Realistic virtual universe

Print edition : May 30, 2014

A large-scale projection through Illustris volume, centred on the most massive cluster. It shows dark matter density overlaid with the gas velocity field. Photo: Illustris Project

ASTRONOMERS have created the first realistic virtual universe using a computer simulation called “Illustris”, which can recreate 13 billion years of cosmic evolution in a cube, 350 million light years on a side, with unprecedented resolution. These results were reported in the latest issue of the journal Nature. “Until now, no single simulation was able to reproduce the universe on both large and small scales simultaneously,” said the lead author Mark Vogelsberger of the Massachusetts Institute of Technology and the Harvard-Smithsonian Centre for Astrophysics (CfA), who conducted the work in collaboration with researchers at several institutions, including the Heidelberg Institute for Theoretical Studies in Germany.

Previous attempts to simulate the universe were hampered by lack of computing power and the complexities of the underlying physics. As a result, those programs either were limited in resolution or forced to focus on a small portion of the universe. Earlier simulations also had trouble modelling complex feedback from star formation, supernova explosions, and supermassive black holes.

Illustris employed a sophisticated computer program to recreate the evolution of the universe in high fidelity. It includes both normal matter and dark matter using 12 billion 3D “pixels”, or resolution elements. The team dedicated five years to developing the Illustris program. The actual calculations took three months of “run time” using a total of 8,000 CPUs (central processing units) running in parallel. If they had used an average desktop computer, the calculations would have taken more than 2,000 years to complete, said the CfA press release.

The starting point for the computer simulation was a mere 12 million years after the Big Bang. When it reached the present day, astronomers counted more than 41,000 galaxies in the cube of simulated space. Importantly, Illustris yielded a realistic mix of spiral galaxies like the Milky Way and football-shaped elliptical galaxies. It also recreated large-scale structures like galaxy clusters and the bubbles and voids of the cosmic web. On the small scale, it accurately recreated the chemistries of individual galaxies. Since light travels at a fixed speed, the farther away astronomers look, the farther back in time they can see. A galaxy one billion light years away is seen as it was a billion years ago. Telescopes like Hubble can give one views of the early universe by looking to greater distances. However, astronomers cannot use Hubble to follow the evolution of a single galaxy over time, whereas “Illustris is like a time machine. We can go forward and backward in time. We can pause the simulation and zoom into a single galaxy or galaxy cluster to see what’s really going on,” said Shy Genel, co-author of the study.

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