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n°19  |  October   2007
Hot topics
Computer simulation unlocks the mysteries of the Universe

A team of French scientists, gathered for the “Horizon Project1” and led by Romain Teyssier, astrophysicist at the CEA (France's Atomic Energy Authority), has achieved the largest simulation ever performed of structure formation in the Universe. This simulation was run on the new BULL supercomputer installed at the CCRT2 Computing Centre. It will enable astrophysicists to compare their models to actual astronomic observations with an unsurpassed realism.

Credit C. Pichon, D. Aubert.

The amazing increase in compute resources allows science to progress at a rapid pace. In astrophysics, the resolution of self-gravitating fluid dynamics equations using increasingly efficient algorithms and increasingly powerful supercomputers now allows to model structure formation in the Universe.
“Starting from the “initial condition” of our universe, that can be observed directly on the fossil radiance at 3 degrees K3, it is possible to follow the individual paths of a large number of particles that are used to make up the cosmological fluid.”, explains Romain Teyssier.

With close to 70 billions particles and more than 140 billions cells, the computation performed at the CCRT is the absolute record for a computer simulated N4-body system. For the first time ever in the history of scientific computing, it is possible to simulate half of the observable universe with enough resolution to describe the Milky Way with more than 100 particles!

To simulate such a huge volume with so much detail, the members of the Horizon Project have used the 6144 Intel® Itanium®2 processors of the Bull NovaScale 3045 computer recently installed at the CCRT, to fully exploit the “RAMSES” code.
“RAMSES” was developed at CEA in partnership with the Project Horizon astrophysicists. It features adaptive mesh refinement5, which allows reaching unprecedented levels of spatial details (the equivalent of a cubic grid with 262144 cell sides!). Thanks to the Bull and CCRT experts, the code was able to utilize computer resources in an optimal way during close to two months, consuming 18 Terabytes of memory and generating close to 50 Terabytes of data stored on disk. The same project run on a PC would have taken more than a thousand years to execute!

With this new simulation, we will be able to predict the distribution of matter in the Universe with an unprecedented level of precision and realism”, continues Romain Teyssier. “We will soon be in a position to compare the model with the actual observations of the entire sky that will be available thanks to the Planck space mission of the European Space Agency, which is planned to be launched in 2008. We are also going to prepare future gravitational6 lens experiments, such as the “Dark UNiverse Explorer” project, which aims at finding out the nature of dark energy”.

1 The Horizon Project is a collaborative initiative of CEA, CNRS and several Universities, gathering some 20 researchers and university teachers, including experts in computer simulation and specialists in structure formation in the universe. For more information, see

2 The CCRT (Centre de Calcul Recherche et Technologie, i.e. Centre for Research and Technology Computing) is a high-performance computing centre installed on the site of the French Atomic Energy Authority's Directorate of Military Affairs (CEA DAM), in the Ile de France, and incorporated within the CEA’s Computing Complex. Its 50 Teraflops computing power is made available to the CEA and to its industrial (EDF, Safran, ONERA, EADS/Astrium) and academic partners, to help them achieve ground-breaking high performance simulation projects.

3 This radiance is the fossil trace of the Universe when it finally became transparent to light. It was then 380 000 year old. This light reaches us 13 Billon years later, and gives us access to the conditions that then prevailed in the cosmological plasma.

4 A “N-body” system is a set of material points, or “particles”, submitted to their mutual gravitational attraction. Powerful computers are needed to compute the path of each particle in a N-body system.

5 To solve fluid dynamics equations with computers, space is discretized in small volume elements, called “cells” or “mesh”. This set of cells is called a “grid” and covers the system to be described, in the present case a large section of the Universe. To improve compute precision, smaller cells are used for dense regions such as galaxies: mesh size adapts automatically to local conditions, hence the term “adaptive mesh refinement”.

6 Light from the farthest galaxies is deflected by the gravitational effect of the matter on its path. This “lens” effect is one of the predictions of Albert Einstein's general theory of relativity. It allows to measure the quantity of matter present between these galaxies and us, and therefore to test the whole theory of structure formation in the Universe.


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