2.3 Λ CDM 20 reionization. When combined with other data, the data obtained from the Planck spacecraft [40] yields values for the practically constant optical depth and the redshift at which reionization occurred (assuming it was sudden) of τ = 0 . 066 ± 0 . 012 (2.44) z$_{reion}$ = 8 . 8 +1 . 2 − 1 . 1 (2.45) The final piece of the standard model concerns the small perturbations in the cosmic fluid density that existed in the early Universe. On large scales, the Universe is homogeneous and isotropic. However, on smaller scales, it is clear that this is violated, as the Universe contains planets, galaxies, empty space etc. Therefore, there must have existed some small differences in the density of the cosmic fluid at very early times, which then grew under the influence of gravity and the Hubble flow. These tiny perturbations present themselves in the CMB, as anisotropies in the average temperature of the microwave radiation. The Planck spacecraft made precise all-sky measure- ments of the CMB, which is displayed in Fig. 2.2. A crucial concept in early Universe Cosmology is that of the Primordial Curvature Perturbation , labeled as ζ . This will be discussed more fully in Section 3.2.1. However, we will briefly discuss two aspects of it here. The spectrum , P $_{ζ}$( k ), of the curvature perturbation conveys how much power is in the perturbation as a function of scale k . We also have the spectral index , n$_{s}$ , which tells us how the spectrum varies with scale k . (The sub- script s denotes that this is for a scalar perturbation.) The spectral index is