Quite apart from determining the ultimate fate of the Universe, relativistic cosmology also provides insight into the origin and evolution of the Universe. The most popular cosmological models all predict that our Universe has evolved over the past twelve or fourteen billion years from an initial state that was hotter and denser and expanding more rapidly than it is now.

The early stages of that expansion are described by the big bang theory, which implies that a hot dense universe, in which matter initially takes the form of a soup of elementary particles (protons, neutrons, electrons and the like) will eventually give rise to a matter-dominated Universe in which about twelve out of every thirteen atoms will be hydrogen, and most of the remainder will be helium - just the kind of Universe we find ourselves living in today. The big bang theory also predicts that since the formation of the first atoms, about 300 000 years after the start of the cosmic expansion, the remainder of the radiation that once dominated the Universe has been cooling and expanding, increasing its wavelength and thus becoming the cosmic microwave background radiation (CMBR) that we can observe coming very uniformly from all directions in space. This explanation of the CMBR is widely regarded as the most convincing evidence in favour of the big bang, and it is detailed investigations of the small-scale unevenness of the CMBR that is expected to settle many of the outstanding questions about space and time over the years ahead.