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A Box of Universe

Watch the cosmos evolve in a cube one billion light-years wide

Brian Hayes

The Bolshoi Intergalactic Ballet

2012-01HayesFC.jpgClick to Enlarge ImageThe domain of the Bolshoi simulation is a cube roughly one billion light-years on a side. For comparison, the diameter of the luminous part of a large galaxy might be 100,000 light years, and the diameter of the observable universe is about 90 billion light-years. Thus the simulation volume is large enough to contain millions of galaxies, but it still represents only a tiny fraction of the visible universe.

The level-0 mesh of the adaptive refinement tree has 2563 (or 16.8 million) cells. These cells can be subdivided to a maximum of 10 levels, at which point the side length of the smallest cells is about 4,000 light years.

What happens at the edges of this big box of universe? Rather than create artificial barriers or let particles leak out, the simulation adopts periodic boundary conditions. If a particle leaves the box on one side, it immediately reenters through the opposite side. This policy avoids disruptive edge effects, but it also introduces another hazard: a sort of gravitational narcissism, where an object is attracted to its own image a billion light-years away. The remedy is to impose a long-range cutoff, suppressing gravitational forces beyond about half this distance. A short-range force cutoff is also needed, to prevent spurious strong scattering if two particles happen to come close together.

The Bolshoi cube is populated with 2,0483 (equal to 8,589,934,592) identical particles. This is a great many objects to be managed in a computer program, but on the other hand it is a very coarse partitioning of the substance in the universe. Each particle represents about 200 million solar masses. Because this quantum of mass is so large, there’s no point in trying to distinguish between baryonic matter and dark matter. All of the particles in the simulation are dark matter. The primary structures formed as the universe evolves are the dark-matter halos in which galaxies are embedded, but the galaxies themselves are not explicitly represented.

At the start of the run, the eight billion particles are distributed almost uniformly in the cube, with only slight fluctuations in density. This configuration represents the state of the universe post-inflation, in an era not too long after the cosmic background radiation was emitted. The simulation begins when the universe is about 23 million years old and progresses to the present in some 400,000 time steps. Snapshots of intermediate states are saved at intervals of 40 to 80 million years.

The simulation was run on a computer called Pleiades at the NASA Ames Research Center in California. Pleiades is currently ranked seventh in the listings of the top 500 supercomputers worldwide. Bolshoi made use of 13,824 processor cores and 13 terabytes of memory.

When a simulation run ends, the computing is not yet finished. The first stage in analyzing the data is to run a program called a halo finder, which identifies regions of elevated density where particles are bound together by gravitational attraction. Finding density peaks is not difficult, but identifying bound groups of particles is a subtle problem. The particles in a halo move like bees in a swarm, so it’s not obvious which particles are members and which are merely passing through.

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