Formation and Evolution of Dark Matter Halos (III. Formation of Cosmic Web and Void Structures)

In the early Universe, the dark matter distribution is nearly uniformly, but small density fluctuations exist. The density fluctuations rapidly grow throughout the Universe via gravitational instability and collapse into gravitationally bound structures, called dark matter halos. Halos merge with each other to form larger halos, in which gas concentrates to form galaxies. Then groups of galaxies known as clusters form. Clusters are connected by galaxies distributed in web-like patterns. This web pattern is also called filaments. The regions without galaxies surrounded by filaments are called voids. Filament and void structures constitute the large-scale structure of the Universe.

This movie visualizes the evolution of dark matter distribution in one of a series of simulations, named Uchuu. In this simulation, we discretized the initial dark matter density fluctuations with almost 2.1 trillion particles and followed their gravitational evolution to the present Universe. By semi-analytically and empirically modeling of the evolution of ingredients for stars and galaxies (baryons) in the dark matter distribution, this simulation enables us to predict the distribution, evolution, and statistical properties of galaxies and active galactic nuclei over an unprecedentedly vast spatial volume. Comparison with future wide-field observations by the Subaru Telescope and other telescopes is expected to reveal the formation and evolution of our Universe with unprecedented resolution.

In the early Universe, the dark matter distribution is nearly uniformly, but small density fluctuations exist. The density fluctuations rapidly grow throughout the Universe via gravitational instability and collapse into gravitationally bound structures, called dark matter halos. In this simulation, the initial dark matter distribution is based on the observations of the cosmic microwave background by the Planck mission.
Many small halos are formed. Halos repeatedly merge with each other and grow. In this simulation, we considered the gravitational evolution of dark matter only. In the actual Universe, the gravity of the halos also collects the ingredients for stars such as hydrogen and helium, and then primordial stars and galaxies are born. Such halos may correspond to galaxy candidates in the early Universe, as reported by observations with the Subaru Telescope and the James Webb Space Telescope.
Halos become more and more massive through repeated mergers. At about one billion years after the birth of the Universe, a number of Milky Way sized halos and groups of halos are born. These halos are thought to correspond to galaxy proto-clusters, the progenitors of galaxy clusters in the current Universe.
Snapshot of the current Universe. A number of galaxies, cluster-sized halos, and large-scale structures have formed. The massive halo in the center is the largest one formed in the simulation. Let's stop time here and walk through the structure of this halo.
The mass of this halo is about 1,000 trillion times the mass of the Sun, corresponding to the mass of a galaxy cluster. Smaller clumps of dark matter orbiting around the halo are called subhalos and are thought to host the member galaxies of the galaxy cluster. The subhalos close to the center of the halo move faster due to the gravitational force. Next, let's walk through the large-scale structure and see the details.
We can see web patterns connecting the halo populations as well as regions without galaxies, called voids, interspersed in the web. These structures constitute the large-scale structure of the Universe. In this movie, the webs are emphasized in the visualization process, making the webs and voids more prominent than in previous movies.
Looking at another massive halo, the shape, number of subhalos and their distribution are different from the one seen earlier. Thus, the properties of each halo are unique and reflect their evolutional track.

Details of Numerical Simulation

PurposeThe formation and evolution of dark matter halos, galaxies, and active galactic nuclei in the standard cosmological model.
ModelLCDM model
Number of Particles12,8003 (2.1 trillion)
ComputerCray XC50 "ATERUI II" (CfCA, NAOJ), 40,000 cores
Time Scale13.8 billion years
Spatial ScaleAbout 3 Gpc (9.6 billion light-years)
ResercherTomoaki Ishiyama (Chiba University)
References"The Uchuu simulations: Data Release 1 and dark matter halo concentrations", Ishiyama, T. et al., 2021, Monthly Notices of the Royal Astronomical Society, 506, 4210-4231

Details of Visualization

Since this simulation is very large scale, using more than 2 trillion particles, it was difficult to visualize. Therefore, the number of particles was thinned out to about 6 billion. Dark matter is naturally invisible, but in this visualization, it is colored based on its velocity. Cool colors (blue, purple, etc.) are slower, and warm colors (yellow, orange, etc.) are faster. Because this video was made as an all-round stereoscopic image, you can view it with a head mounted display (HMD). And there is another version (short, 4K), so you can view more details by using a 4K monitor.


  • Simulation: Tomoaki Ishiyama
  • Visualization: Hirotaka Nakayama
  • Four-Dimensional Digital Universe Project, NAOJ

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