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iSGTW Feature - A virtual universe

Feature - A virtual universe

Structure formation in a computer-simulated Universe covering a dynamic range of a factor of 10000 in linear scale. Left image shows the Millennium simulation which models the distribution of dark matter on very large scales. Center image
shows the results of a simulation of a particular region taken from of the Millennium simulation which has been resimulated at higher resolution and includes baryonic matter. The right most image shows one example out of many of a disc galaxy forming within the GIMIC high resolution region. Image courtesy of Virgo Consortium

With the aid of the grid, researchers are conducting the largest-ever calculation to follow the formation of the dark haloes that seed galaxies.

To understand the properties of the galaxies themselves, it is necessary to simulate
how gas cools and forms stars in such haloes, says Carlos Frenk, director of the Institute for Computational Cosmology (ICC) and principal investigator of the Virgo Consortium, an international group of cosmologists that performs large numerical simulations on the formation of galaxies and other cosmic large-scale structures.

A key project of the consortium is the Galaxies Intergalactic Medium Interaction Calculation (GIMIC), which simulates the formation of galaxies in five key regions of the universe, allowing Virgo members to obtain unprecedented insight into how galaxies form.

GIMIC makes full use of DEISA's Extreme Computing Initiative common data repository and coordinated scheduling, using a work farm approach to computation scheduling and post-processing. (GIMIC is a collaboration project between the Max Planck Institute for Astrophysics in Germany and the ICC, in the UK. GIMIC includes also colleagues at Leiden in the Netherlands, at Sussex and at Nottingham in the UK.)

Carlos Frenk explains the running of the Galaxies Intergalactic Medium Interaction
Calculation (GIMIC) computer simulatio. Image courtesy of DEISA

A big problem to tackle

Simulating the formation and evolution of galaxies and other structures in the universe from cosmological initial conditions, is one of the most difficult problems in computational physics. The non-local nature of the gravitational interaction makes it difficult to split the computational domain into more or less isolated areas that can be computed in parallel. In order to simulate the evolution of a patch of universe, one needs to account for the contribution of all matter in the rest of the universe. This requires subtle parallelization strategies.

The non-locality of the computational domains is the major bottleneck for the efficient use of cosmological computational codes on grid-based technology; one needs to have access to the largest parallel supercomputers with low latency interconnection, as available in DEISA.

GIMIC has now revealed that astrophysical processes separate the ordinary or "baryonic" matter from dark matter even on large scales. As gas collapses to make a galaxy, the energy liberated by stars can blow powerful winds which heat the surrounding gas and pollute it with the products of nuclear fusion in the centers of stars - what astronomers call "heavy elements"

"Nobody in the world has yet succeeded in producing a realistic spiral galaxy like the Milky Way in a computer."- Carlos Frenk

"We now have an inventory of the distribution and thermodynamic state of the baryonic matter in the universe and its heavy element content. This will serve to guide astronomical searches for the currently missing bulk of the mass in the Universe," explains Frenk.

In spite of this advance, the problem of galaxy formation remains largely unsolved. "Nobody in the world has yet succeeded in producing a realistic spiral galaxy like the Milky Way in a computer. We do not yet know if the reason for this is our poor understanding of the physics of galaxy formation or if our cosmological model is somehow incomplete. For example, the cosmological model that has been so successfully explored in the Millennium simulation assumes a particular kind of dark matter, the so-called cold dark matter. Since the particles that would make up this cold dark matter have not yet been discovered in the laboratory, we cannot be sure that our assumptions are correct. Petaflop machines will simultaneously allow us to model the physics of galaxy formation with increasing realism and to explore alternative assumptions for the cosmological model, including the nature of the dark matter. Ultimately, we would like to simulate a representative region of the Universe with full gas physics - in short to create a virtual universe," concludes Frenk.

-Excerpted from DEISA Newsletter. Click for full text

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