In 1917, just a year after the theory of general relativity was published, Einstein suggested that the theory might be applied on a cosmic scale to describe the space and time of the Universe as a whole. According to general relativity, the overall distribution and movement of the contents of the Universe (i.e. the distribution of energy and momentum, and the flow of momentum from place to place) should determine the large scale properties of space and time.

In particular, it should determine whether space is infinite or not, and whether the Universe, which is currently expanding, will go on expanding forever, or whether it will eventually recollapse. The idea of applying general relativity on such a large scale is a bold one, but it has been very fruitful. In particular it has spawned the subject of relativistic cosmology, which is largely concerned with the study of mathematical models of the Universe based on simple assumptions about the contents of the Universe and their behaviour.