Large Scale Structure of the Universe Introduction

The changes of main paradigms on the structure and evolution of the Universe are reviewed. Two puzzles of the modern cosmology, the mean density of matter and the regularity of the Universe on large scales, as well as the possibility to solve these puzzles by the introduction of more complicated form of inflation, are discussed. 1 Changes of paradigms in cosmology Until mid-70's it was generally believed that galaxies form clusters and groups, and the remaining field galaxies are more-or-less randomly spaced in the Universe. In late-70's and early 80's this simple picture was radically changed. It was found that field galaxies form elongated chains or filaments, clusters also are often located along chains; they form together filamentary superclusters of galaxies 1. The space between such filaments is devoid of any visible galaxies. This new picture was reviewed by Zeldovich, Einasto and Shandarin 2 and Oort 3. The distribution of galaxies and clusters was confronted with theoretical predictions by Zeldovich et al. It was found that this distribution has some similarity with the distribution of particles found in the adiabatic theory of structure formation developed by Zeldovich and collaborators. According to this scenario the structure evolution is determined by the dominating dark matter population of the Universe. If this population is due to massive neutrinos as it was expected in early 80's, then perturbations on small wavelength are damped, and the large-scale structural units, such as superclusters, will form first. Matter flows from low-density regions which have positive gravitational potential, to high-density regions forming gravi-tation wells, and builds up pancake-like superclusters. In low-density environment the contraction of matter to form galaxies is impossible, and the matter remains in some pre-galactic form. Superclusters and voids form a continuous network of alternating high-and low-density regions; the mean diameter of voids between rich clusters of galaxies is about 100 h −1 Mpc 2. Zeldovich et al. noticed also some problems with the neutrino dominated Universe: in such picture only very rich superclusters form and there are no systems of galaxies of intermediate richness; and, as a result, voids should be completely empty. The observed structure is more complicated: there exist intermediate sized systems of galaxies that form rarified filaments between superclusters. This failure of the neutrino-dominated Universe seems to be fatal, and it is overcomed by a new candidate for the dark matter introduced by Peebles 4. It is called cold since in …