Ground-breaking simulation for researching the "dark" Cosmos

June 08, 2017 - by Simone Ulmer

The Universe is a cradle of the unknown. Twenty-three per cent of it is made up of dark matter, while 72 percent is dark energy, the source and composition of which remains an unsolved mystery for science. Dark energy is thought to influence the formation of structures in the Cosmos and cause the Universe to expand at an ever-increasing rate. The Euclid satellite is set to lift off into space in 2020 and gather data over a period of around six years. New ground-breaking simulations by astrophysicists at the Center for Theoretical Astrophysics and Cosmology at the University of Zurich carried out on Piz Daint supercomputer at Switzerland’s National Supercomputing Centre (CSCS) will provide an important basis for the mission. The objective is to research the development of the Universe over the past 10 billion years, uncover the source of dark energy and find out why it is accelerating the expansion of the Universe.

The aim of the scientists Joachim Stadel, Douglas Potter and Romain Teyssier at the University of Zurich was to simulate trillions of particles within a useful timescale that facilitate the creation of the galaxy catalogue necessary for the project. According to the researchers, the code had to be optimised in detail and adjusted for this purpose in such a way as to exploit memory and computing processors in the best possible way. And they succeeded: in just 80 hours, which the researchers say is the fastest time-to-solution simulation with such a high number of particles, the astrophysicists used Piz Daint to simulate halos of the dark matter from one tenth of the Milky Way mass. Using another computer model, the researchers in the Euclid consortium then calculated, purely statistically, the galaxies that should be located in the halos.

Researchers use weak lensing to measure the material density, and through this also the percentage of dark matter. Weak lensing is the very subtle distortion of the light from background galaxies through the fluctuations in mass density, mainly of dark matter, taking place in the foreground. “It is comparable with the distortion you would see through a glass pane that is not quite even”, Stadel says. The notion that mass can affect light in the same way as a lens (gravitational lensing) was one of the predictions made by Albert Einstein in his Theory of General Relativity. As Stadel explains, it is very difficult to determine this distortion in the picture of the galaxies with high precision. However, this is essential in order to determine the fundamental properties of dark matter and dark energy, which is why Euclid has to observe lots of galaxies across large areas of the sky.

In addition to weak lensing, Euclid also uses the effect of what is known as galaxy clustering. The new simulations conducted by the researchers at the University of Zurich will also be used for this method. As Stadel explains, however, these measurements are less direct and based on more assumptions.

Beginning of a paradigm shift

Besides the simulation on Piz Daint, the researchers also tested their new code on the US supercomputer Titan at Oak Ridge National Laboratory in Oak Ridge, Tennessee. Like Piz Daint, the system is a hybrid supercomputer with graphics processors (GPUs) and traditional processors (CPUs) but is more powerful at the time of simulation. Here, the researchers succeeded in getting the newly revised code for the simulation of no less than eight trillion particles to run on up to 18,000 compute nodes with a peak performance of 10 Petaflops. For the scientists at the University of Zurich, the new simulation means the beginning of a paradigm shift in the field of simulations. In another decade, they expect to be able to carry out simulations like this in eight hours or less instead of in 80. It will then no longer be possible to store data and process it later. Stadel and his colleagues believe that analysis instruments would have to practically be installed in the code and used on the fly.