Cosmology – the study of the evolution of the universe as a whole – is becoming an ever more exact science with the recent precision observations by BOEMERANG, MAXIMA and WMAP. Within a few percent uncertainty, we know that the universe is open, flat and contains only a few percent of baryonic matter. The universe is primarily filled with Cold Dark Matter (CDM) and dark energy (a cosmological constant). If this is not a coincidence, the cosmological constant is time-varying, and exchanges energy and momentum with CDM and, possibly, baryonic matter. The imprint of the earliest epoch of the universe that at present can be probed, is the Cosmic Microwave Background (CMB). The CMB is a relic of the last surface of scattering at time 379 kyr[45]. Its extreme homogeneity is well accounted for by a preceeding inflationary phase. A recent review of cosmometry is compiled by L. M. Kraus[308].

The early universe may well have produced a stochastic background in gravitational waves. If so, these relic waves could provide the earliest signature of the universe at an epoch much earlier than the CMB and the preceding phase which produced the initial light element abundances[360]. At present, this relic in gravitational waves is largely unknown, except that its spectrum should be smooth. It may or may not have a thermal component.