A billion light years, in 3D

Posted: 2013-05-29

The universe is a big place. There are an estimated 170 billion galaxies, averaging hundreds of billions of stars each. The largest structures in the universe are giant "sheets" and "filaments" of matter, comprised by galaxies and shaped by mutual gravitational forces.

In 2005, the largest n-body computer simulation ever, dubbed the "Millenium Run," simulated only about 0.01% of the total.

Using three.js and some of the visualization techniques behind Asterank, I've created a visualization of part of the Millenium Run, spanning about 5 million galaxies and a billion light-years.

Getting the data

I prototyped with the milli-millenium database, a tiny version of the Millenium dataset made available to the general public. I requested full access after I was satisfied that I could make a decent visualization. Gerard Lemson, one of the people in charge of the Millenium Run, was very kind and made the full database available for my use.

The amount of data is huge. Due to query constraints, I wound up slowly scraping most of the database and storing it locally in flat files. I chose to scrape/visualize a cube within the larger simulation spanning a billion light years.

Performance tricks

This is by far the most GPU-intensive simulation I've done. It won't run well without an ok graphics card. And it definitely won't run on your phone.

Reducing points with spatial compression

It's not necessary to actually render millions of points for typical screen resolutions. I created a preprocessing algorithm for combining particles that overlapped visually with one another. Particles representing masses of dark matter are combined based on their size and luminosity. Using an R-tree, nearby neighbors are fitted and combined in the visualization coordinate system to reduce extra rendering.

This reduced the number of particles rendered by an order of magnitude without significantly affecting the overall appearance of the visualization. As a result, decent laptops (eg. my 11" macbook air) can run the simulation.

Adjusting simulation based on fps

The simulation also changes based on user fps. Someone whose computer and graphics card are more powerful will see a tilt shift and better rotation. Despite the small change, this enabled the simulation to run on a class of laptops that otherwise would not have had access to it.

The code uses a simple FPS counter, available on github.

Next steps

Unfortunately I haven't come up with a way to visualize the passage of time without drastically cutting down on the scope and overall accuracy of the visualization. Frankly, this is why the visualization is merely interesting instead of really amazing. It's about 40,000 data points per timestep, with about 50 timesteps. I'll be looking at ways to improve this.

Until then, it's not very useful, but it looks nice:

Ian Webster

About the author

Ian is a software engineer based in the Bay Area. He has helped Google, NASA, and a dozen governments around the world improve their data pipelines and visualizations. He maintains a handful of cool websites like DinosaurPictures.org, MeteorShowers.org, and this one.

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