Chapter 6: Application: Cosmology

This is the second application we wish to discuss, and it is called cosmology or the study of the universe as a whole. You may ask: how can they study the whole universe? The answer is: very coarsely. One ignores most details of it, and concentrates on a few of them. In the simplest example, the only degree of freedom considered is the scale of the universe. This is actually not a bad approximation: if one looks at things at a large scale, the universe is pretty homogeneous and isotropic (it looks the same in all places and in all directions; see Figure 6.1). That is not the case on a small scale: obviously the Solar System is different close to the Sun than close to Pluto, for example. But if one goes at the scales of galaxies or, even further, clusters of galaxies, things look pretty uniform…