Public Outreach

Modern cosmology has been shaped by three unexpected observations:

• The gravitational force on the scales of galaxies and galaxy clusters is stronger than can be explained from the observable amounts of matter within the general theory of relativity. This lead cosmologists to suggest the existence of an unknown form of matter - “dark matter” - that would have to make up most of the matter density in the Universe.

• On even larger cosmic distances gravity becomes repulsive, leading to an accelerated expansion of the Universe during its recent history. This lead cosmologists to propose another mysterious substance: “dark energy”, which would need to have an unusual (negative) proportionality between its energy density and pressure in order to explain the observed acceleration of the cosmic expansion.

• The Universe is surprisingly homogeneous, with regions that were seemingly never in causal contact over the entire history of the Universe nevertheless conspiring to have almost the same average matter density. This (along with other observations) has lead cosmologists to propose an epoch of exponential cosmic expansion in the early Universe - the epoch of cosmic inflation.

As a result of these developments, three mysterious new ingredients (dark matter, dark energy and the epoch of cosmic inflation) are now the cornerstones of the cosmological standard model. But despite the exceptional importance of these concepts, very little is known about their details. How warm or cold is dark matter? Is dark energy dynamical or a cosmological constant? Do dark matter and dark energy take part in interactions other than gravity? What is the exact mechanism behind inflation? Do these three novel concepts indeed correctly describe the evolution of the Universe or do we need to consider alternative explanations for the above mentioned observations?

To answer these questions, large-scale structure cosmology is studying the evolution of cosmic structures such as galaxies, galaxy cluster or cosmic voids over the history of the Universe. Together these structures for the so called cosmic web, and at researchers at USM are involved at leading positions in a number of projects aimed at deciphering that web of structures. We do so by a number of different means: by mapping the locations of galaxies and galaxy clusters, by studying gravitational lensing effects caused by cosmic mass concentrations and by measuring the absorption features that large cosmic gas clouds imprint into the spectra of luminous distant light sources. International experiments that USM is involved in include - among others - the following:

• The Dark Energy Survey, https://www.darkenergysurvey.org/
• The Euclid Mission, https://www.esa.int/Science_Exploration/Space_Science/Euclid
• Vera Rubin Legacy Survey of Space and Time, https://www.lsst.org/
• The Dark Energy Spectroscopic Instrument, https://www.desi.lbl.gov/

At the beginning of its life, a star is surrounded by a rotating disk of gas and dust - the protoplanetary disk. Properties of the disk such as its temperature and density profiles and its material composition are intricately linked to the properties of its central star and to the composition of the molecular cloud from which both star and disk formed. And these relationships between seed material, star and disk determine what kind of processes of planet formation can be fueled in the protoplanetary disk.

Today, we are aware of thousands of extra-solar planets, i.e. of planets that revolve around stars other than our sun. This has brought us closer to understanding the mechanisms of planet formation and the variety of planets that these mechanisms can produce. This also brings us closer to answering a central question of modern astrophysics: how typical are the conditions in our solar system and in particular how typical is planet earth? Scientists at the USM work at the forefront of efforts to answer this and other questions. These efforts include

• studying exoplanets and protoplanetary disks in observational data;
• simulating planet formation mechanisms in protoplanetary disks;
• modeling how geophysical processes on exoplanets shape the atmospheres of those planets, and deriving what kind of signatures this imprints onto light spectra of planet atmospheres.

An important part of understanding planet formation and evolution is to understand the formation and evolution of their host stars. But stellar astrophysics has of course a much more fundamental role in modern astrophysics: countless astronomical observations - e.g. colour-luminousity diagrams of star clusters, light spectra of distant galaxies, supernova observations, measurements of metallicity and metallicity-gradients in galaxies - can only be accurately interpreted with the help of stellar evolution models. USM is hosting scientists who are world leading in deriving such models.

At first glance, galaxies are simply accumulations of Billions of stars in one place on the nightsky. With deep imaging surveys we have by now spotted hundreds of Millions of these objects (with many Billions more estimated to exist in the observable Universe), and this cosmic galaxy population displays a huge variety, indicating that there are in fact many different types of galaxies. Observational properties that can vastly differ between galaxies are e.g. the following.

• Morphology: Galaxies can drastically differ in their shapes and general structure. There are elliptical galaxies with an at first glance completely featureless distribution of stars. There are spiral galaxies whose appearance it dominated by vast arms of increased stellar density spiraling into the galactic center. Others show very irregular shapes. Some galaxies are accompanied by huge jets of material that shoot from the galactic center right into the intergalactic space. And many more structureal details can be used to distinguish different morphology types.
• Light spectra: The type of radiation emitted by galaxies can strongly vary between different galaxy types. Some galaxy spectra are dominated by blue light - like that produced by young, hot stars, while other spectra are dominated by longer, colder wavelengths. Some galaxies can display strong emission lines in their spectra. Some galaxies have incredibly bright light sources in their centers that can even outshine the entires rest of the galaxy's stars. And this is still just a selection of the many different spectral features that can characterise galaxy spectra.
• Composition: Galaxies can vary strongly in the materials out of which they are made. Some galaxies are rich with young stars, dust as well as molecular and atomic gas (along side with hotter, ionised gas), while other galaxies are mostly devoid of colder gas phases and host mainly older stars. There are also variations in the amount of metallicity of galaxies, i.e. in the fraction of their materials that is made up of elements heavier than Lithium.
• Dynamics: Different galaxies will display different stellar and gas dynamics. Some galaxies rotate in a coherent thin disk, while in other galaxies the stars revolve on incoherent, seemingly chaotic orbits. The typical speed at which stars move in a galaxy is also closely related to the overall size of that galaxy as well as its concentration (a measure for how tightly packed the stars are in a galaxy).

The dimensions in which galaxies can be distinct from each other are not independent, but they are (causally) related and are remnants of the individual formation history that defines each galaxy. Scientists at the USM are at the forefront of exploring the processes that drive galaxy evolution - both observationally, and with the help of advanced computer simulations. This way they are uncovering the fascinating stories that are behind the vast variety of galactic phenomena. And they are also testing our understanding of fundamental physics: They compare galaxy observations to the standard cosmological paradigm, in which galaxies form in halos of dark matter and in which the properties of galaxies are closely related to the history of mergers between different dark matter halos. And they uncover relations between galaxie properties and the properties of the supermassive black holes that live in the centers of those galaxies.

Modern astronomy needs modern instruments! These are manufactured, among others, at the LMU Observatory (USM).The cryogenic optical setup of the MICADO camera for the upcoming ELT (ESO), large parts of the NISP instrument for the Euclid satellite and several projects at the VLT (ESO) are just a few examples of this.

In addition to software and technology for international projects, the USM workshop also builds proprietary observation instruments such as the Wendelstein Wide Field Imager (WWFI) and the 3-channel camera (3KK), which are located and active at LMU's state-of-the-art Wendelstein Observatory.