Astronomy’s gift

21 February 2012

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James Martin Fellow Andrew Pontzen has used computer simulations to create videos showing the evolution of the cosmos. Recently featured on BBC2’s Stargazing Live, Pontzen explains the importance of distilling numbers into images.

Astronomy has a gift. Whenever we want to convince people we're doing good work with telescopes we can turn to a rich archive of gorgeous imagery. While not a substitute for good science, it certainly helps. But for those of us who work on modelling the universe inside a computer, things are a little harder.

I've been developing methods for turning computer simulations of galaxies into images for a while, as part of my open source project pynbody. It's hugely important in scientific analysis: whether we want to create a literal analogue to a telescopic picture, or a more metaphorical representation of the contents of our virtual universe, distilling millions of numbers into an image is a critical step.

But these days that's not enough. Static images just don't cut it in the age of YouTube. And the cosmos really is a fairly static place on human lifetimes – galaxies evolve only over billions of years. This is where we computer nerds have the edge over observers of the real universe: the process of simulation involves calculating and storing the state of the computerised cosmos from its early stages through to the present day. That means there's nothing to stop you from taking a picture of it at any stage of its development. Or, better, taking a whole series of pictures at subsequent steps. To jazz it up, you can start moving the virtual telescope through space. And, before you know it, you have a Hollywood-style movie of how galaxies form and evolve -- but all based on real physics.

Last November luck sent the production team of BBC2's Stargazing Live in my direction. They were looking for ways to discuss the evolution of the cosmos. Having seen a movie I'd made based on a previous paper, the production commissioned a series of videos taking us from the Big Bang through to a present day 'Milky Way'-like galaxy. Using a portion of Oxford's Berg supercomputer, it was possible to generate 10TB of data and distil it into 50,000 separate images to create the final footage in just over 6 weeks. For the first time the full original videos – just as we discussed in the live show – are available here.

Clip 1 shows the beginning of the virtual Universe (not as large as our real visible Universe, but rather a representative cut-out). Two things happen here: first, you see the overall expansion of the Universe. Then the matter, which starts out evenly spread through the cut-out sphere, starts to clump together into intricate structures which we call the 'cosmic web'. The Universe doesn't spin, by the way; the camera is just moving around it. After thirty seconds or so, we freeze the evolution of the Universe so you can see the structure that has formed.

If you were wondering why the Universe is green: it's not. Actually, clip 1 shows the behaviour of the mysterious dark matter which makes up four fifths of the total matter in the Universe. The remaining fifth is shown in clip 2, viewed at same time at which we froze clip 1 (about one tenth of the way to the present day). The stars are shown in white, while gas is artificially coloured blue.

Clip 3 flies through the frozen-in-time cosmos. After a few seconds, we reintroduce the dark matter to show how it's dictating the positions of stars and gas. You'll notice that the individual dots of light in clip 2 were not actually single stars, but whole collections of millions of stars -- in other words, the ancestors of today's galaxies.

Clip 4 follows the evolution of a small portion of our virtual universe. We see the small 'proto-galaxies' fall together under the influence of gravity, and ultimately merge into larger 'proto-galaxies'. This process repeats many times, with each generation of galaxies being larger than the last. The overall process has become known as 'hierarchical merging'.

Finally, in clip 5, we catch up with a virtual galaxy that has made it through to the present day. It is now a fully-fledged disk galaxy -- much like our own milky way -- and if we journey about half way from the edge of the disk to the centre, we reach a view remarkably like that from our own Earth on a dark night.