Introduction to Cosmology

The universe can be approximated as being homogeneous and isotropic only if we smooth it with a filter across. On smaller scales, the universe contains density fluctuations ranging from subatomic quantum fluctuations up to the large superclusters and voids, across, which characterize the distribution of galaxies in space. If we relax the strict assumption of homogeneity and isotropy that underlies the Robertson–Walker metric and the Friedmann equation, we can ask (and, to some extent, answer) the question, “How do density fluctuations in the universe evolve with time?”

The formation of relatively small objects, such as planets, stars, or even galaxies, involves fairly complicated physics. Because of the greater complexity when baryons are involved in structure formation, we will postpone the discussion of galaxies and stars until the next chapter. In this chapter, we will focus on the formation of structures larger than galaxies – clusters, superclusters, and voids. Cosmologists use the term large scale structure of the universe to refer to all structures bigger than individual galaxies. A map of the large scale structure of the universe, as traced by the positions of galaxies, can be made by measuring the redshifts of a sample of galaxies and using the Hubble relation, to estimate their distances. For instance, Figure 11.1 shows two slices through the universe based on data from the 2dF Galaxy Redshift Survey (2dFGRS). From our location near the midplane of our galaxy's dusty disk, we get our best view of distant galaxies when we look perpendicular to the disk, toward what are conventionally called the “north galactic pole” (in the constellation Coma Berenices) and the antipodal “south galactic pole” (in the constellation Sculptor). The 2dFGRS selected two long narrow stripes on the sky, one near the north galactic pole and the other near the south galactic pole. In each stripe, the redshift was measured for galaxies. By plotting the redshift of each galaxy as a function of its angular position along the stripe, a pair of two-dimensional slices through the universe were mapped out.