New supercomputer simulation to test model behind Universe’s formation

An international team of astrophysicists has simulated galaxy formation and large-scale cosmic structure with unprecedented statistical detail to investigate how the Universe formed. 

The team, including Dr Sownak Bose, Professor Carlos Frenk and other researchers at Durham University, say their MillenniumTNG supercomputer simulations will allow scientists to carry out precision tests of the standard cosmological model. 

Known as Lamba-CDM, the standard cosmological model is used by physicists to explain the formation of the Universe following the Big Bang. 

The researchers say their simulations are essential for interpreting existing and new observational studies – such as the surveys being carried out by the James Webb Space Telescope and the recently launched Euclid satellite – allowing scientists to investigate the nature of dark energy and dark matter by comparing the actual Universe to virtual universes created in a supercomputer. 

Dark energy is thought to be behind the accelerating expansion of the Universe, while dark matter is the structural backbone — not visible through telescopes — upon which galaxies eventually form. 

Both make up the majority of the Universe’s total content (with the remaining five per cent being stars, planets and galaxies) but scientists do not know what they are made of. 

The first results of the MillenniumTNG project will be published in a series of ten articles in the journal Monthly Notices of the Royal Astronomical Society. 

Dr Sownak Bose, Assistant Professor (Research), in Durham University’s Institute for Computational Cosmology, said: “The nexus between high precision observational data and ambitious, state-of-the-art cosmological simulations like MillenniumTNG is critical in advancing our understanding of how galaxies form and evolve over cosmic history.  

“This is an important step in the pathway to realising cosmologists’ ultimate ambition: to use the observed galaxy population to test the standard model of cosmology, and decode the mysterious entities of dark matter and dark energy.”  

MillenniumTNG is led by researchers at the Max Planck Institute for Astrophysics, Germany, Harvard University, USA and Durham University. It also includes researchers at York University, Canada, and the Donostia International Physics Center, Spain. 

Building upon previous successes with the Millennium and IllustrisTNG projects, they developed a new suite of simulation models – named MillenniumTNG (Millennium, the next generation)– which trace the physics of cosmic structure formation with considerably higher statistical accuracy than previously possible. 

They employed the code AREPO to follow the processes of galaxy formation directly, throughout volumes still so large that they could be considered representative of the Universe as a whole. 

Comparing simulations with and without galaxies gives a precise assessment of the impact of ”normal” matter related to supernova explosions and supermassive black holes on the total matter distribution.  

This is important when interpreting upcoming observations correctly, such as so-called weak gravitational lensing effects – where light is warped by the mass of another object – which respond to matter whether it is dark or normal.  

The researchers used two extremely powerful supercomputers, the SuperMUC-NG machine at the Leibniz Supercomputing Centre in Garching, Germany, and the Cosma 8 machine hosted by Durham University on behalf of the UK’s DiRAC High-Performance Computing facility.  

MillenniumTNG is tracking the formation of about one hundred million galaxies in a region of the Universe around 2,400 million light-years across. This calculation is about 15 times bigger than the previous best in this category, the TNG300 model of the IllustrisTNG project.  

Using COSMA 8, the team also computed an even bigger volume of the Universe — covering a region nearly 10 billion light-years across — filled with more than a trillion particles to represent dark matter and more than 10 billion particles to track massive neutrinos. To tackle this enormous computational challenge, the team used the advanced cosmological code, GADGET-4, which was custom-built for this purpose. 

Neutrinos are subatomic particles that rarely interact with normal matter and previous simulations had usually omitted them for simplicity, because they make up at most one to two per cent of dark matter’s mass and do not clump together. 

However, cosmological surveys such as Euclid and the Dark Energy Spectroscopic Instrument (DESI) survey, in both of which Durham plays a key role, will be precise enough to detect the percent-level effects. This raises the prospect of measuring the neutrino mass itself, a profound open question in particle physics. 

Professor Volker Springel, of the Max Planck Institute for Astrophysics, said: “MillenniumTNG combines recent advances in simulating galaxy formation with the field of cosmic large-scale structure, allowing an improved theoretical modelling of the connection of galaxies to the dark matter backbone of the Universe.  

“This may well prove instrumental for progress on key questions in cosmology, such as how the mass of neutrinos can be best constrained with large-scale structure data.” 

Dr Bose’s role in the research was funded through a UK Research and Innovation Future Leaders Fellowship grant. Professor Frenk is the recipient of a European Research Council Advanced Investigator Grant. COSMA/DiRAC is funded by the UK’s Science and Technology Facilities Council. 

Figure 1: Projections of gas (top left), dark matter (top right), and stellar light (bottom centre) for a slice in the largest hydrodynamical simulation of MillenniumTNG at the present period of time. The slice is about 35 million light-years thick. The projections show the vast physical scales in the simulation from size, about 2,400 million light-years across, to an individual spiral galaxy (final round inset) with a radius of approximately 150,000 light-years. The underlying calculation is presently the largest high-resolution hydrodynamical simulation of galaxy formation, containing more than 160billion resolution elements. Credit: MPA  

Figure 1(a) – 100mpc.jpg: Top section of Figure 1 minus annotation. Image shows projections of gas (top left), dark matter (top right), and stellar light (bottom centre) from the MillenniumTNG simulation at 100 megaparsecs. Credit: MPA. 

Figure 1 (b) – 10mpc.jpg: Bottom left section of Figure 1 minus annotation. Image shows projections of gas (top left), dark matter (top right), and stellar light (bottom centre) from the MillenniumTNG simulation at ten megaparsecs. Credit: MPA. 

Figure 1 (c) – 1mpc.jpg: Bottom right section of Figure 1 minus annotation. Image shows projections of gas (top left), dark matter (top right), and stellar light (bottom centre) from the MillenniumTNG simulation at one megaparsec. Credit: MPA. 

Figure 2: Comparison of the neutrino (top) and dark matter (bottom) distributions on the past backwards lightcone of a fiducial observer positioned at the centre of the two horizontal stripes. As cosmic expansion slows down the neutrinos at late times (small redshift/distance), they start to weakly cluster around the biggest concentrations of dark matter as shown by a comparison of the zoomed insets. This slightly increases the mass and further growth rate of these largest structures. Credit: MPA 

Figure 3: Galaxy distribution on the past backwards lightcone in MillenniumTNG, where the galaxies are predicted with a sophisticated semi-analytic model on top of the dark matter backbone. Credit: MPA

Full press release available here.