If cosmologists were to make a movie of the universe's entire history, the show would start, of course, with the scorching blast of the Big Bang. The universe—absolutely every bit of mass we can detect or even infer today—would expand at unfathomable speeds, going from smaller than a proton to larger than a galaxy in the blink of an eye. As the expansion continued, the universe would cool down, and by the time the opening credits of the movie finished scrolling, a superhot soup of elementary particles would fill the whole cosmos, ready to cook the first protons and neutrons.But what would happen next?

The fact is, cosmologists are still working out the rest of the plot—what exactly took place during the more than 13 billion years since that primeval blast. For this article, in keeping with the current trend in international scientific publishing, IEEE Spectrum uses the words \"billion\" to mean 109 and \"trillion\" to mean 1012. A particular piece of the story that has kept researchers scratching their heads is how galaxies formed and evolved. How did that amorphous particle soup transform itself into billions and billions of galaxies of breathtakingly different shapes and sizes? Why did these galaxies gather together in clusters, and clusters of clusters, embedded along unimaginably enormous structures of matter shaped like bubbles, filaments, and sheets?

To answer these and other fundamental questions in cosmology, an international group of scientists from Canada, Germany, the United Kingdom, and the United States has been working on an ambitious project whose goal is to simulate on a supercomputer the evolution of the entire universe, from just after the Big Bang until the present.

The group, dubbed the Virgo Consortium—a name borrowed from the galaxy cluster closest to our own—is creating the largest and most detailed computer model of the universe ever made. While other groups have simulated chunks of the cosmos, the Virgo simulation is going for the whole thing. The cosmologists' best theories about the universe's matter distribution and galaxy formation will become equations, numbers, variables, and other parameters in simulations running on one of Germany's most powerful supercomputers, an IBM Unix cluster at the Max Planck Society's Computing Center in Garching, near Munich.

Late this year, the group plans to begin storing all of its output data in public repositories available to researchers around the world see \"Downloading the Sky\" in this issue. This accessibility, according to Simon D.M. White, a research director at the Max Planck Institute for Astrophysics who leads the German participation in the Virgo Consortium, will allow researchers to compare each simulated universe to its ultimate benchmark: the universe itself, as observed with ground and space telescopes.

If the simulation produces a bizarre universe that doesn't resemble ours, the assumptions that underpin the simulation are probably flawed or in need of adjustment. On the other hand, if the virtual cosmos is like the one we see, researchers will know they are on the right track. In this way, they hope to see some of the deepest mysteries of the cosmos solved on their computer screens.

By peering into great distances with powerful telescopes, astronomers have discovered a startling pattern of, literally, cosmic proportions: galaxies gather into clusters of varying sizes that don't float in isolation in the universe but rather are linked to one another by long tendrils of matter. What's more, these agglomerations of clusters are collected onto incredibly huge bubbles, filaments, and sheetlike structures, millions of light-years in size.

These structures, the grandest that are known, form a three-dimensional cosmic web that fills the universe. If you could shrink the cosmos until a galaxy cluster were the size of a grain of sand, a chunk plucked out of this universal web would resemble a piece of a kitchen sponge; the air pockets in the sponge would represent the huge cosmic voids that contain almost no matter see illustration, \"Spongy Universe\".

To find out how and when this giant clumpy web formed, the Virgo group is simulating how matter dispersed in space over almost the entire course of the universe's existence.

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