6 The angular distribution of the 3 K radiation
6.1 Basic isotropy
As we have said, the photons in the 3 K background have been practically free from interaction with anything since about 4 × 105 years after the instant of the big bang. The present angular distribution of the microwave radiation – the way in which it is spread across the sky – is therefore almost the same as it was then. The spectrum we find today depends on the temperatures at that time – for the intensity of the radiation in a particular region of the early Universe depended only on the temperature. If the Universe at that time was inhomogeneous (in the sense of being hotter in some regions than others) the 3 K radiation observed today would be more intense in some directions than in others.
In fact, as we have indicated, the radiation is largely isotropic. There are, however, some small – some very small – but significant departures from perfect isotropy. But before these can be identified, we have to take into account the effect of the Earth's motion. This would in any event impose an apparent asymmetry on the radiation. Indeed, as we shall now show, this effect can be turned to our advantage and provide us with a means for measuring the speed of the Earth relative to the average distribution of matter and energy in the Universe.