In their pursuit of understanding cosmic evolution, scientists rely on a two-pronged approach. Using advanced instruments, astronomical surveys attempt to look farther and farther into space (and back in time) to study the earliest periods of the Universe. At the same time, scientists create simulations that attempt to model how the Universe has evolved based on our understanding of physics. When the two match, astrophysicists and cosmologists know they are on the right track!

In recent years, increasingly-detailed simulations have been made using increasingly sophisticated supercomputers, which have yielded increasingly accurate results. Recently, an international team of researchers led by the University of Helsinki conducted the most accurate simulations to date. Known as SIBELIUS-DARK, these simulations accurately predicted the evolution of our corner of the cosmos from the Big Bang to the present day.

In addition to the University of Helsinki, the team was comprised of researchers from the Institute for Computational Cosmology (ICC) and the Centre for Extragalactic Astronomy at Durham University, the Lorentz Institute for Theoretical Physics at Leiden University, the Institut d’Astrophysique de Paris, and The Oskar Klein Centre at Stockholm University. The team’s results are published in the Monthly Notices of the Royal Astronomical Society.

Images of the SIBELIUS-DARK simulation. Credit: McAlpine et al. (2021)

This simulation is the first study conducted as part of the “Simulations Beyond the Local Universe” (SIBELIUS) project and was performed using the DiRAC COSmology MAchine (COSMA), a distributed computer network operated by the ICC. The simulation covers a volume of space up to a distance of 600 million light-years from Earth and is represented by over 130 billion simulated ‘particles’, which required thousands of computers several weeks to produce.

The team used known physics to describe how Dark Matter and cosmic gas evolved during the history of the Universe. Specifically, they sought to determine if what we observe today is consistent with the standard model of cosmology – the Cold Dark Matter (CDM) model. For the past few decades, astrophysicists have used this model to explain the properties of the Cosmic Microwave Background (CMB) to the number and spatial distribution of the galaxies we see today.

Previous CDM simulations have typically modeled random patches of the Universe that are similar to what we observe today. By using advanced generative algorithms, these simulations were conditioned to reproduce our specific patch of the Universe. This allowed the team to see if their simulation reproduced the present-day structures in the vicinity of the Milky Way that astronomers have observed for decades.

After meticulously comparing the virtual Universe they created to a series of observational surveys, they found that the simulation matched the locations and properties of structures like the Virgo, Coma, and Perseus galaxy clusters, the “Great Wall,” and the “Local Void.” Most importantly, at the center of the simulation were the two most important and familiar structures to astronomers: the virtual counterparts of the Milky Way and the neighboring Andromeda galaxy.

At the very center of the simulation is the Milky Way galaxy (MW) and our nearest massive neighbour, the Andromeda galaxy (M31). Credit Dr Stuart McAlpine

As co-author Professor Carlos Frenk (the Ogden Professor