Numerical simulations
As can be deduced from many of the preceding paragraphs, numerical cosmological simulations have become key in the study of the large scale structure of the Universe, whether it be for theoretical modeling, interpretation of observations or the planning of future projects. In an astronomical context two different categories of simulations can be identified, N-body and hydrodynamic. N-body simulations uniquely describe the gravitational growth of structures in an expanding Universe, starting from the initial conditions. Such simulations essentially describe the behaviour of dark matter, which dominates the ordinary matter, but don't take into account the dissipative effects within the so-called baryonic material. To do this hydrodynamic simulations must be used, that along with the gravitational evolution, try to reproduce, within certain approximations, the behaviour of the gas (heating and cooling) and the triggering of star formation. Each of these general categories can be further sub-divided between large-scale simulations of cosmological volumes of several hundreds of Mpc and those that focus on the high resolution study of single objects (like galaxies and clusters of galaxies).
Star formation, supernova explosions, the release of energy from accretion onto super-massive black holes, magnetic fields, and the injection of cosmic rays, are examples of phenomena, included in today's most advanced simulations, that are absolutely fundamental in the determination of the evolution of galaxies and their interaction with the diffuse gas of the intergalactic medium (IGM). Even though the effects of these phenomena manifest themselves on relatively large scales and are well resolved in the simulations, they originate on scales of a few light years, that in cosmological simulations are absolutely inaccessible, due simply to computational limitations. For this reason, these physical phenomena are usually modeled via analytical recipes that allow the effects on the large scales, dealt with by the models, to be estimated. In this context, the semi-analytical models (SAM) of galaxy formation represent a completely complementary approach to direct numerical simulations. Finally, the physical description of the intergalactic medium through the phases of re-ionisation and successive evolution, requires the detailed interaction between gas and the electromagnetic radiation emitted by astrophysical sources to be taken into account. This field is still to be considered rather young, and so prone to significant developments in the near future. Also in this field, the Italian community finds itself among the world's best, even though penalised with respect to other nations by the limited number of available super-computing facilities.
