The Illustris cosmological simulation projection. Towards a predictive theory of galaxy formation and evolution with hydrodynamical simulations of large cosmological volumes on a moving mesh.
The standard model of cosmology posits that the mass-energy density of the Universe is dominated by unknown forms of dark matter and dark energy. Testing this extraordinary scenario requires precise predictions for the formation of structure in the visible matter, which is directly observable as stars, diffuse gas, and accreting black holes. These components of the visible matter are organized in a 'Cosmic Web' of sheets, filaments, and voids, inside which the basic units of cosmic structure - galaxies - are embedded. To test our current ideas on the formation and evolution of galaxies, we strive to create simulated galaxies as detailed and realistic as possible, and compare them to galaxies observed in the real universe. By probing our successes and failures, we can further enhance our understanding of the galaxy formation process, and thereby perhaps realize something fundamental about the world in which we live.
The Illustris project is a set of large-scale cosmological simulations, including the most ambitious simulation of galaxy formation yet performed. The calculation tracks the expansion of the universe, the gravitational pull of matter onto itself, the motion or "hydrodynamics" of cosmic gas, as well as the formation of stars and black holes. These physical components and processes are all modeled starting from initial conditions resembling the very young universe 300,000 years after the Big Bang and until the present day, spanning over 13.8 billion years of cosmic evolution. The simulated volume contains tens of thousands of galaxies captured in high-detail, covering a wide range of masses, rates of star formation, shapes, sizes, and with properties that agree well with the galaxy population observed in the real universe. We are currently working to make detailed comparisons of our simulation box to these observed galaxy populations, and some exciting promising results have already been published.
The Lambda Cold Dark Matter (Lambda-CDM) paradigm of cosmology, currently favored by observations of the large-scale distribution of galaxies in space, implies that the cosmos is filled with three distinct components: normal matter (which astronomers term 'baryons'), dark matter, and dark energy. The mathematical models which govern the physical behavior of these components are sufficiently complex that they can only be solved exactly for very particular, simplified "test" problems. Understanding how the nearly uniform, primordial universe evolved into the many diverse phenomena we observe in the night sky today therefore requires the use of computer simulations, which can numerically evolve a representation of some fraction of the universe forward in time.
Simulations of the combined evolution of dark matter and dark energy, which only include the force of gravity, have been run to great success over the past few decades. Recently, such simulations have reached staggering scale, including on the order of 1 trillion particles, each of which exerts a gravitational force on every other. However, such "DM-only" simulations cannot predict the distribution of galaxies made up of normal matter, severely limiting their utility as a means to directly connect with the observations. Our approach for establishing this link is through directly accounting for the baryonic component (gas, stars, supermassive black holes, etc.) in cosmological simulations that calculate fluid motion ("hydrodynamics") as well as gravity, in principle offering a self-consistent and fully predictive methodology. For example, the image below shows two galaxies (out of thousands of similar systems), one on each row, evolving in time from left to right, from when the universe was a quarter its current age, to the present. The top galaxy shows the massive, red, elliptical-shaped galaxy forming after a series of mergers with other systems, whereas the bottom galaxy reveals the formation of a smaller, bluer, disk-shaped galaxy forming after a less violent history of interactions.