Simulating the Entire Universe

Image modified from http://www.deus-consortium.org

 

An international consortium of Universities is working together to build an n-body simulation to model the entire visible cosmos. The project is current being run on a French supercomputer with 11,520 cores, 1440 of which are eight core Nehalem. If run on a single processor the task would require 30 million hours (about 3500 years). Fortunately, it won’t take quite that long for their computations to run. This is the first time the entire universe has been run at once. The Millennium simulation modeled a volumetric region but not all of space. Cosmologists are using computational models to determine how the stuff we can’t see (dark matter & dark energy) shaped the stuff that we can see.

The group has made substantial progress towards defining how galactic dark matter haloes work. Basically, our galaxy (and others) don’t rotate as you would expect them to. There’s essentially only two ways this could happen. One, gravity acts differently than we currently understand it. Or two, our galaxy rotates within a cloud of weakly interacting massive particles. So invisible stuff with gravity. Physics doesn’t have a solid answer for what dark matter is; but observationally, gravitational lensing does support dark matter over modified gravity. Here is a good Michio Kaku video if you want to learn more.

 

So in building a model universe, where does one start? Reconstructing the entire universe from observation would seem easy enough. You step out into your backyard, look up into the sky and try to copy what you observe into a list of coordinates. Okay, now try to find the distance to each of those objects. Parallax works in the beginning, for stars in our galaxy, but using parallax to determine distances to other galaxies would be like trying to determine how far away a mountain is by looking at it from your neighbor’s yard and comparing it against the view from yours. Luckily astronomers have something called standard candles which are a set of empirical rules that stack up to allow ballpark determination of distances.

But don’t worry, it gets tougher. Even though astronomers have an a tape measure for the cosmos, the universe is expanding at an accelerating rate. Since space itself is expanding, that means that the astronomers’ ruler also needs to “stretch.” There’s something called the comoving distance. This essentially means that the distance between tickmarks on the ruler will be different depending on how far away something is. Let me blow your mind a little bit. The universe is 13.7 billion years old. Since light only travels at a finite speed, and the expansion of space has accelerated; the observable universe becomes closer to 45 billion years wide. Another trippy but important feature is that as space expands, the light wave traveling through space dilates. Since longer wavelengths are red, this means stuff that is further away becomes more red. This is called cosmological redshift. This is what originally allowed Hubble to guess the age of the universe.

But back to the story at hand, the DEUS consortium is one of the largest computational experiments in the world; it will have a huge impact in cosmology. The code is even publicly available, and so is the data. So keep an eye on this project, the result will be nothing short of incredible. I’m anxious to see the types of visualizations that can be done with the output data. And even though this project aims to cover the entire universe, there will always be room to improve on the resolution of that simulation. Who knows what level of simulations will exist 20 years from now if exponential growth in computing performance persists. It’s cool to think people can reorganize sand into silicone and use it to remake a reflection of the entire cosmos. It appropriately addresses that saying “a universe in a grain of sand.”

Here’s video from the group’s website discussing specs and outcome.

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