There is an old joke among astronomy students about a question on the final exam for a cosmology class. It looks like this: “Describe the universe and give three examples.” Well, a team of researchers in Germany, the United States, and the United Kingdom has taken a giant leap toward giving at least one accurate example of what the universe looks like.
To do this, they used a set of simulations called “MillenniumTNG”. It tracks galaxy accumulation and cosmic structure through time. It also provides a new view of the standard cosmological model of the universe. It’s the latest in cosmological simulations, and it joins such ambitious efforts as the AbacusSummit project two years ago.
This simulation project takes into account as many aspects of cosmic evolution as possible. It uses simulations of ordinary (baryonic) matter (which is what we see in the universe). It also includes dark matter, neutrinos, and dark energy whose mechanisms for forming the universe are still obscure. This is a long request.
More than 120,000 computer centers at SuperMUC-NG in Germany went to work on the data for MillenniumTNG. This was followed by the formation of about one hundred million galaxies in a region of space with a diameter of about 2,400 million light-years. Then Cosma8 in Durham went to work calculating a universe larger than the size but filled with a trillion simulated dark matter particles and another 10 billion tracking the action of massive neutrinos.
The result of this number of crunches was a simulated region of the universe that reflects the composition and distribution of galaxies. The size was large enough that cosmologists could use it to extrapolate assumptions about the entire universe and its history. They can also use it to search for “cracks” in the standard cosmological model of the universe.
Cosmological model and prediction
Cosmologists have this basic model they propose to explain the evolution of the universe. It goes like this: the universe has different kinds of matter. There is ordinary baryonic matter, which is what all of us, stars, planets, and galaxies are made of. It is just under 5% of the “stuff” of the universe. The rest is dark matter and dark energy.
The cosmological community calls this strange set of cosmic conditions the “cold Lambda dark matter” (LCDM, for short) model. It actually describes the universe pretty well. However, there are some inconsistencies. This is what simulation should help solve. The model draws on data from a large variety of sources, including cosmic microwave radiation to the “cosmic web,” where galaxies are arranged along an intricate web of dark matter filaments.
Still missing is a good understanding of what exactly dark matter is. And for dark energy, it’s a challenge. And astrophysicists and cosmologists are looking to better understand the LCDM and the existence of two big unknowns. That requires a lot of sensitive new observations from astronomers. On the other side of the coin, they also need more detailed predictions of what the LCDM model actually suggests. It’s a big challenge and it’s what drives MillenniumTNG’s big simulations. If cosmologists can successfully simulate the universe, they can use those simulations to help understand what is happening “in real life.” This includes properties of galaxies in both the modern and very early universe.
Understanding and predicting the trends of galaxies in the universe using MillenniumTNG
The MillenniumTNG simulations follow the previous simulation projects called “Millennium” and “IllustrisTNG”. This newer group provides a tool to point out some of the gaps in their understanding of things like galaxy evolution and shapes (or morphology).
Astronomers have long known about something called “intrinsic galactic alignment.” This is basically a tendency for galaxies to orient their shapes in similar directions, for reasons no one fully understands.
It turns out that weak gravitational lensing affects how we see the galaxy alignment. MillenniumTNG simulations could allow astronomers to measure such alignments in the “real world” using simulated alignments. This is a huge step forward, according to team member Ana Maria Delgado. “Perhaps our determination of the intrinsic alignment of galaxy directions can help resolve the current discrepancy between the amplitude of cluster matter inferred from the weak lensing and the cosmic microwave background,” she said.
As with other fields of cosmology, the MillenniumTNG group is examining the very young universe through simulations. This is the time after the era of reionization when the first stars were already shining brightly and the first galaxies evolved. Some of those early galaxies are very large, which seems out of the context of the young universe. They have been seen by the James Webb Space Telescope (JWST) and the question remains: How did they become so massive in such a short time after the Big Bang?
The MillenniumTNG simulation appears to replicate this tendency of some early galaxies to grow exponentially in a short time. Typically, this would be about 500 million years after the Big Bang. So, why are these galaxies so massive? Astronomer Rahul Kannan offers a few ideas to explain this. “Perhaps star formation was more efficient shortly after the Big Bang compared to later times, or massive stars may have formed at higher rates at that time, making these galaxies unusually bright,” he explained.
Now that the JWST is looking at earlier times in cosmic history, it will be interesting to see if the simulations predict what it finds. Keenan suggests that there may be a rift between the real universe and the simulation. If that happens, it will pose another perplexing question for cosmologists about the earliest eras of cosmic history.
The future of simulated and real universe exploration
The coming decades of cosmological studies will greatly benefit from simulations such as the Millennium TNG. However, simulations are only as good as the data they receive and the assumptions their science teams make. MillenniumTNG benefits from vast databases of information, as well as the capabilities of supercomputers to process its data. According to the team’s principal investigator, Professor Volker Sprengel of the Max Planck Institute, simulations that have generated more than 3 petabytes of data are a major asset to cosmology.
“MillenniumTNG combines recent advances in simulation of galaxy formation with the field of large-scale cosmic structure, allowing for improved theoretical modeling of the connection of galaxies to the dark matter backbone of the universe,” he said. “This could be very useful for advancing key questions in cosmology, such as how the mass of neutrinos can be better constrained by large-scale structure data.”
His predictions certainly align with the goals of the MillenniumTNG Project. The teams continue to build on the success of the IllustrisTNG project, which ran hydrodynamic simulations in addition to the dark-matter-only Millennium simulation created nearly a decade ago. The team’s simulations have been used to study a number of different galactic subjects. They include the aggregation of matter and halos of galaxies, galaxy clusters and their distribution, models of galaxy formation, galaxy clusters in the early universe, those intrinsic alignments of galaxies, and other related topics. While they may not be able to fully define the universe (and give three examples), the MillenniumTNG team is making huge strides in understanding its origin and evolution.
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