The latest computer model of the cosmos involves 400 billion particles in a box about two thirds of the volume of the universe
Back in 1970, Jim Peebles at Princeton University carried out a ground-breaking experiment. He used the new-fangled technology of computing to simulate the behaviour of a cluster of galaxies under the force of gravity.
This simulation was tiny by modern standards: it involved just 300 'particles'. But it showed that computer models could give an important insight into the formation of structures on a grand scale. It's fair to say that this and other early simulations revolutionised cosmology.
Today, Juhan Kim at the Korea Institute for Advanced Study in Seoul, and a few pals, show just how far this technique has come. These guys have carried out the largest simulation of the universe ever undertaken, consisting of 374 billion particles in a box some 10 gigaparsecs across. That's roughly equivalent to about two thirds the size of the observable universe.
This took some 20 days of computing time on the Tachyonii supercomputer in Korea, the 26th fastest in the world in the last set of rankings.
By contrast, the 2005 Millennium simulation, which was the biggest cosmological simulation of its time, consisted of 10 billion particles in a 500 megaparsec box. The new simulation called Horizon Run 3 is 8800 time bigger.
The purpose of the simulation is to reproduce the entire evolution of a universe made largely of cold dark matter to see whether it produces same structures that we see in ours--from the galactic structures to galactic clusters, galactic superclusters and beyond.
The most recent theories predict that our universe ought to have structures on the very largest scales called acoustic baryon oscillations. These are essentially the leftovers of waves in the plasma that existed in the very early universe which became frozen in place as they cooled.
Clearly, it is only possible to model these objects in simulations that cover a large fraction of the universe.
There also ought to be structures that are the precursors of galaxies. These formed when dark matter formed into haloes that then attracted visible matter which went on to form the galaxies we see today.
These structures are too old and therefore too distant to see. But it should be possible to simulate them in models that contain a significant fraction of the universe at very high resolution. To date that hasn't been possible, at least not reliably, since low resolution simulations can easily become biased in unrealistic ways. .
So the new simulation opens to way to studying these objects in detail for the first time. In effect, astronomers peer into these simulations as if they were observing from a virtual Earth at the centre of a virtual universe. They look to see whether the simulations reproduce the universe (on average) that we see and if so, what more distant observations with the next generation of telescopes should look like.
That's impressive. But clearly, Horizon Run 3 is just one step in an ongoing race to make bigger simulations of the universe in greater resolutions. And one day, it will look as primitive to future cosmologists as Peeble's galaxy cluster run looks today.
Ref: arxiv.org/abs/1112.1754: The New Horizon Run Cosmological N-Body Simulations
