ABSTRACT
This chapter looks at the observations of the cosmic microwave background radiation (CMB) primordial anisotropy. This is a powerful tool to explore the characteristics of the primordial density perturbations, which are responsible for the formation of structure in the Universe, such as galaxies and galactic clusters. The primordial density perturbations are responsible for the formation of structure in the Universe. The principle contender to inflation for the formation of structures in the Universe used to be cosmic strings. That however, corresponded to a causal mechanism, which would not produce peaks in the CMB spectrum. When the acoustic peaks were observed the cosmic string paradigm collapsed, which demonstrates how closely cosmology is connected to the real world. Apart from the CMB temperature perturbations, more information is encoded in the polarisation of CMB light. We are in a position to define the primordial curvature perturbation and relate it with the density perturbations and the perturbation in the temperature of the CMB.
