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After my talk at the Caucasus Winter School Zeldovich offered me collaboration in the study of the universe. He was developing a theory of formation of galaxies (the pancake theory, Zeldovich 1970). A hierarchical clustering theory was suggested by Peebles (1971). Zeldovich asked for our help in solving the question: can we find some observational evidence which can be used to discriminate between these theories?

In solving the problem we used our previous experience in the study of galactic populations: kinematical and structural properties of populations hold the memory of their previous evolution. Random velocities of galaxies are of the order of several hundred km/s, thus during the whole lifetime of the Universe galaxies have moved from their place of origin only by about 1 h-1 Mpc (the Hubble constant is used in units of H0 = 100 h km s-1 Mpc-1). In other words - if there exist some regularities in the distribution of galaxies, these regularities must reflect the conditions in the Universe during the formation of galaxies.

Thus we had a leading idea how to solve the problem of galaxy formation: We have to study the distribution of galaxies on larger scales. The three-dimensional distribution of galaxies, groups and clusters of galaxies can be visualised using wedge-diagrams, invented just when we started our study. We prepared relatively thin wedge diagrams, and plotted in the same diagram galaxies, as well as groups and clusters of galaxies. In these diagrams regularity was clearly seen: isolated galaxies and galaxy systems populated identical regions, and the space between these regions was empty. This picture was quite similar to the distribution of test particles in a numerical simulation of the evolution of the structure of the Universe.

We reported our results (Jõeveer & Einasto 1978) at the IAU symposium on Large-Scale Structure of the Universe in Tallinn 1977, the first conference on this topic. The main results were:

  1. galaxies, groups and clusters of galaxies are not randomly distributed but form chains, converging in superclusters;

  2. the space between galaxy chains contains almost no galaxies and forms holes (voids) of diameter up to approx 70 h-1 Mpc;

  3. the whole picture of the distribution of galaxies and clusters resembles cells of a honeycomb, rather close to the picture predicted by Zeldovich (1978).

However, some important differences between the Zeldovich pancake model and observations were detected. First of all, there exists a rarefied population of test particles in voids absent in real data. This was the first indication for the presence of biasing in galaxy formation - there is primordial gas and dark matter in voids, but due to low-density no galaxy formation takes place here (Jõeveer et al. 1978, Einasto et al. 1980). The second difference lies in the structure of galaxy systems in high-density regions: in the model large-scale structures (superclusters) have rather diffuse forms, real superclusters consist of multiple intertwined filaments (Zeldovich et al. 1982, Oort 1983, Bond et al. 1996).

The difficulties of the neutrino-dominated model became evident in early 1980s. A new scenario was suggested by Blumenthal et al. (1982) and others, where hypothetical particles like axions, gravitinos or photinos play the role of dark matter. Numerical simulations of structure evolution for neutrino and axion-gravitino-photino-dominated universe were made and analysed by Melott et al. (1983). All quantitative characteristics (the connectivity of the structure, the multiplicity of galaxy systems, the correlation function) of this new model fit the observational data well. This model was called the Cold Dark Matter (CDM) model, in contrast to the neutrino-based Hot Dark Matter model. Presently the CDM model with some modifications is the most accepted model of the structure evolution. The properties of the Cold Dark Matter model were analysed in detail by Blumenthal et al. (1984).

The modern cosmological paradigm includes Dark Energy as the basic component of the matter/energy content of the Universe. Direct observational evidence for the presence of Dark Energy comes from distant supernova observations (Perlmutter et al. 1999, Riess et al. 1998) and CMB observations. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite allowed the measurement of the CMB radiation and its power spectrum with a much higher precision (Spergel et al. 2003). The position of the first maximum of the power spectrum depends on the total matter/energy density. Observations confirm the theoretically favoured value 1 in units of the critical cosmological density. Combined CMB, supernova and large-scale distribution data yield for the density of baryonic matter, Omegab = 0.041, the dark matter density OmegaDM = Omegam - Omegab = 0.23, and the dark energy density OmegaLambda = 0.73. These parameters imply that the age of the Universe is 13.7 ± 0.2 gigayears.

Dark energy act as a repulsive force, thus the Universe is presently expanding with an increasing speed. Dark energy also has the effect of freezing the cosmic web. This explains the smoothness of the Hubble flow. The nature of dark matter particles and dark energy is still unknown.

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