There is an old joke among astronomy students about a question on the final exam for a cosmology course. It sounds like: “Describe the universe and give three examples.” Well, a team of researchers in Germany, the US and the UK have taken a giant leap to provide at least one concrete example from the universe.

To do this, they used a set of simulations called “MillenniumTNG”. It tracks galaxy accretion and cosmic structure over time. It also provides a new view of the standard cosmological model of the universe. It is the latest in cosmological simulations and joins ambitious efforts like the AbacusSummit project from a few years ago.

This simulation scheme takes into account as many aspects of cosmic evolution as possible. It uses simulations of ordinary (baryonic) matter (what we see in the universe). These include matter, neutrinos, and dark energy, whose mechanisms of dynamical universe formation remain unclear. It is very challenging.

Cosmic simulation

More than 120,000 SuperMUC-NG computer centers in Germany worked on the data for MillenniumTNG. This came after about a hundred million galaxies formed in a region of space about 2400 million light-years in diameter. Cosma8-Durham then worked to calculate a larger universe, but one would monitor the activity of 1 trillion simulated dark matter particles and another 10 billion massive neutrinos.

The result of this numerical calculation is a simulated region of the universe that reflects the composition and distribution of galaxies. It was so large that cosmologists used it to develop hypotheses about the entire universe and its history. They can use it to probe for cracks in the standard cosmological model of the universe.

Cosmological Modeling and Prediction

Cosmologists have this basic model to explain the evolution of the universe. It goes like this: The universe consists of different types of matter. There is ordinary baryonic matter of which we are all composed, stars, planets, and galaxies. It makes up less than 5% of the “stuff” in the universe. The rest is dark matter and dark energy.

The cosmological community calls these strange cosmic conditions the “cold lambda dark matter” (LCDM, for short) model. In fact, it describes the universe very well. However, there are some contradictions. This is what simulation should help solve. The model draws on data from a variety of sources, including cosmic microwave radiation and the “cosmic web,” where galaxies are arranged in a complex web of dark matter filaments.

What is still missing is a better understanding of what exactly dark matter is. And when it comes to dark energy, that’s a challenge. Astrophysicists and cosmologists are trying to better understand the existence of the LCDM and the great unknowns. This requires a lot of sensitive new observations from astronomers. On the other side of the coin, they also need detailed predictions of what the LCDM model actually represents. It’s a big challenge and it’s what drives the best MillenniumTNG simulations. If cosmologists can successfully simulate the universe, they can use those simulations to understand what happens in “real life.” These include the properties of galaxies in both the modern and very ancient universes.

Understanding and predicting trends in galaxies in the Universe using MillenniumTNG

MillenniumTNG simulations follow from previous simulation programs called “Millennium” and “IllustrisTNG”. The new team provides a tool to fill some of the gaps in our understanding of things like the evolution and shapes (or morphology) of galaxies.

Astronomers have long known about so-called “intrinsic galactic alignments.” It’s basically a tendency for galaxies to point their shapes in similar directions, for reasons no one really understands.

Weak gravitational lensing affects how we see galaxy alignment. MillenniumTNG simulations will allow astronomers to measure such alignments in the “real world” using simulated alignments. According to team member Ana Maria Delgado, this is a big improvement. “Perhaps our determination of the intrinsic alignment of galaxies’ orientations will help resolve the current discrepancy between the degree of material accretion inferred from the faint lens and the cosmic microwave background,” he said.

Look at the past

Like other fields of cosmology, the MillenniumTNG team studies the very young universe through simulations. This is a time after the reionization time when the first stars were already shining brightly and the first galaxies were forming. Some of these early galaxies are so massive that they seem out of the context of the nascent universe. The James Webb Space Telescope (JWST) spotted them, and the question remains: How did they become so massive in such a short time after the Big Bang?

The MillenniumTNG simulation reflects this tendency for some ancient galaxies to grow larger in a shorter period of time. Typically, about 500 million years after the Big Bang. Why are these galaxies so big? Astronomer Rahul Kannan offers some ideas to explain this. He explained, “Perhaps star formation became more efficient shortly after the Big Bang, or massive stars formed at a higher rate at that time, making these galaxies unusually bright.”

Now that JWST is looking at earlier periods of cosmic history, it will be interesting to see if the simulations predict what it will find. Keenan suggests that there may be a divide between the real universe and the simulation. If this happens, it will raise another intriguing question for cosmologists about the earliest epochs of cosmological history.

The future of simulated and real space exploration

The next few decades of cosmological studies will benefit greatly from simulations like Millennium TNG. However, the quality of the simulations depends on the data they receive and the assumptions of their scientific teams. MillenniumTNG benefits from vast databases of information and the capabilities of supercomputers to process its data. According to the team’s principal investigator, Professor Volker Sprenkel of the Max Planck Institute, the simulation, which generated more than 3 petabytes of data, is a huge boon for cosmology.

“MillenniumTNG combines the latest advances in galaxy formation simulations with the large-scale cosmic structure, allowing for better theoretical modeling of the connection of galaxies to the dark matter backbone of the Universe,” he said. “This will be very useful for advancing key questions in cosmology, such as how to deduce the neutrino mass with large-scale structural data.”

His expectations certainly match the goals of the MillenniumTNG project. The teams continue to build on the success of the IllustristicNG project, which ran hydrodynamic simulations in addition to the dark-matter-Millennium simulation developed nearly a decade ago. Group simulations have been used to study various Hungarian subjects. Includes the composition of galaxies and haloes, clusters and their distribution, patterns of galaxy formation, clusters in the early universe, these intrinsic galactic alignments, and other related topics. While they may not be able to fully define the universe (and give three examples), the MillenniumTNG team is making great progress in understanding its origin and evolution.

For more information

Looking for cracks in the standard cosmological model
MillenniumTNG project webpage