The large-scale smoothness of the Universe

The Universe is inhomogeneous-and essentially fractal-on the scale of galaxies and clusters of galaxies, but most cosmologists believe that on larger scales it becomes isotropic and homogeneous: this is the 'cosmological principle'. This principle was first adopted when observational cosmology was in its infancy, and was then little more than a conjecture. The data now available offer a quantitative picture of the gradual transition from small-scale fractal behaviour to large-scale homogeneity.