How does Cosmic Microwave Background Radiation provide evidence for the Big Bang?
The reason why background radiation (whether light or microwave) is useful to demonstrate the expanding nature of the universe is because of it demonstrates that distant objects are moving, but their speed is not constant, but a function of the distance from earth. The change in frequency of the radiation (Doppler effect) received from those objects shows this type of movement, and the reason that supports the Big Bang theory is because by calculating the previous position of all those objects one can conclude that all of them were at some point in time located at the same point in space. Hopefully this helps. The theory is a bit more complicated than that but I hope this guides you towards an answer. A lot of this theory has been developed after the discoveries of Sir Edward Hubble in 1929.
