STRUCTURES AND CORRELATIONS IN THE LARGE SCALE UNIVERSE

One of the main assumptions to explain structure formation in an expanding universe is that a dominant mass component in the form of non-baryonic dark matter plays a central role. However, although dark matter is supposed to provide with more than 0.9 of the total fraction of the mass-energy in the universe, its amount and properties can only be defined a posteriori. In this context, a crucial point concerns the identification of a possible clear feature of dark matter fields which is expected to hold on very general theoretical conditions. This property, in standard cosmological models, is represented by super-homogeneity, i.e. a very fine tuned balance between negative and positive (weak) correlations of density fluctuations, which must be imprinted both in the anisotropies of the Cosmic Microwave Background Radiation and in the large scale distribution of galaxies. We discuss the fact that the complex clumpiness characterizing galaxy structures corresponds to power-law correlations, i.e. a regime where the main feature is represented by scale-invariance of galaxy clustering. The transition between such a regime and the super-homogeneous one has not been detected yet. This would represent the main observational verification of standard models of galaxy formation, linking the early universe physics, through the statistical properties of the initial density field, to the non-linear structures observed today in galaxy structures.