Special & General Relativity Questions and Answers
Why is the Doppler shift the only explanation for cosmological redshifts?
It isn't. It is a common misconception among non-astronomers that the cosmological redshift is a Doppler shift that involves the motion of the distant galaxies. In fact there are three kinds of 'redshifts', and the Doppler shift is only one of these. The other two were successfully predicted by Einstein's general theory of relativity. They involve the gravitational redshift, which has noting to do with motion, and the cosmological redshift, which has to do with the motion of space and not the motion of particles. It is the cosmological redshift which provides the only explanation for what we see in distant galaxies. None of the other redshift' mechanisms are relevant because they lead to false predictions given the data we have.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.