Visible matter constitutes only 16% of the total mass of the universe.
Little is known about the nature of the rest of this mass, called dark matter. Even more surprising is the fact that the total mass of the universe is only 30% of its energy. The rest is dark energy, which is completely unknown but is responsible for the accelerated expansion of the universe.
To learn more about dark matter and dark energy, astrophysicists use large-scale surveys of the universe or detailed studies of the properties of galaxies. But they can only interpret their observations by comparing them to the predictions of theoretical models of dark matter and dark energy. But these simulations take tens of millions of hours of computation on supercomputers.
The Extreme-Horizon collaboration was able to simulate the evolution of cosmic structures from the first moments after the Big Bang to the present day, on the Joliot-Curie supercomputer, which offers a computing power of 22 petaflops (22 x 1015 floating point operations per second). The volume of digital data processed has exceeded 3 TB (1012 bytes) at each stage of the calculation, justifying the use of new writing techniques (RAMSES code with adaptive mesh refinement) and reading of simulation data.
Cosmology: correcting the data of the Lyman-α forest
The first result of the simulation concerns the interpretation of large structures in the distant universe: intergalactic hydrogen clouds. Astrophysicists detect them by measuring the light absorption of quasars, which are extremely bright due to the presence of a supermassive black hole that attracts matter into its accretion disk. Each of the clouds along the line of sight produces an absorption line (Lyman-α) with a specific redshift, due to the expansion of the universe. All these lines form a dense forest, revealing the one-dimensional distribution of hydrogen clouds, and therefore matter, at distances between 10 and 12 billion light years (ly).
However, many black holes between these quasars and us expel a considerable amount of energy into the intergalactic medium, altering its thermal state and the properties of the Lyman-α forest. The physical model used in the Extreme-Horizon simulation describes this feedback in detail, which skews the estimates of cosmological parameters by several percent. The calculated correction factor will be essential, in particular for the DESI (Dark Energy Spectroscopic Instrument) experiment under construction in Arizona (US), because the bias can exceed 5%, while the target accuracy is 1%.
Ultra-compact massive galaxies shaped like a beehive
The high resolution of the Extreme-Horizon simulation in the low density regions allowed to describe the accretion of cold gas by the galaxies and the formation of ultra-compact massive galaxies when the universe was only 2 to 3 billion years. These atypical galaxies, recently observed with the Alma radio telescope (Atacama Large Millimeter / Submillimeter Array) in Chile, are formed by the rapid clustering of many very small galaxies. It was only possible to identify this “hive” growth method thanks to the exceptional resolution of Extreme-Horizon.
Big challenge on the Joliot-Curie supercomputer
Designed by the company Atos for GENCI (the French high-performance computing center), the Joliot-Curie supercomputer, based on Atos’ BullSequana architecture, reached peak computing power of 22 petaflops in 2020.
The great challenges are exceptional simulations and calculations performed during the period of the Grand Challenge following the installation of a new computer partition. This three month period provides a unique opportunity for a small number of users to access a large portion of the machine’s resources. They benefit from the support of the TGCC and manufacturer teams, working together to optimize the operation of the computer during this start-up phase.
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