How does cosmic background radiation show that the universe is changing?
I may be wrong but I think it tells us that the universe is "cooling down"...
From what I know about cosmic background radiation it should have a frequency into the microwave range and it was originated at the moment of Big Bang when its frequency was incredibly high; we can imagine this radiation being emitted in the incredibly "hot" instant of the Big Band and having a very high frequency. After the passing of time the frequency of this radiation would have "cooled down" decreasing up to our present period reaching the comparatively low frequency in the microwave region.
I would say that this reduction in frequency should tell us that our universe is cooling down.