Free Astronomy Magazine July-August 2014

COSMOLOGY completed. The 12 billion ini- tial virtual elements were grad- ually modelled into 41,416 galaxies, which faithfully in- corporate the morphological, physical and chemical proper- ties of all types of observable galaxies, with the only discrep- ancy being the timing of the formation of the first low-mass galaxies, which appear too early compared to what sug- gested by telescopic observa- tions. Going briefly into detail, the real universe and the universe generated by Illustris have among other things, for the equal-volume case, the follow- ing common characteristics: the current ratio between the quantity of stars with respect to the dark matter for any type of galaxies; the star for- mation rate as a function of time; the stellar mass and gal- axies luminosity functions; the amount of intergalactic gas; the abundance of hydrogen, helium and metals in galaxies; the number of satellite gal- axies in clusters as a function of distance from the centre of gravity of the clusters themselves. Last but not least, the almost perfect proportion between the number of elliptical galaxies and the num- ber of spiral galaxies, sore subject of all pre- vious simulations. The dimensions and, above all, the resolution of the universe created by Illustris are such that the team of researchers who worked on it was only able to explore a small part of it and many aspects of that virtual world still remain to be discovered. In thinking of the many further progresses that in the future will inevitably charac- terize the various aspects of simulations such as Illustris, one wonders if there will come a day when astrophysicists will find more useful, and quick, studying a com- plete virtual version of the universe rather than the real universe. E volution back- wards in time (from z=0 to z=4) of a portion of the universe, re- produced by Illus- tris on the basis of the dark matter density and some properties of the intergalactic gas. It can be clearly seen how the structures of the young virtual uni- verse appear less organized, colder and less metallic. [Illustris Project] internal objects and phenomena of indi- vidual galaxies have on the same galaxies hosting them. Illustris retraces the entire cosmic evolution, from 12 million years after the Big Bang to the present day. The simulation thus begins in a time when in the universe there were not yet any large- scale organized structures and progresses to its more familiar present form. To cover such a long period of time (more than 13 billion years), with the extraordinary maxi- mum resolution of just 156 light-years, less than the distance separating the Sun from Spica, were deployed supercomputers of the U.S., Germany and France, which, al- though using 8192 CPUs running in parallel, they had to work continuously for 3 months before producing their result. Calculations of this type done on any personal computer would have taken thousands of years to be n