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May 7, 2014

Simulation Details 13.8 Billion Years of Cosmic Evolution

Tiffany Trader
Illustris Simulation image Vogelsberger

Astrophysicists and cosmologists have come up with a new time-lapse simulation of the universe’s evolution that is the most comprehensive and detailed yet. The Illustris simulation, as it’s called, spans 13.8 billion years of cosmic evolution and follows thousands of galaxies taking into account gravity, hydrodynamics, cooling, the course of stellar population and other complex processes.

Developed by a team of scientists from the Massachusetts Institute of Technology and several other institutions and executed on powerful supercomputers, the model traces the history of the universe, starting soon after the Big Bang and continuing through to the present day, capturing 13.8 billion years of change with unprecedented fidelity.

The research team reports that the massive simulation once again confirms the standard theory of the universe and matches key astronomical observations, including the distribution of galaxies and the richness of neutral hydrogen gas in galaxies of all sizes.

A paper describing the research appears in the May 7 issue of the journal Nature. Besides MIT, the 10-author team includes researchers from Harvard-Smithsonian Center for Astrophysics (CfA); the Heidelberg Institute for Theoretical Studies in Germany; the University of Heidelberg, the Kavli Institute for Cosmology and the Institute of Astronomy, both in Cambridge, England; the Space Telescope Science Institute in Baltimore; and the Institute for Advanced Study in Princeton, N.J.

Aside from being a stunning achievement in its own right, Illustris offers important insight into the rate at which certain types of galaxies develop.

“Some galaxies are more elliptical and some are more like the Milky Way, [spiral] disc-type galaxies,” explains Mark Vogelsberger, an assistant professor of physics at MIT and lead author of the Nature paper. “There is a certain ratio in the universe. We get the ratio right. That was not achieved before.”

The model also provides clues on the tendency of matter to redistribute in the universe, prodded by supernovas and other phenomenon. This finding could be used to fine-tune experiments performed with space-based telescopes, such as NASA’s WFIRST survey, and EUCLID, the European Space Agency’s program.

Illustris showcases a cubic chunk of the universe measuring 350 million light-years on each side, which is found to contain 41,416 galaxies. The amount of data is such that the complete simulation required several months of computing time at multiple computing centers, including the Harvard Odyssey and CfA/ITC cluster; the Ranger and Stampede supercomputers at the Texas Advanced Computing Center; the CURIE supercomputer at CEA/France; and the SuperMUC computer at the Leibniz Computing Centre in Germany. The largest run incorporated 8,192 compute cores, and spanned 19 million CPU hours. For comparison’s sake, it would take the best desktop computers of the day 2,000 years to execute the entire simulation.

Adding to the simulation’s complexity are 12 billion visual “resolution elements,” which enabled the researchers to compare “snapshots” from the simulation with images of the known universe. “[There was] agreement with observational data on small scales and large scales,” affirms Vogelsberger. A close match like this serves as validation of the study’s correctness.

Illustris diverges from earlier efforts in both scope and fidelity. Its predecessor, Millennium, only tracked the evolution of the dark matter web; ordinary matter and galaxies were tacked on in an ad hoc approach. But the Illustris simulation incorporates ordinary matter from the start. Where the former visualization was relatively calm-looking, Illustris is packed with explosions, including hot blasts of gas that emanate from supermassive black holes at the center of galaxies. These ejections are an essential part of galaxy formation, acting as a brake to star formation.

As Simon White, a cosmologist at the Max Planck Institute for Astrophysics in Garching, Germany, who worked on the Millennium Simulation, explains: Illustris is the first simulation that is large enough to capture a representative segment of the universe and also fine-grained enough to incorporate individual galaxies. “It’s the combination of those two things that is new,” he tells Science.

Advances in supercomputing power are what enabled the simulation to handle the 350 million light-year span and all the additional features. “Previous simulations of the growth of cosmic structures have broadly reproduced the ‘cosmic web’ of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies, because of numerical inaccuracies and incomplete physical models,” the research team explains in the Nature article.