Some naive thoughts provide a first insight into Newtonian cosmology. Olbers' paradox, introduced in Chapter 6 (6.7), suggests that the Universe is finite, either in space or in time, or both. If the Universe were finite in space and infinite in time, the whole Universe would have collapsed gravitationally. Therefore, the Universe is finite in time. Are there other simple arguments with which to assess the fmiteness of space, that is, of matter content of the Universe? The cosmological principle may provide such an argument.

The cosmological principle is widely accepted as a reasonable, basically compatible with observations, philosophically attractive principle. From the Newtonian point of view its statement and interpretation does not present any difficulty: the point in space at which we are in the Universe is not a special point; all points in the Universe are similar, or, more precisely, all thermodynamic parameters have the same value at any point in the Universe: the Universe is homogeneous. The cosmological principle also ensures the isotropy of the Universe, that is, all directions are equivalent. Isotropy implies homogeneity, but homogeneity does not imply isotropy. For example, if the Universe were embedded in a constant magnetic field, it could be homogeneous but anisotropic. However, the Newtonian interpretation of the cosmological principle usually states that the Universe is both homogeneous and isotropic. This would only be true on a very large scale in the Universe, that is, larger than characteristic sizes of superclusters.