From Big Bang to present: Supercomputer builds largest simulation of the universe

Chinese scientists and international collaborators have used one of the largest cosmological simulations ever created to "fast-forward" the universe from the Big Bang to the present day inside a supercomputer.

The project, called HyperMillennium and led by the National Astronomical Observatories of the Chinese Academy of Sciences, simulates a cubic volume of space 12 billion light years on each side. That is roughly equivalent to lining up about 120,000 Milky Way galaxies end to end.

Inside this digital universe, 4.2 trillion virtual dark matter particles evolve under gravity, reproducing 13.8 billion years of cosmic history. Dark matter refers to an invisible form of matter that does not emit or absorb light but is believed to make up about 85 percent of all matter in the universe and to provide the gravitational "framework" on which galaxies form.

"Due to the vast scale of the universe and the long course of its evolution, the distant galaxies observed through telescopes are actually static snapshots dating billions or even tens of billions of years ago, making it impossible to track their evolution in real time," said Gao Liang, a professor at the National Astronomical Observatories who leads the project.

Gao said ordinary matter that emits light represents only a "visible surface" of the cosmos, while dark matter forms its "invisible skeleton." Because dark matter does not interact with electromagnetic radiation, it cannot be directly observed with telescopes.

"Cosmological simulations are the key to solving this puzzle. By calculating the gravitational interactions among vast numbers of virtual dark matter particles, they can reconstruct — with high precision — the evolution of the universe from its early days to the present, allowing us to directly 'see' how dark matter particles cluster, distribute, and form structures over time," he said. "By comparing simulated virtual universes with real telescope observations, scientists can test and refine theoretical models of dark matter and dark energy."

Wang Qiao, a researcher at the observatory, said next-generation sky surveys are increasing demand for such simulations. These include the European Space Agency's Euclid mission and the China Space Station Telescope.

He said simulations must balance two competing requirements: a large enough volume to match the scope of sky surveys and extremely fine resolution to capture the formation of small galaxies.

"The emergence of ultra-large-scale cosmological simulations like HyperMillennium aims to bridge the gap driven by observational progress, providing a crucial link in humanity's quest to understand the universe's ultimate mysteries," Wang said.

Globally, he noted, there are now three major simulations tracking trillions of dark matter particles: Japan's Uchuu simulation, Europe's Flagship 2, and China's HyperMillennium. Flagship 2 emphasizes large cosmic volume, while Uchuu focuses on high resolution. Wang said HyperMillennium combines both strengths.

Wang added that HyperMillennium runs on China's ORISE supercomputer, using software optimized for that system.

The simulation of 4.2 trillion particles produced about 13 petabytes of data — roughly equivalent to 13 million high-definition movies. A petabyte is 1,000 trillion bytes of data. Processing this dataset required 16,000 accelerator computing cards working for 18 days, underscoring advances in high-performance computing.

The first academic paper from the project was recently published in the Monthly Notices of the Royal Astronomical Society. It focused on Abell 2744, a massive galaxy cluster known for its complex structure and extreme gravitational interactions. The study found that the simulation closely matches real observations, supporting its accuracy in modeling rare and complex cosmic systems. The first batch of data will be made available worldwide later.

Mike Boylan-Kolchin, a professor at the University of Texas in the United States, described the simulation as "a computational marvel that will help unlock fundamental physics from observations of the cosmos."

"It has an unprecedented range of volume and mass resolution, enabling detailed predictions about how huge numbers of relatively common galaxies are distributed across the cosmic web and the properties of inherently rare and interesting objects that are inaccessible with smaller volumes. The HyperMillennium Simulation will be a touchstone for the galaxy formation and cosmology communities for years to come," he said.

Volker Springel, director of the Max Planck Institute for Astrophysics in Germany, said he was struck by the simulation's scale and precision, adding that it "redefines what is nowadays possible in numerical cosmology."