Frontier Adventure

An Enormous Cosmological Simulation Wraps Up, Recreating Even More of the Universe

There’s an old joke among astronomy students about a question on the final exam for a cosmology class. It goes like this: “Describe the Universe and give three examples.” Well, a team of researchers in Germany, the U.S., and the UK took a giant leap toward giving at least one accurate example of the Universe.

To do it, they used a set of simulations called “MillenniumTNG”. It traces the buildup of galaxies and cosmic structure across time. It also provides new insight into the standard cosmological model of the Universe. It’s the latest in cosmological simulations, joining such ambitious efforts as the AbacusSummit project of a couple of years ago.

This simulation project takes into account as many aspects of cosmic evolution as possible. It uses simulations of regular (baryonic) matter (which is what we see in the Universe). It also includes dark matter, neutrinos, and the still-mysterious dark energy on the formation mechanisms of the Universe. That’s a tall order.

Simulating the Universe

More than 120,000 computer cores in the SuperMUC-NG in Germany went to work on the data for MillenniumTNG. That tracked the formation of about a hundred million galaxies in an area of space about 2,400 million light-years across. Then the Cosma8 at Durham went to work computing 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.

 Projections of gas (top left), dark matter (top right), and stellar light (bottom center) for a slice in the largest hydrodynamical simulation of MillenniumTNG at the present epoch. The slice is about 35 million light-years thick. Courtesy MPA.

The result of this number crunching was a simulated area of the Universe that mirrored the formation and distribution of galaxies. The size was big enough that cosmologists can use it to extrapolate assumptions about the entire Universe and its history. They can also use it to probe for “cracks” in the Standard Cosmological Model of the Universe.

The 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 types of matter. There’s ordinary baryonic matter, which is what all of us and the stars, planets, and galaxies are made of. It’s just under 5% of the “stuff” of the cosmos. The rest is dark matter and dark energy.

A composite model of matter distribution (with dark matter overlay) in a galaxy formation simulation made by the TNG Collaboration.

The cosmology community calls this strange set of cosmic circumstances the “Lambda Cold Dark Matter” model (LCDM, for short). It actually describes the Universe pretty well. However, there are some discrepancies. Those are what the simulations should help solve. The model is based on data from a huge variety of sources, including cosmic microwave radiation to the “cosmic web”, where galaxies are arranged along an intricate network of dark matter filaments.

What’s still missing is a good understanding of exactly what dark matter is. And, as for dark energy, well, it’s a challenge. And, astrophysicists and cosmologies are looking for a better understanding of LCDM and the existence of the two big unknowns. That requires a lot of sensitive new observations from astronomers. On the other side of the coin, it also needs more detailed predictions for what the LCDM model actually implies. It’s a huge challenge and is what’s driving the big MillenniumTNG simulations. If cosmologists can successfully simulate the Universe then they can use those simulations to help understand what’s happening “in real life.” That
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