A Code for the Heavens

A slice, 70 million light-years thick, taken from the Millennium Run. The overlaid panels zoom in each time by a factor of four. CREDIT: Max-Planck-Institute for Astrophysics.

A highly detailed computer model has captured the birth of galaxies and giant black holes.  It lets astronomers follow the subsequent growth of these massive structures in the largest cosmological simulation to date.

The so-called "Millennium Run" took 28 days of intense computation to generate its 25 terabytes (25 trillion bytes) of data.  The simulation - named after the 2000-time-frame in which the idea was conceived - tracks the evolution of matter inside a cube 2 billion light-years on a side. 

A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers).

"One of the main advances here is size, which does matter in this business," said August Evrard of the University of Michigan. "We are able to connect the first structures in the universe with the galaxies we see nearby." 

The simulation starts when the universe was 10 million years old and evolves it all the way to the present - 13 billion odd years later.   The cube contains roughly 10 billion "particles" - each with the mass of a billion Suns.  These colossal blobs of matter interact gravitationally with each other in cyberspace.

Gravity will cause some of the particles to merge.  In the center of these matter clumps, galaxies can form, but exactly what type of galaxy will depend on the size of the clump and the history of mergers.  It would take a clump of a few thousand particles to house a Milky-Way-sized galaxy.

A paper describing the Millennium Run appeared in the June 2 issue of Nature.

Invisible skeleton

The Millennium Run does not actually go into all the messy details of forming stars and accreting gas.  Instead, it essentially provides the framework, or skeleton, for all that galaxy business by concentrating on the elusive dark matter, which is the dominant form of matter in our universe.

The light-emitting stuff - that we are all familiar with - only makes up about a tenth of the matter.  The other 90 percent does not react with light.  This dark matter has yet to be detected directly, but astrophysicists find it indispensable for explaining the cosmos.

"At present, cosmologists can simulate dark matter, which we can't see, better than galaxies and gas, which we can," said Nickolay Gnedin from the University of Colorado in a separate commentary.

Dark matter is easier to work with because it does not interact with anything, except through gravity.  Although computing the gravitational interactions of 10 billion dark matter clumps is no small feat, it becomes significantly harder when you throw in the radiation and gas dynamics needed to make stars.

In some sense, then, the Millennium Run is just the first step in creating a digital universe.  Once the dark matter "template" was finished, the international team of investigators - that calls itself the Virgo Consortium - was able to tack on a galaxy formation model, which basically told the computer where to stick bright, shiny things amongst the dark clumps. 

Is it possible to separate the dark matter evolution from galaxy formation?  Evrard admitted that there are complications, but simulations like the Millennium Run have compared favorably with full hydrodynamical simulations, which incorporate everything at once but are so computationally expensive that the represented volumes are considerably smaller.

Quasar lineage

Of particular interest in the "bright and shiny" category are quasars - the most luminous objects in the universe.  They are believed to be giant black holes - some of them billions of times more massive than our Sun - which are gobbling up very hot, glowing material.

Recent observations by the Sloan Digital Sky Survey (SDSS) have found big booming quasars so far away that we are seeing them when the universe was just a tenth of its age. Making black holes this big, this early, had seemed implausible in the currently favored cosmology.

"Yet when we tried out our galaxy and quasar formation modeling, we found that a few massive black holes do form early enough to account for these very rare SDSS quasars," said lead author Volker Springel of the Max Planck Institute for Astrophysics.

These black hole quasar candidates can be traced from when the universe was only a few 100 million years old, all the way to the present.  If the simulation is correct, the first quasar galaxies later turned into the massive galaxies that now sit in the center of the biggest galaxy clusters.

This finding was not surprising, but the Millennium Run allows scientists the opportunity to watch the entire life cycle of these behemoth structures - as well as other, more modest galaxy types.

Try out your own pet theory

One advantage of calculating the cosmic web of dark matter separately is it allows you the freedom to explore different ways of building up galaxies. 

"The really cool thing is that in the future, when the data is made public, you can go in and insert your own rules for galaxy formation," Evrard said. 

This is seen as a much more efficient use of computer time, as different researchers - and the ambitious amateur cosmologist - can use the dark matter skeleton from the Millennium Run to hang their own galaxy models. 

"For this reason, the simulation will have staying power," said Evrard.  "Maybe not for a millennium," he joked, "but for a decade, at least, and perhaps longer."

This article is part of SPACE.com's weekly Mystery Monday series.

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