Teach Astronomy - Cosmological Principle

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A very basic assumption about the universe which forms the basis for modern cosmology is called the cosmological principle, that the universe is isotropic and homogeneous. Isotropic means the same in all directions. This means that in any direction we look we tend to see the same structures and numbers of galaxies, and that is in fact confirmed by observation. It also means that the Hubble expansion is the same in every direction we look, that the expansion rate is smooth and not faster in one direction of the sky than in another direction, and this is also confirmed by observations. The second part of the cosmological principle, the homogeneity of space, is much more difficult to test because as we look out in space we look back in time. So when we view distant parts of the universe we are viewing parts of the universe as they were earlier when the universe was smaller, but we can basically test the idea by showing that the universe contains more or less the same structures everywhere we look and that on the largest scales, over a hundred megaparsecs or three hundred million light years, the average amount of material in any volume of space from one direction to the other is about the same. So the universe is indeed smooth on the very largest scales.