When Hubble did his early work on the expanding universe, he knew nothing about the cosmological redshift. He used the Doppler shift equations instead, but these gave him the correct answer. Why?
A) These two sets of equations (Doppler shift and cosmological redshift) are the same and always give the same result for all cases.
B) Hubble was using only ordinary galaxies. If he had used galaxies with AGNs, the equations would not have worked.
C) Hubble was fortunate enough to use only galaxies for which the gravitational redshift and the cosmological redshift nearly cancel, so the Doppler shift provided essentially the whole effect.
D) Hubble was using only relatively nearby galaxies for which the nonrelativistic cosmological redshift and the Doppler shift provide the same result.

Question

Quizplus · Accepted Answer

Hubble was observing relatively nearby galaxies where the differences between the cosmological redshift and the Doppler shift are negligible, thus the Doppler shift equations were sufficient to describe the observed redshifts accurately.

When Hubble Did His Early Work on the Expanding Universe