Next to theory and experiments, simulations are one of the most important tools used in research today. Occasionally, scientists develop theories that cannot be tested using today’s practice or technology. Here, a simulation might then be able to point the theoretical physicist where he or she needs to look. Other times, it might happen that there are two different theories that could be suitable for describing reality. If simulations are built based on both theories, their results can sometimes separate the significant from the useless. And sometimes it also happens that there isn’t any theory yet, only data from measurements. If it’s possible to create a simulation that produces the same results as the experiment, then sometimes it’s also possible to derive a theory from the simulation.
Astronomers who want to simulate the cosmos are normally confronted with a choice: either they use their computational power for details or for the largest possible space in their simulation. Both methods have their drawbacks in their informational value: simulations of small numbers of galaxies cannot provide good statistical results, and large-scale simulations lack the details compared with reality. With the TNG50 astronomical simulation, researchers were able for the first time to combine a large-scale cosmological simulation with the high resolution of a detailed simulation, like those that were previously possible only for studies of individual galaxies.
Incidentally, the simulation also produced very fascinating images from the history of its simulated universe, which you can view on the project’s website. TNG50 consisted of a cube-shaped section of space with a side length of 230 billion light-years. Inside of this space, the simulation tracked the simultaneous development of thousands of galaxies over their 13.8-billion-year cosmic history and here took into account up to 20 billion particles, corresponding to dark matter, stars, cosmic gas, magnetic fields, and supermassive black holes. The computation itself was performed on 16,000 computer cores of the Hazel Hen supercomputer in Stuttgart, the third fastest computer in Germany (as of June 2019), which ran for more than a year around the clock (Hazel Hen has a total of 185,000 cores).
The first scientific results from TNG50, which have now been published by a team led by Annalisa Pillepich (Max Planck Institute for Astronomy, Heidelberg, Germany) and Dylan Nelson (Max Planck Institute for Astrophysics, Garching, Germany) also include some completely unexpected phenomenon. “Numerical experiments of this kind are particularly successful when you get more out of them than you put in,” says Nelson. According to the press release, “phenomena that the scientists had not explicitly programmed appeared in the simulation.”
The new simulation produced two exciting examples. One involves the formation of disc galaxies like our own Milky Way. With TNG50 as a “time machine,” the researchers could rewind cosmic history and then watch as the quickly rotating disc galaxies with their orderly star motions emerged from the chaotic, unordered and highly turbulent gas clouds of earlier epochs. Little by little the gas settles down. Stars formed from this gas thus find themselves more and more frequently on circular orbits, finally forming a large spiral galaxy as a kind of galactic carousel.
“TNG50 shows that our own Milky Way galaxy with its thin disc is at the height of current fashion,” says Annalisa Pillepich. In the past ten billion years, at least the galaxies that produce new stars have all become more and more disk shaped; in addition, their chaotic internal motions have significantly decreased. In a few words: “The universe was much more chaotic when it was only a few billion years old than it is today.”
In the simulated evolution of their model’s galaxies, the astronomers have discovered another phenomenon: gas and particle winds flowing out from the galaxies at high velocity. These winds are caused by supernova explosions, as well as the activity of the supermassive black holes at the centers of the galaxies. The gas initially leaves the galaxies in arbitrary directions, but over time, the gas flows find the path of least resistance. Given enough time, the gas then typically flows in two opposite directions within conical regions. These look something like two ice-cream cones – placed tip to tip – with the galaxy in the center. Such structures have also been found in real astronomical observational data.
Under the influence of dark matter’s gravity, which envelopes the galaxy, these winds then become slower and slower. Like the water of a fountain, they can fall back into the original galaxy and supply it with recycled gas. This process also provides for a redistribution of the gas from the center of a galaxy to its outer regions, thus accelerating the transformation of the galaxy into a thin disk: galactic structures produce galactic fountains and vice versa.