M ar 2 01 1 Inhomogeneities in the universe

Galaxy structures represent one of the most challenging observations in cosmology. Whether or not these structures are found to be compatible with the standard model of galaxy formation crucially depends on the a-priori and assumptions encoded in the statistical methods employed to characterize the data and on the a-posteriori hypotheses made to interpret the results. We present strategies to test the most common assumptions, i.e. spatial homogeneity and statistical homogeneity and isotropy. These tests provide evidences that, in the available samples, galaxy distribution is spatially inhomogeneous but statistically homogeneous and isotropic. Different conclusions are obtained through statistical tools based on a-priori assumptions that not verified in the data when directly tested. Thus we find that the observed galaxy structures are not compatible with the standard model of galaxy formation, e.g. LCDM, that predicts spatial homogeneity at small scales (i.e., r < 10 Mpc/h), structures of relatively limited size (i.e. r < 100 Mpc/h) and anti-correlations at large scales (i.e. r > 150 Mpc/h). While the observed inhomogeneities pose a fundamental challenge to the standard picture of cosmology they also represent an important opportunity which may open new directions for many cosmological puzzles. PACS numbers: 98.65.-r,98.65.Dx,98.80.-k Inhomogeneities in the universe 2