A supercomputer simulation has tracked the evolution of the universe, from a mere 12 million years after the big bang until the present day. It is the first to produce realistic-looking galaxies by the thousands – and a triumph for our current understanding of the history of the universe.
"If you take 40,000 galaxies from the simulation and compare them to 40,000 real galaxies, you would have a very hard time distinguishing them," says one of the simulation's creators, Mark Vogelsberger of the Massachusetts Institute of Technology. "This tells you that our understanding of galaxy evolution is so complete now that we can make models that predict a universe that just looks like ours."
Previous simulations had focused on the properties of individual galaxies, or on the web-like scaffold of dark matter that connects groups of galaxies. Recent advances in supercomputing and a fresh computational approach let the recent simulation, called Illustris, do both at once.
"Our main goal was to have a statistical sample of galaxies but also be able to say something about the characteristics of each galaxy," says Vogelsberger.
He and his colleagues modelled a cube of space with sides that stretched to 347 million light years, a fraction of the size of the observable universe today. They divided it up into 12 billion smaller units, and used equations that describe ordinary matter's behaviour in cosmic structures to calculate what should happen in each unit.
Instead of forcing the units to stay in a rigid grid, the team allowed them to shift, making the simulation more accurate, as well as more computationally efficient, than previous ones.
2000 years of data
The simulation handles ordinary matter as well as enigmatic dark matter and dark energy. It also accounts for complex phenomena such as cooling gas, star formation, supernova explosions and supermassive black holes pulling in material that gets too close.
To run the simulation, the team used several supercomputers in Europe and the US, each of which contained many central processing units, or CPUs. By contrast, an ordinary computer might have just one. The entire simulation took 16 million CPU hours, which means that running it on a single normal computer would take nearly 2000 years.
The resulting cosmos was almost indistinguishable from the real one we see today. As a demonstration, the team compared a simulated version of the Hubble Ultra Deep Field observation to the real thing, which was made when the Hubble Space Telescope stared at one spot in the sky for nearly 12 days.
The code also managed to reproduce different types of galaxies, including spiral galaxies like the Milky Way and blob-shaped elliptical galaxies.
"This is like simulating the whole US, where previously it was like just simulating your neighbourhood," says Michael Boylan-Kolchin at the University of Maryland in College Park, who led one of the largest previous simulations, called Millennium-II.
Some details weren't perfect, however. For instance, small galaxies in the simulation produced stars too early, so they appeared older than we observe them to be today. "This is one of the failures of the model," Vogelsberger says.
What's more, Boylan-Kolchin notes that some of the processes through which galaxies grow up such as star formation and black hole radiation, are still not very well understood – making it difficult to tell whether the simulation gets them right.
However, models like Illustris can also help elucidate these processes, he says. "The more we can get at these processes by trying to make realistic galaxies, and seeing what works and what doesn't, that will give us information about how these processes work on smaller scales."
Journal reference: Nature, DOI: 10.1038/nature13316
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