simulate only a small region of the universe,” he acknowledges. The COMPLEX group is still working to develop new, more
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 simulations, and to see what changes.
see that mixing and evolution inside of a cluster changes when we include viscosity. How much does that then change our global picture?” he outlines.
The COMPLEX group has also been involved in simulating the Local Universe, a cosmic neighbourhood which can be thought of as next door to the Milky Way, at least in cosmological terms. This demonstrates the ability to simulate a cluster representative
detailed simulations, with researchers now able to include magnetic fields and the treatment of high-energy particles, which represents significant progress. “We are among the very first groups able to simulate the radio emission of galaxy clusters to a high level of detail. This is because we have been able to include all the physical processes which are required to predict that,” says Dr Dolag.
Project Director, Dr. Klaus Dolag
Ludwig-Maximilians-Universität München Universitäts-Sternwarte München Scheinerstr. 1
D-81679 München
T: +49 89 2180 5994
E: dolag(at)usm.lmu.de W: https://www.origins-cluster.de/en/newsevents/news/detail/erc-advanced-grantfuer-klaus-dolag
Böss et al. 2023, https://ui.adsabs.harvard.edu/ abs/2023MNRAS.519..548B/abstract
Groth et al., 2023 , https://ui.adsabs.harvard.edu/ abs/2023arXiv230103612G/abstract
Marin-Gilabert et al., https://ui.adsabs.harvard.edu/ abs/2022MNRAS.517.5971M/abstract
Klaus Dolag
Klaus Dolag is a staff member of the computational astrophysics section at Ludwig Maximillian University (LMU) Munich. He performed the world’s largest cosmological hydro-dynamical simulation earlier in his career, the Magneticum Pathfinder, and holds deep experience in numerical algorithms and code development.
