I have so far taken the early universe to be completely homogeneous. It’s time now to deal with the slight inhomogeneity, which causes the CMB anisotropy displayed in Figure 14.1 and leads eventually to the formation of galaxies. The quantities which define the slight departure from homogeneity are called perturbations. I will call the idealised, homogeneous Universe that we have been studying up till now, the unperturbed Universe.