The universe simulation involves nearly 60,000 billion particles


Collectively numbering nearly 60 trillion particles, a new set of cosmological simulations is by far the largest ever produced.

The simulation suite, dubbed AbacusSummit, will help extract the secrets of the universe from future studies of the cosmos, predict its creators. They present AbacusSummit in several articles published on October 25 in Monthly Notices of the Royal Astronomical Society.

AbacusSummit was produced by researchers at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City and the Center for Astrophysics | Harvard and Smithsonian. Composed of over 160 simulations, it models how gravitational attraction causes particles to move in a box-shaped universe. Such models, known as N-body simulations, capture the behavior of dark matter, which makes up most of the matter in the universe and only interacts by gravity.

“This sequel is so large that it probably contains more particles than all other N-body simulations that have ever been run combined – although that’s a difficult claim to confirm,” says Lehman Garrison, lead author of one. new articles. and a researcher from CCA.

Garrison led the development of the AbacusSummit simulations with graduate student Nina Maksimova and professor of astronomy Daniel Eisenstein, both of the Center for Astrophysics. The simulations were run on the US Department of Energy’s Summit supercomputer at the Oak Ridge Leadership Computing Facility in Tennessee.

AbacusSummit will be useful soon, as several surveys will produce maps of the cosmos in unprecedented detail in the years to come. These include the Dark Energy Spectroscopic Instrument, the Nancy Grace Roman Space Telescope, and the Euclid spacecraft. One of the goals of these big budget missions is to improve estimates of the cosmic and astrophysical parameters that determine the behavior and appearance of the universe.

Scientists will make these improved estimates by comparing the new observations to computer simulations of the universe with different values ​​for various parameters, such as the nature of the dark energy that separates the universe. With the improvements offered by next-gen surveys, better simulations are needed, Garrison says.

“Galaxy surveys provide extremely detailed maps of the universe, and we need equally ambitious simulations that cover a wide range of possible universes in which we could live,” he says. “AbacusSummit is the first suite of such simulations that has the scale and fidelity to compare to these astonishing observations.”

The project was intimidating. N-body calculations – which attempt to calculate the motions of objects, like planets, interacting gravitally – have been among the greatest challenges in physics since the days of Isaac Newton. They are delicate because each object interacts with all other objects, regardless of their distance. This means that as you add more objects, the number of interactions increases rapidly.

There is no general solution to the N-body problem for three or more massive bodies. The calculations available are only approximations. A common approach is to freeze time, calculate the total force acting on each object, and then push each one based on the net force it experiences. The time is then slightly advanced and the process repeats.

Using this approach, AbacusSummit processed a colossal number of particles thanks to intelligent code, a new numerical method and great computing power. The Summit supercomputer was the fastest in the world when the team performed the calculations.

The team designed their codebase – called Abacus – to take full advantage of Summit’s parallel processing power, which allows multiple calculations to be performed simultaneously. Summit has many graphics processing units, or GPUs, that excel at parallel processing.

Performing N-body calculations using parallel processing requires careful algorithm design, because an entire simulation requires a substantial amount of memory to store. This means that Abacus cannot just make copies of the simulation for the different supercomputer nodes to work on. So the code instead divides each simulation into a grid. A first calculation provides a good approximation of the effects of distant particles at a given moment of the simulation. (Distant particles play a much smaller role than nearby particles.) Abacus then groups nearby cells and separates them so that the computer can work on each group independently, combining the approximation of the distant particles with precise calculations of the particles. relatives.

For large simulations, the researchers found that the Abacus approach offers a significant improvement over other N-body code bases, which divide simulations irregularly based on particle distribution. The uniform divisions used by AbacusSummit make better use of parallel processing, the researchers report. In addition, the smoothness of the Abacus grid approach allows a large part of the distant particle approximation to be calculated before the simulation even begins.

Thanks to its design, Abacus can update 70 million particles per second per node of the Summit supercomputer (each particle represents a cluster of dark matter with 3 billion times the mass of the sun). The code can even analyze a running simulation, looking for patches of dark matter indicating the bright star-forming galaxies that are the focus of future investigations.

“Our vision was to create this code to provide the simulations necessary for this particular new brand of galaxy investigation,” says Garrison. “We wrote the code to perform the simulations much faster and with much more precision than ever before. “

Eisenstein, who is a member of the Dark Energy Spectroscopic Instrument collaboration – which recently began its study to map an unprecedented fraction of the universe – says he’s looking forward to using Abacus in the future.

“Cosmology is taking a leap forward thanks to the multidisciplinary fusion of spectacular observations and advanced computing,” he says. “The coming decade promises to be a wonderful time in our study of the historical sweep of the universe.”

Reference: Maksimova NA, Garrison LH, Eisenstein DJ, Hadzhiyska B, Bose S, Satterthwaite TP. AbacusSummit: A massive set of high-precision, high-resolution N-body simulations. Mon Royal Astron Notice. Share. 2021; 508 (3): 4017-4037. doi: 10.1093 / mnras / stab2484

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