There’s an old joke among astronomy students about a question on the final exam for a cosmology course. 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 have taken a giant leap forward to provide at least one specific example from the Universe.
To do this, they used a set of simulations called “MillenniumTNG”. It traces the accumulation of galaxies and cosmic structure through time. It also provides new insight into the standard cosmological model of the Universe. It is the latest of the cosmological simulations, joining efforts as ambitious as the Project AbacusSummit from a few years ago.
This simulation project takes into account as many aspects of cosmic evolution as possible. It uses simulations of regular (baryonic) matter (what we see in the Universe). It also includes dark matter, neutrinos and the still mysterious dark energy on the mechanisms of formation of the Universe. It’s a big challenge.
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Simulate the universe
More than 120,000 computer cores from the SuperMUC-NG in Germany worked on the data for MillenniumTNG. This followed the formation of about one hundred million galaxies in an area of space about 2,400 million light-years in diameter. Then Durham’s Cosma8 got to work calculating a larger volume of the Universe, but filled with a trillion simulated dark matter particles and another 10 billion that tracked the action of massive neutrinos.
The result of this numerical calculation was a simulated area of the Universe that reflected the formation and distribution of galaxies. The size was large enough that cosmologists could use it to extrapolate hypotheses about the entire universe and its history. They can also use it to probe “cracks” in the standard cosmological model of the universe.
The cosmological model and the prediction
Cosmologists have this basic model that they propose to explain the evolution of the Universe. It goes like this: The Universe has different types of matter. There is ordinary baryonic matter, which we are all made of as well as stars, planets and galaxies. It’s a little less than 5% of the “stuff” of the cosmos. The rest is dark matter and dark energy.
The cosmology community calls this strange set of cosmic circumstances the “Lambda Cold Dark Matter” model (LCDM, for short). In fact, it describes the universe quite well. However, there are some discrepancies. This is what simulations should help solve. The model is based on data from a wide variety of sources, including microwave cosmic radiation to the “cosmic web”, where galaxies are arranged along an intricate network of dark matter filaments.
What is still missing is a good understanding of exactly what dark matter is. And, when it comes to dark energy, well, that’s a challenge. And, astrophysicists and cosmologies seek to better understand the LCDM and the existence of the two great unknowns. This requires a lot of sensitive new observations from astronomers. On the other side of the coin, it also needs more detailed predictions of what the LCDM model actually entails. It’s a huge challenge and it’s what drives the great MillenniumTNG simulations. If cosmologists can successfully simulate the Universe, they can use those simulations to help understand what is happening “in real life.” This includes characteristics of galaxies both in the modern Universe and in very ancient times.
Understand and predict the orientations of galaxies in the universe using MillenniumTNG
The MillenniumTNG simulations follow on from earlier simulation projects called “Millennium” and “IllustrisTNG”. This most recent set gives a tool to fill in some gaps in their understanding of things like the evolution of galaxies and their shapes (or morphology).
Astronomers have long been familiar with what are called “intrinsic galactic alignments.” It’s basically a tendency for galaxies to orient their shapes in similar directions, for reasons no one really understands.
It turns out that weak gravitational lensing affects how we see the alignment of galaxies. The MillenniumTNG simulations could allow astronomers to measure such alignments in the “real world” using its simulated alignments. It’s a big step forward, according to team member Ana Maria Delgado. “Perhaps our determination of the intrinsic alignment of galaxy orientations can help resolve the current discrepancy between the magnitude of material aggregation inferred from the faint lens and the cosmic microwave background,” she said.
Investigate the past
As in other areas of cosmology, the MillenniumTNG group probes the very young Universe through simulations. It is a time after the time of reionization when the first stars were already shining and the first galaxies were evolving. Some of these early galaxies are quite large, which seems a bit out of context with a nascent universe. The James Webb Space Telescope (JWST) saw them and the question remains: how did they get so massive in such a short time after the Big Bang?
The MillenniumTNG simulations actually appear to replicate this tendency for some ancient galaxies to grow larger in a short time. Typically, it’s about 500 million years after the Big Bang. So why are these galaxies so massive? Astronomer Rahul Kannan suggests a few ideas to explain this. “Perhaps star formation is much more efficient soon after the Big Bang than later, or perhaps massive stars formed in higher proportions at the time, making these galaxies unusually bright,” he explained.
Now that JWST is probing even earlier times in cosmic history, it will be interesting to see if the simulations predict what it finds. Kennan suggests that there may well be a disagreement between the real universe and the simulations. If this happens, it will pose another puzzling question to cosmologists about the earliest epochs of cosmic history.
The future of simulated and real universe exploration
The next few decades of cosmological studies will greatly benefit from simulations such as Millennium TNG. However, the quality of the simulations depends on the data they receive and the assumptions made by their scientific teams. The MillenniumTNG benefits from vast databases of information, as well as the capabilities of supercomputers to process their data. According to the team’s principal investigator, Professor Volker Springel of the Max Planck Institute, the simulations that produced more than 3 petabytes of data are a major boon for cosmology.
“MillenniumTNG combines recent advances in the 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 well prove instrumental for progress on key questions in cosmology, such as how best to limit neutrino mass with large-scale structure data.”
His predictions certainly match the goals of the MillenniumTNG project. The teams continue to build on the success of the IllustrisTNG project, which performed hydrodynamic simulations as well as 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 matter aggregation and galaxy halos, galaxy clusters and distribution, galaxy formation patterns, galaxy populations in the early universe, these intrinsic galaxy alignments, and other related topics. While they may not yet be able to fully define the Universe (and give three examples), the MillenniumTNG team is making huge strides in understanding its origin and evolution.