First thing is that it is not due to Doppler’s Effect. Now suppose we have no alternative explanation. So should we accept Doppler’s Effect as winner simply because there is no contestant?
Doppler’s effect is “redshift-speed” relationship. Cosmological Redshift is “redshift-distance” relationship.
It is possible that a car is receding from us at uniform speed. At near or far distance, the speed remains the same. No matter what is the distance, speed is same; therefore redshift shall also be the same.
Thus we see that Doppler’s Effect has no direct bearing with distance. Therefore in isolation, redshift-distance relationship is not the indication of Doppler’s Effect.
Given the assumption that we have no alternative explanation, we should not adopt Doppler’s interpretation. But exactly this wrong course was suggested by Edwin Hubble. He was not satisfied with Doppler’s interpretation. He even said that redshifts could be due to unrecognized principle of nature. But he advised that as long as correct reason is found, Doppler’s meaning may be adopted but only for practical reasons.
Following is from his book “The Realm of the Nebulae” (1936)
So only for the sake of “convenience”, velocity meaning of redshift is advocated. I am against this “convenience” approach. If we have no alternative, then reason of redshifts is “indeterminate” and there is no need of wild speculations that Universe is Expanding.
Having said that now I come to the alternative which is correct to my satisfaction at least and I have explained it in book. In my opinion, the reason of redshifts is Huygens’ Principle.
See that wave at t2 is greater in size. According to Huygens’ Principle, light propagates in this way. With propagation, size of wave gets increased. Somehow light itself is expanding by way of expansion in wavelengths.