Large-Scale Structure in the Universe

The emergence of large-scale structure from the small density fluctuations in the early Universe (as evident from CMB observations) is one of the greatest challenges in contemporary cosmology. In our current understanding of the matter content of the Universe, ~85% of the matter in the Universe exists in the form of dark matter, a weakly interacting particle that remains unidentified. Initial fluctuations in this matter, amplified by gravity during the expansion of the Universe, eventually collapsed to form the galaxies seen in the Universe today. Modeling this process is done by representing the matter as a collisionless N-body system evolving under self-gravity.

The largest simulation of this kind was recently carried out by physicists at the Max Planck Institute for Astrophysics, whose simulation included 10 billion particles (an order of magnitude greater than any previous simulation) in a periodic box simulation volume of 500 Mpc/h to a side, resulting in a particle mass of 860 million/h solar masses. Thereby, dwarf galaxies were represented by ~100 particles, galaxies like the Milky Way by ~1000 particles, and rich galaxy clusters by several million particles. The results of this simulation, named the Millennium Simulation, are illustrated in the above images. The simulations show a rich population of halos, connected to each other by dark matter filaments. This structure has been named the "Cosmic Web," as seen in the top image above. In the bottom image, one of the rich galaxy clusters created in the simulation can be seen.

Some absolutely unbelievable animations of these simulations are shown here. Note that many of these visualizations are .avi files, so you may need to view them with e.g. VLC Media Player.