• Wed. Feb 28th, 2024

A massive cosmological simulation wraps around, recreating more of the universe

A massive cosmological simulation wraps around, recreating more of the universe

There is an old joke among astronomy students about the final exam question in a cosmology class. It goes like this: “Describe the universe and give three examples.” A team of researchers from Germany, the US, and the UK has taken a giant leap toward providing at least one accurate example of the universe.

To do that, they used a set of simulations called “MillenniumTNG.” It traces the buildup of galaxies and cosmic structure over time. It also provides new insight into the Standard Cosmological Model of the Universe. It is the latest in a series of cosmological simulations, joining ambitious efforts like the Abacus Summit project of 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). That includes dark matter, neutrinos, and the mysterious dark energy in the universe’s formation systems. That’s a tall order.

Imitating the universe

More than 120,000 computer cores at Germany’s SuperMUC-NG went to work on MillenniumTNG’s data. It tracked the formation of about a hundred million galaxies in space within a radius of about 2,400 million light-years. Then Cosma8 at Durham went to work calculating a larger volume of the universe, but it was filled with a trillion simulated dark matter particles and another 10 billion tracking the activity of massive neutrinos.

Projections of gas (top left), dark matter (top right), and stellar light (bottom center) for a slice in the largest hydrodynamic simulation of the Millennium TNG in the current epoch. This slice is about 35 million light-years thick. Courtesy MPA.

The result of this number crunching was a simulated area of ​​the universe that reflected the formation and distribution of galaxies. It was large enough that cosmologists could use it to explain hypotheses about the entire universe and its history. They can also use it to look for “cracks” in the standard cosmological model of the universe.

Cosmological model and prediction

Cosmologists have this basic model to explain the evolution of the universe. It goes like this: The universe has different types of matter. There is a common baryonic matter, which is what we, stars, planets, and galaxies are made of. This is less than 5% of the “stuff” in the universe. The rest is dark matter and dark energy.

An integrated model of the matter distribution (with dark matter overlay) in a galaxy formation simulation produced by the TNG collaboration.
An integrated model of the matter distribution (with dark matter overlay) in a galaxy formation simulation produced by the TNG collaboration.

The cosmology community calls this strange cosmological scenario the “Lambda Cold Dark Matter” model (LCDM, for short). It actually describes the universe quite well. However, there are some discrepancies. They are what simulations help solve. The model is based on data from a variety of sources, from cosmic microwave radiation to the “cosmic web,” where galaxies are arranged in a complex network of dark matter filaments.

What is still missing is a good understanding of what dark matter is. This is a challenge for dark energy. Also, astronomers and cosmologists are trying to better understand the LCDM and the existence of two great unknowns. That would require highly sensitive new observations from astronomers. On the other side of the coin, more detailed predictions are also needed for what the LCDM model actually implies. It’s a big challenge, and that’s what drives the big MillenniumTNG simulations. If cosmologists can successfully simulate the universe, they can use those simulations to understand what happens in “real life.” It includes the properties of galaxies in the modern universe and early ages.

Understanding and predicting galaxy orientations in the universe with MillenniumTNG

MillenniumTNG simulations follow previous simulation projects called “Millennium” and “IllustrisTNG”. This latest set provides a tool to close some of the gaps in their understanding of things like galaxy evolution and their shapes (or morphology).

Astronomers have long known about something called the “internal galaxy alignment.” It’s basically the tendency of galaxies to point their shapes in similar directions, for reasons no one understands.

Weak gravitational lensing affects how we see galaxy alignment. Millennium TNG simulations allow astronomers to measure such alignments in the “real world” using its simulated alignments. According to team member Ana Maria Delgado, this is a big step forward. “Perhaps our determination of the internal alignment of galaxy orientations will help resolve the current discrepancy between the extent of matter clustering inferred from weak lensing and the cosmic microwave background,” they said.

Investigating the past

Like other areas of cosmology, the Millennium TNG group investigates the very young universe through simulations. This is the time after the age of reionization when the first stars were already shining and the first galaxies were evolving. Some of those early galaxies are so massive that they seem out of context for an infant universe. The James Webb Space Telescope (JWST) spotted them, and the question remains: How did they get so big in such a short time after the Big Bang?

The MillenniumTNG simulations actually seem to replicate this tendency for some early galaxies to grow larger in a shorter time. Typically, this is about 500 million years after the Big Bang. So, why are these galaxies so massive? Astronomer Rahul Kannan suggests two ideas to explain it. “Perhaps after the Big Bang, perhaps more efficiently than at a later time, or massive stars formed at a higher rate, 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 be a discrepancy between the real universe and the simulations. If that happens, it will hand cosmologists another puzzling question about the early stages of the universe’s history.

The future of simulated and real space exploration

Cosmological studies in the coming decades will greatly benefit from simulations like the Millennium TNG. However, the simulations are just the data they get and the assumptions their science teams make. MillenniumTNG benefits from vast databases of information and the data crunching capabilities of supercomputers. According to the team’s principal investigator, Professor Volker Springel of the Max Planck Institute, the simulations, which produced more than 3 petabytes of data, are an important asset for cosmology.

“Millennium TNG integrates recent advances in simulating galaxy formation with cosmic large-scale structure, allowing improved theoretical modeling of the binding of galaxies to the dark matter backbone of the Universe,” he said. “This may help advance key questions in cosmology, such as how to constrain the mass of neutrinos using large-scale structural data.”

Galaxy distribution in MillenniumTNG predicting galaxies and positions as part of the simulation. Actual observations of galaxies can be perturbed by Doppler effect shifts, which can be incorporated into simulations. Courtesy MPA.

Certainly his predictions are in line with the goals of the Millennium TNG project. The teams continue to build on the success of the IllustrisTNG project, which was created a decade ago with hydrodynamic simulations and the dark-matter-only Millennium Simulation. The team’s simulations were used to study different galaxy subjects. They include clustering of matter and galactic halos, galaxy clusters and distribution, galaxy formation models, the early galaxy population of the Universe, internal alignments of galaxies, and other related topics. Although they have yet to fully define the universe (give me three examples), the MillenniumTNG team is making great strides in understanding its origins and evolution.

For more information

Looking for cracks in the Standard Cosmological Model
MillenniumTNG Project web page

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