The characterisation of the large scale structure of the Universe is largely based on measurements of the redshifts of large samples of galaxies, that is, the reconstruction of the three-dimensional distribution of luminous objects via the relation between distance and motion towards the red of spectral lines (caused by the expansion of the Universe). This line of research has seen an exponential growth since the end of the '70s of the last century, until the Sloan Digital Sky Survey (SDSS), that measured the distances to a million galaxies to distances of up to a few billion light years, mapped the large scale structure within what is now considered the "local Universe", at redshift z<0.2. At the same time, these large samples of galaxies have allowed the properties of the galaxy population to be established with great precision (luminosities, colours, masses, morphologies) and to connect them to the distribution of dark matter, which instead dominates in the construction of the cosmic skeleton. In this scenario, galaxies form by cooling and contraction of baryonic material (that with which we are familiar) within potential wells (halos) created by cold dark matter. The theory is able to provide very robust and precise predictions as to the mean density and inhomogeneity (clustering) of the dark matter halos, both of which depend on fundamental cosmological parameters. To derive constraints on these parameters, the theory needs to be connected with the observations of baryonic matter, the only type visible to us thanks to its emission of radiation at various wavelengths, that we collect with our telescopes on the ground and in space.

Over the last ten years the Italian community has played an ever more important role in deep redshift surveys, that is, large spectroscopic samples extended to the most remote epochs of the Universe, to study not only the distribution and properties of the galaxies, but also, and above all, their evolution in time. The contribution in this field began at the start of 2000 with the construction by an Italo-French consortium of VIMOS, installed on ESO's Very Large Telescope (VLT) in Chile. VIMOS has completed two of the largest deep surveys currently in existence, VVDS and ZCOSMOS, that measured redshifts for tens of thousands of galaxies, and in which the contribution of the Italian community was dominant. These surveys led, two years ago, to the new Italian-driven survey called VIPERS (VIMOS Public Extragalactic Redshift Survey), that, again with VIMOS, will measure 100,000 redshifts at an epoch when the Universe was half its present age, exploring a volume similar to that already explored locally. VIPERS, will produce over the next three years unprecedented measures of the clustering of galaxies and the cosmological parameters that regulate the growth of structure, with a precision similar to that reached in the local Universe. The ability of Italian research groups to propose and manage a project of this kind has been made possible by two fundamental factors. On one hand, the gaining of a leadership role in the fields of observational cosmology and galaxy evolution over the last ten years. On the other, the development of the specific skills and infrastructure necessary to calibrate, analyse and archive these large quantities of data in an optimal way. It is the sum of these technical and scientific capabilities that have permitted the same Italian research groups, along with analogous European groups, to propose the cosmological satellite EUCLID to ESA, presently in the last phase of study before construction. EUCLID will simultaneously cover the entire extragalactic sky with high resolution images in the visible and infrared and infrared spectroscopy, measuring the redshifts of more than 50 million galaxies. The aim is to use measures of gravitational lensing and galaxy clustering to trace, respectively, the distribution of dark and luminous matter in space and time, to solve the mystery of the dark components in the Universe. There is no doubt that projects such as VIPERS and EUCLID place Italy at the forefront of this kind of study, that until ten or so years ago were dominated essentially by the Anglosaxon community (USA and UK).

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