When it comes to reconstructing the past, you might think that astrophysicists have it easy. After all, the sky is awash with evidence. For most of the universe’s history, space has been largely transparent, so much so that light emitted by distant galaxies can travel for billions of years before finally reaching Earth. It might seem that all researchers have to do to find out what the universe looked like, say, 10 billion years ago is to build a telescope sensitive enough to pick up that ancient light.
A good 95 percent of the cosmos is made up of two very different kinds of invisible and as-yet-unidentified stuff that is “dark,” meaning that it emits and absorbs no light at all. One of these mysterious components, called dark matter, seems immune to all fundamental forces except gravity and perhaps the weak interaction, which is responsible for some forms of radioactivity. We know dark matter must exist because it helps bind rapidly moving stars to their host galaxies and rapidly moving galaxies to even larger galaxy clusters. The other component is “dark energy,” which seems to be pushing the universe apart at an ever-increasing rate.
To identify these strange dark substances, cosmologists require more than just the evidence collected by telescopes. We need theoretical models of how the universe evolved and a way to test those models. Fortunately, thanks to progress in supercomputing, it’s now possible to simulate the entire evolution of the universe numerically. The results of these computational experiments have already been transformative, and they’re still only in their early days.
Such a simulation, dubbed Bolshoi simulation - the Russian word for “great” or “grand - has recently been completed. Bolshoi was started in a state that matched what the universe was like some 13.7 billion years ago, not long after the big bang, and simulated the evolution of dark matter and dark energy all the way up to the present day. This whole project was done by 14 000 central processing units (CPUs) on the Pleiades machine at NASA’s Ames Research Center, in Moffett Field, Calif., the space agency’s largest and fastest supercomputer.