With the discovery of the Cosmic Background Radiation by Penzias and Wilson (1965), cosmology became a branch of physics: there was a well defined framework within which to formulate models and confront them with observational data. Prior to that there had been a few important observations and a few important solutions to the Einstein Field Equations for General Relativity. We suspected that these were somehow connected: that the Friedman-Lemaitre solutions of the Einstein field equations described the cosmological redshift law discovered by Hubble.
With the discovery of the background radiation we were left in no doubt that the Universe had a hot singular origin a finite time in our past. That important discovery also showed that our Universe, in the large, was both homogeneous and isotropic, and it also showed the appropriateness of the Friedman-Lemaitre solutions.
The establishment of the "Big Bang" paradigm led to a search for answers, in terms of known physical laws, to key questions: why was the Universe so isotropic, how did the structure we observe originate? and so on. Cosmologists built models involving only known physics and confronted them with the data. Cosmology became a branch of physics with a slight difference: we cannot experiment with the subject of our discussion, the Universe, we can only observe it and model it.
With the current round of cosmic microwave background anisotropy maps we are able to see directly the initial conditions for galaxy formation and for the formation of large-scale structure. That observed structure is thought to reflect directly the fluctuations in the gravitational potential that gave birth to cosmic structure and it is a consequence of the physics of the early universe. The goal is to link those initial conditions with what we see today.
The aim of this article is to show how the "homogeneous and isotropic Universe with a hot singular origin" paradigm has emerged, and to explain how, within this framework, we can quantify and understand the growth of the large scale cosmic structure.