Monday, September 26, 2022

Record-breaking simulations of large-scale structure formation in the universe

Computer simulations that capture the complex dynamics of elusive neutrinos left over from the Big Bang.

The ghost-like particles, called neutrinos, are much lighter than all other known particles. Though, their exact mass remains a mystery.

Determining the mass of the neutrinos could help scientists develop theories that go beyond the standard model of particle physics and test explanations for how the Universe evolved.

Studying the impact of cosmic relic neutrinos on large-scale structure formation is one way to determine their mass. This way includes creating simulations and comparing the results with observations. But these simulations must be exact.

Existing simulations of cosmic structure formation do not accurately reproduce the properties of neutrinos. A research team from Japan has devised an approach that solves this problem.

Scientists at the University of Tsukuba, Kyoto University, and the University of Tokyo have created simulations that precisely follow the dynamics of such cosmic relic neutrinos.

Dr. Naoki Yoshida, Principal Investigator at the Kavli Institute for the Physics and Mathematics of the Universe, the University of Tokyo, said, “Standard simulations use techniques known as particle-based N-body methods, which have two main drawbacks when it comes to massive neutrinos. First, the simulation results are susceptible to random fluctuations called shot noise. And second, these particle-based methods cannot accurately reproduce collisionless damping—a key process in which fast-moving neutrinos suppress the growth of structure in the Universe.” 

Scientists avoided such issues by following the dynamics of the massive neutrinos by directly solving a central equation in plasma physics known as the Vlasov equation. Unlike previous studies, they solved this equation in full six-dimensional phase space, which means that all six dimensions associated with space and velocity were considered. 

The team coupled this Vlasov simulation with a particle-based N-body simulation of cold dark matter—the main component of matter in the Universe. They performed their hybrid simulations on the supercomputer Fugaku at the RIKEN Center for Computational Science.

The lead author of the study, Professor Koji Yoshikawa, said“Our largest simulation self-consistently combines the Vlasov simulation on 400 trillion grids with 330 billion-body calculations, and it accurately reproduces the complex dynamics of cosmic neutrinos. Moreover, the time-to-solution for our simulation is substantially shorter than that for the largest N-body simulations, and the performance scales extremely well with up to 147,456 nodes (7 million CPU cores) on Fugaku.”

This study could help scientists study phenomena involving electrostatic and magnetized plasma and self-gravitating systems.

Journal Reference:

  1. Kohji Yoshikawa et al. A 400 trillion-grid Vlasov simulation on Fugaku supercomputer: large-scale distribution of cosmic relic neutrinos in six-dimensional phase space. DOI: 10.1145/3458817.3487401
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