The entire scientific Astronomical and Cosmological community assumes that it's the Doppler effect, since that's the only existing scientific explanation for the shifts in wavelengths away from what they're supposed to be. The whole theoretical structure of the "Big Bang" and the expanding universe is built on that understanding.
If anybody ever comes along and discovers another way for electromagnetic waves to get longer or shorter on their way to us through space, then the whole big bang and expanding universe may be out the window.
