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Simulating the Universe

May 21, 2014

In a previous article (Our Simulated Universe, January 2, 2013), I examined the idea that we might be inhabitants of a computer simulation. The old adage, "If all you have is a hammer, everything looks like a nail," now codified as the Law of the Instrument, might apply here. Since we've just discovered information theory and are into our early years with computers, we might have a tendency to think that everything is information, and information is best processed by computers.

Claude E. ShannonI've always liked this image of Claude Shannon, the founder of information theory, demonstrating maze-solving by a mechanical mouse.

(Still image from a YouTube video by BinarycoreMedia, April 30, 2013, modified for artistic effect.)

A computer powerful enough to simulate our entire universe would be quite a computer, indeed! The whole thing seems fantastic. However, we can see how far computing has progressed in the last half century, so we would be shortsighted to dismiss the idea that the universe is a simulation without giving it some thought. If the technological singularity does arrive in the next few decades, whatever intelligence survives after that point might be able to simulate sizable universes willy-nilly a short time later.

Theoretical physicists are able to simulate small pieces of our universe, up to about 10-14 meter in size, or 1020 Planck lengths (1.6 x 10-35 meter). Simple molecules of chemical compounds such as methane are a few tenths of a nanometer in size, we're within a few orders of magnitude from simulating those.[1] The next step up would be viruses and biological cells.

Would it be possible for us to determine whether we are living in a simulation? In 2012, physicists from the University of Washington and the Universität Bonn, suggested that any such simulation would have its limits. The highest energy cosmic rays have an energy of 1011 GeV, so we should look at the distribution of the highest energy cosmic rays for anomalies.[2-3]

Such bottom-up simulations will capture every nuance of physical reality, but we're often just interested in what we can glean from some broad strokes. That's why we have such tools as the finite element method, which divides objects into a much coarser grid than the Planck length, but still gives us an idea whether our bridge will endure, collapse under its own weight, or as the result of environmental affects.

The Illustris simulation is a joint project of scientists from the Massachusetts Institute of Technology (Cambridge, Massachusetts), the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), the Heidelberg Institute for Theoretical Studies (Heidelberg, Germany), the Zentrum für Astronomie der Universität Heidelberg (Heidelberg, Germany), the Kavli Institute for Cosmology (Cambridge, UK), the Space Telescope Science Institute (Baltimore, Maryland) and the Institute for Advanced Study (Princeton, New Jersey). Its object, the simulation of galaxy formation in the universe, was completed in late 2013, and its results have been recently reported.[4-10]

Through a comprehensive physical model and quite a bit of computation, this simulation has achieved high resolution simulations of large volumes of space that agree favorably with physical reality. It's an advance over previous simulations that didn't create the proper mix of elliptical and spiral galaxies.[4-5]

The Illustris simulation starts 12 million years after the Big Bang, and it reproduces 13.8 billion years of cosmic evolution with 12 billion resolution elements in a cube of 106.5 megaparsecs (350 million light-years) on a side. Along with its reproducing the proper mix of elliptical and spiral galaxies, Illustris reproduces the observed distribution of galaxies in clusters, the density of hydrogen on large scales, and the hydrogen content of galaxies on small scales.[4-5]

The Illustris simulation includes normal matter and dark matter, and its calculations are on 12 billion voxel (volume pixel) elements.[6] Such high resolution allows the Illustris data to be presented visually, so it can be compared with actual astronomical observations.[7] Program development occurred over the course of five years, with the actual computation taking somewhat less than six months on a variety of supercomputers.[6,9]

Illustris example images
Left image, a still frame from the Illustris simulation, centered on the most massive galactic cluster known today. The blue-purple filaments show the location of dark matter, while the red, orange and white regions are areas of gas. Right image, an expanded view of the central area of the left image. (Illustris simulation images)[6]

Computers used for the Illustris simulation were the Harvard Odyssey and CfA/ITC cluster; the Ranger and Stampede supercomputers at the Texas Advanced Computing Center; the CURIE supercomputer at CEA/France; and the SuperMUC computer at the Leibniz Computing Centre in Germany.[7] These calculations would have taken more than 2,000 years to complete on a typical home computer.[6,8-9]

Starting from an initial condition for a universe twelve million years after the Big Bang, the Illustris simulation proceeds to the present day, 13.8 billion years later. The simulation volume contained 41,416 galaxies at the end, with galactic development closely following what's observed. The simulation recreated the observed mix of spiral and elliptical galaxies, as well as the occurrence of galactic clusters, bubbles and voids.[6-7]

Illustris comparison with actual observation
Illustris image (right) compared with an actual observation by the Hubble Space Telescope (left). The sizes, types, and colors of galaxies are closely simulated. (Images: NASA / Illustris Collaboration.)

Dark matter figures prominently in the Illustris calculations. Over the course of the simulation, dark matter coalesces to seed the first galaxies.[8] Mark Vogelsberger, an assistant professor of physics at MIT and principal author of the article in Nature describing the simulation, is quoted by BBC News as saying, "If you don't include dark matter, it will not look like the real Universe."[8]

Although the Illustris simulation gives results that amazingly mirror reality, its creators still see a need for improvement. Says Vogelsberger, "Low-mass galaxies just form too early in the simulation... so the stars are too old compared to the ones in the real universe."[7,9]

A high-definition video of the simulated evolution of the universe is available online at the Illustris Web Site (http://www.illustris-project.org).[6] Other researchers are welcome to the data, but there's a small technical problem. These data would take months to download, even with a fast Internet connection. The Illustris simulation was funded by the German Research Foundation, the European Research Council, NASA, and the Alexander von Humboldt Foundation.[7]


  1. Note that this a priori simulation is different from molecular orbital theory, which is routinely practiced by chemists.
  2. Vince Stricherz, "Do we live in a computer simulation? UW researchers say idea can be tested," University of Washington Press Release, December 10, 2012.
  3. Silas R. Beane, Zohreh Davoudi and Martin J. Savage, "Constraints on the Universe as a Numerical Simulation," arXiv Preprint Server, November 9, 2012.
  4. M. Vogelsberger, S. Genel, V. Springel, P. Torrey, D. Sijacki, D. Xu, G. Snyder, S. Bird, D. Nelson and L. Hernquist, "Properties of galaxies reproduced by a hydrodynamic simulation," Nature, vol. 509, no. 7499 (May 8, 2014), pp. 177–182.
  5. M. Vogelsberger, S. Genel, V. Springel, P. Torrey, D. Sijacki, D. Xu, G. Snyder, S. Bird, D. Nelson and L. Hernquist, "Properties of galaxies reproduced by a hydrodynamic simulation," arXiv Preprint Server, May 6, 2014.
  6. Astronomers Create First Realistic Virtual Universe, Harvard-Smithsonian Center for Astrophysics Press Release No. 2014-10, May 7, 2014.
  7. Peter Dizikes, "The universe in a cube," MIT Press Release, May 7, 2014.
  8. Pallab Ghosh, "Universe evolution recreated in lab," BBC News, May 7, 2014.
  9. Ian Sample, "Universe recreated in massive computer simulation," The Guardian (UK), May 7, 2014.
  10. The Illustris Simulation Web Site.

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