This chapter describes the first steps toward an understanding of large structures, which are observed in the universe at all scales—galaxies, groups of galaxies, and galactic clusters. It does so by studying the evolution of perturbations at linear order in Friedmann–Lemaître spacetimes. To simplify the discussion, the chapter limits the scope to the textbook case where the spatial sections of the background space are Euclidean (K = 0), and anisotropic perturbations and entropy perturbations are absent. This basically means that the matter reduces to a single fluid. The relativistic and Newtonian theories of cosmological perturbations differ. Finally, the chapter discusses the limit in which they converge.
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