The Largest Simulation of the Universe Ever Made

It’s about time to retire the old astronomy joke: “Define the Universe and give three examples.” That’s because recent simulations are answering that question pretty well. Nowadays, the answer could just very well be, “See the FLAMINGO simulations.”

They’re a product of the VIRGO consortium for cosmological supercomputer simulations. FLAMINGO stands for Full-hydro Large-scale structure simulations with All-sky Mapping for the Interpretation of Next Generation Observations. Its main science goals are to carry out a suite of massive cosmological simulations. These latest and largest model universe simulations were executed on a supercomputer cluster at Durhan University in the UK.

A Sizeable Simulation

The largest simulations in the suite use 300 billion resolution elements. These are particles with the mass of a small galaxy, all arrayed in a cubic volume with edges of ten billion light-years. It’s likely the largest cosmological computer simulation with ordinary matter ever completed. Matthieu Schaller (Leiden University): “To make this simulation possible, we developed a new code, SWIFT, which efficiently distributes the computational work over 30 thousand CPUs.”

The background image shows the present-day distribution of matter in a slice through the largest FLAMINGO simulation, which is a cubic volume of 2.8 Gpc (9.1 billion light years) on a side. The luminosity of the background image gives the present-day distribution of dark matter, while the color encodes the distribution of neutrinos. The insets show three consecutive zooms centered on the most massive cluster of galaxies; in order, these show the gas temperature, the dark matter density, and a virtual X-ray observation. Josh Borrow, the FLAMINGO team and the Virgo Consortium.

The VIRGO consortium focuses on the evolution of the intergalactic medium, the formation and evolution of galaxies, galaxy clusters, large-scale structures, the formation of dark matter haloes, and the large-scale distribution of dark matter. The FLAMINGO simulations go further, tracking not only dark but also ordinary matter (such as planets, stars, and galaxies). The result is a glimpse of how the Universe may have evolved.

Simulation of the Cosmos

So, how do you go about simulating the Universe? First, you need data about everything that’s out there. Observatories on the ground and in space collect huge amounts of information about everything they can detect. Next, all that information goes into a series of models that are incorporated into the larger simulations.

The simulations utilized data about all the regular (baryonic) matter there is. That includes planets, stars, galaxies, gaseous nebulae—essentially everything detectable. But, that’s not all there is in the cosmos. There’s dark matter, even though it’s only detectable through its gravitational influence on baryonic matter. And, you have to account for the mysterious dark energy.

The surface density of cold dark matter is modeled in a 20 Mpc slice of the Universe. This is part of the FLAMINGO simulation. Courtesy Joop Schaye, et al.

Okay, so that’s it, right? Well, not so fast. Stuff in the Universe interacts, usually via gravity, but also other processes. So, you have to take gravity into account. In addition, ordinary matter reacts to gas pressure. This sends matter out of galaxies by way of active black holes or supernova explosions. Then, there are more esoteric factors, such as the gravitational back-reaction of dark matter due to the redistribution of baryons. You need models of the relations between galaxy properties and various physical processes at work in galaxies and clusters. Other models take into account the star-formation activity of a central galaxy, which may correlate with the distribution of gas around it.

You’d also need hydrodynamical simulations of cosmology and galaxy cluster physics and data about the effects of gravitational lensing. Oh, and neutrinos, because they also play a role. All of this information has to be modeled and put into the simulation suite. The result is a greater large-scale understanding of our cosmos.

Simulations and Tensions

Aside from coming up with a full-scale model of the Universe, FLAMINGO’s work is also a way to take all cosmic data and connect various predictions and theories about the universe to actual observations. Theories include a set of properties about the Universe called the “cosmological parameters”. Those can be measured and compared to other observations. If they don’t match, that introduces a “tension” between measurements.

For example, there’s one called the “Hubble tension”. It refers to a parameter called H0 (pronounced “H-naught”) that really affects distance measurements in the Universe. Its value is the expansion rate of the Universe. H0 has been variously measured at 67 kilometers per second per megaparsec all the way up to 74.

There are other “tensions” in cosmological measurements, as well. One involves properties of the cosmic microwave background. That’s the light essentially left over from the earliest epochs of cosmic history. Some measurements of those properties yield different values so astronomers need to resolve that tension as well. Otherwise, they’d have to completely revise the standard model of cosmology which relies on a cold dark matter model. So, simulations are an important way to resolve tensions experimentally. They also allow researchers to play with different tweaks to their models.

Opening a Virtual Window on the Universe

The FLAMINGO project is really giving astronomers a new window into cosmic evolution. It’s based on real data to populate a virtual universe. The result is virtual data that researchers can test using new data analysis techniques and machine learning. Astronomers involved with the project have published three papers: one describing the methods, another presenting the simulations, and the third examining how well the simulations reproduce the large-scale structure of the Universe.

For More Information

The FLAMINGO Project
The VIRGO Consortium
Astronomers Carry Out Largest-ever Cosmological Computer Simulation
The FLAMINGO Project: Cosmological Hydrodynamical Simulations for Large-scale STructure and Galaxy Cluster Surveys
“FLAMINGO: Calibrating Large Cosmological Hydrodynamical Simulations with Machine Learning”.

“The FLAMINGO Project: Revisiting the S8 Tension and The Role of Baryonic Physics”

Carolyn Collins Petersen

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