2.3 Λ CDM 18 nation . At this point, the photons fell out of thermal equilibrium with the electrons (the latter are now bound inside neutral atoms) and thus became decoupled, which allowed them to travel freely, following a geodesic. There- fore, there should exist a last scattering surface , corresponding to the time of recombination. (In reality, recombination did not occur instantaneously and so the last scattering surface actually has a thickness to it.) The CMB radiation was discovered in 1964 by Arno Penzias and Robert Wilson, a discovery which earned them the 1978 Nobel Prize for Physics. When combined with other data, the data obtained from the Planck space- craft [40] yields values of the corresponding blackbody temperature and red- shift of the last scattering surface of The redshift of the last scattering surface corresponds to a time of ≈ 378000 years after the birth of the Universe. A diagram depicting the main topics of our discussion so far in the history of the Universe is shown in Fig. 2.1. All of what has been talked about so far forms part of the current standard model of Cosmology. However, to complete the model, we need to consider the effects of reionization. Reionization amounts to the partial ionization 2.3 Λ CDM T$_{0}$ = 2 . 718 ± 0 . 021 K (2.41) z$_{ls}$ = 1089 . 90 ± 0 . 23 (2.42)