COBE also discovered the required inhomogeneities almost embarrassingly on the nose, ten parts per million. During the decade since, a number of ingenious experiments have confirmed this basic feature. A comprehensive experiment WMAP more or less fingerprinted the whole sky when it measured the CMB temperature in any direction of viewing to within a micro-Kelvin. Largest visible fluctuations are in milli-Kelvin.
These measurements provide the detailed spectrum of the inhomogeneities, which must have pre-existed the formation of neutral Hydrogen. Now armed with the knowledge of the fundamental forces we can extend our theoretical study of the Universe much farther back. But in nothing that we know is there a mechanism for producing these inhomogeneities exactly of this extent. A theory called Inflationary Universe proposes that these inhomogeneities originated from the quantum fluctuations of some fields populating the early Universe. This would require us to build a Grand Unified Theory to correctly predict the fluctuations.
An alternative is that the inhomogeneities arose with the birth of the Universe itself, governed by that unknown realm of Physics called Quantum Gravity. The only theory where Quantum Gravity seems amenable to any calculation is Superstring Theory. In this theory what appears to be a violent and singular event (the Big Bang) has alternative mathematical description where it would be tame and calculable.
A number of cosmology experiments, some operational, some in planning, will pin down the detailed structure of the fluctuations within the coming decade. They are aimed at allowing us to distinguish the origin of the fluctuations, whether from the Quantum Gravity era or a little later from the era of Grand Unification. They will then provide further insight into the nature of the unified forces.
