Supercomputer "feels" magnetic fields in the cosmos

The supercomputer Piz Daint has enabled researchers to perform high-resolution simulations of cosmic magnetic fields and thus confirm existing theories about their formation.

October 16, 2014 - by Simone Ulmer

Around six months after the high-performance computer Piz Daint was officially cleared for research, the first scientific results are in. A team of researchers from Germany, Switzerland, Italy and the USA have studied the formation of cosmic magnetic fields by simulating the development of cosmic structures such as galaxies, galaxy clusters and turbulence in the plasma that surrounds the structures. The simulations, which were performed on CSCS’s flagship supercomputer within a “CHRONOS” project (Computationally-Intensive, High-Impact Research on Novel Outstanding Science), confirmed the hypothesis that cosmic magnetic fields are reinforced and sustained by certain dynamo processes in the cosmos. The results have been published recently in Monthly Notices of the Royal Astronomical Society.

The distribution of cosmic magnetic fields for the simulated cosmic web using "Piz Daint". (Source: F. Vazza, M. Brüggen, C. Gheller, P. Wang, 2014)

Where do the magnetic fields come from?

One of the most puzzling phenomena of the cosmos is the magnetic fields that pervade our universe. These magnetic fields surround galaxies, galaxy clusters and the so-called cosmic web, the thread-like structure composed mostly of dark matter and neutral gas (mostly hydrogen and helium) that extends through the universe for hundreds of millions of light years. The junction points of the cosmic web form galaxies and galaxy clusters.

According to radio telescope observations in the last few decades, the magnetic fields around galaxies and galaxy clusters are a few microgauss in strength, i.e. five to six orders of magnitude weaker than Earth’s magnetic field at the equator. Although small compared to Earth’s, the galactic magnetic field is unexpectedly intense – and scientists are still seeking an explanation for its origin.

The simulations tested the scenario whereby very tiny magnetic fields were generated shortly after the Big Bang, and later amplified by dynamo processes during the formation of structures. According to Franco Vazza, the principal investigator of the CHRONOS project and a post-doc under Marcus Brüggen at the Hamburger Sternwarte, these latest simulations show, however, that in the absence other mechanisms for the seeding of magnetic fields in the Universe, these fields would remain extremely low just outside of galaxy clusters and for most of the volume of the cosmos.

Weaker than previously assumed

Nevertheless, theoretical considerations and computer simulations have revealed that very weak magnetic fields might well have developed through certain processes during inflation (when the universe expanded extremely quickly at the beginning of its formation) and at its end (the transition that led to so-called symmetry breaking). The structure-forming processes in the cosmos coupled with turbulences that develop as a result could subsequently act as a dynamo – due to the turbulent motions of the plasma that surrounds galaxies and fills the volume of cluster of galaxies. This dynamo effect was believed to have strengthened and sustained the seed fields – which is precisely what the researchers have just succeeding in demonstrating. However, the results of the new simulations also reveal that the strengthening of the magnetic fields is weaker than speculated in previous publications. The simulations were conducted at an unprecedented resolution, greatly exploiting the graphics processors (GPU) of Piz Daint and resulting in a quadrupling of performance with respect to the CPU code. “This was made possible by a major rewriting of the most consuming parts of the numerical code, performed by co-author Peng Wang, a support scientist at NVIDIA”, mentioned Vazza.

Too weak to be observed

The simulations showed that the magnetic fields around the cosmic web’s filaments only reach an intensity of one nanogauss – that’s three orders of magnitude lower than that of galaxies, which is a pity, says Claudio Gheller, a CSCS staff member and co-author of the study, as it will barely be possible to observe these weak magnetic fields with the coming generation of radio telescopes. As soon as we are capable of doing so, however, these weak magnetic fields could yield a wealth of information about the early epochs of cosmic evolution, such as the extremely energy-rich phenomena at the beginning of the universe’s formation. After all, according to the researchers information about these dynamic physical processes should be stored and readable in the filaments’ current magnetic field.

The scientists are planning further simulations as part of the CHRONOS project at CSCS to research the magnetic fields interacting with cosmic radiation. “We also want to use even more precise algorithms to verify the previous results”, says Marcus Brüggen, a professor of extragalactic astrophysics and observational cosmology at the Hamburger Sternwarte.

These high-resolution simulations require large parallel computer systems. The simulations were conducted on Piz Daint with the cosmological code ENZO. The scientists simulated a cubic volume with a side length of 150 million light years, with which they were able to model the birth of galaxies and galaxy clusters in great detail. The largest simulation (24'003 cells) run on 1'024 Piz Daint computing nodes required about 5 days (2.5 million CPU hours).