Illustris Collaboration

In the scientific community, it's the age of

the virtual map. Neurologists recently charted connections in the human brain, and environmental researchers routinely parse databases into maps to model climate change. But today, a paper published in Nature presents arguably the most ambitious virtual map to date: a computer simulation, tracking the evolution of the entire universe.

"Our computer program accounts for the laws of nature—gravity, the formation of supernovae, black holes—and then evolves the universe until the present," says Shy Genel, a post-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics and coauthor on the study. "For the first time, we have a simulation that covers a large fraction of the universe but also has enough resolution to look at the internal structure of individual galaxies."

Starting 12 million years after the Big Bang, this virtual model covers 13 billion years of cosmic evolution with 12 billion resolution elements. Part map and part mathematical model, the simulation could help astrophysicists study the formation and content of the cosmos.

A Better Map

Virtual maps are especially useful to astrophysicists, who make their calculations based on starlight emitted from distant galaxies. Scientists routinely translate dim telescope images into hard, physical properties, but sometimes a galaxy in question is so distant that any kind of direct observation is impossible. "Converting light into real, physical properties can be difficult," Genel says. "We typically make a lot of assumptions."

In an effort to take some of the guesswork out of astrophysics, researchers have spent years trying to produce an accurate virtual model of the universe. Previous attempts have broadly simulated the "cosmic web" of galaxies, but generally failed to reproduce a reliable mixture of galaxies with varied gas and metal content.

This particular virtual model begins 12 million years after the Big Bang. Genel and his team programmed their computers to consider a bevy of physical factors, from gravity to black holes, and then simulate the following 13 billion years of cosmic evolution. The virtual map could help scientists perfect their cosmic calculations and study phenomena that would be impossible to observe.

"We can use our simulations to simulate observations," says Genel. "And we can compare our simulations to real galaxies to help us figure out how light should be translated into physical applications."

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Mapping Galaxies and Dark Matter in Detail

"This is a great step forward," says Philip Hopkins, and astrophysicist at Caltech. "It's an impressive demonstration of just how far the simulations have come, especially in terms of computing power."

Hopkins notes that the astrophysics community will turn to this virtual map to examine the histories of certain galaxies in more detail. "Even one galaxy's history can be chaotic," he says. "In the future, we may use this simulation to investigate the factors that control a galaxy's morphology."

One of the most interesting applications, Hopkins says, involves dark matter, the elusive, mysterious material that scientists believe makes up around 80 percent of the universe. Astrophysicists hypothesize that swaths of dark matter collide with one another frequently, but galaxies and stars embedded in dark matter should, in theory, run into each other just as often.

Direct observation, however, suggests otherwise. "That's one of the questions that I'm hoping to use this simulation to address," Hopkins says. "What are stars doing differently that they don't collide as often as dark matter."

Next Steps

This virtual map of the universe is not quite ready for primetime. Genel and his team presented their simulation today in the journal Nature, but they will spend the next few months testing their results against hundreds of direct observations before they pass along their data to fellow astrophysicists.

"First of all, we need to compare our simulation to observations," Genel says. "If we find that our simulated universe reproduces observations in the real universe, we can be more confident that our model represents reality."

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