How can one tell the difference between a gravitational red shift of light and a red shift caused by motion? Could the red shift of quasars be from gravity and not speed, therefore making the quasars closer than we think they are? – FG
At astronomical distances, there is no way to tell the difference between the two red shifts. An object that is deep in the gravitational potential well of a very massive object experiences time slowly and its light appears shifted toward the red (low frequency and long wavelength) when it reaches us. The light from an object that is moving away from us rapidly also appears red shifted (low frequency and long wavelength), but this time it’s due to the Doppler effect.
Quasars exhibit enormous red shifts and one explanation for those red shifts is that the quasars are located near the other side of the universe. If so, they would be moving away from us rapidly, along with their surroundings in the expanding universe, and their light would appear highly red shifted. Moreover, their light would have been traveling almost since the beginning of the universe so that we would be observing very ancient objects. However, it’s also possible that quasars are much near to us and that their red shifts are caused by gravitational effects rather than relative motion. As far as I know, this possibility can’t be ruled out and remains a concern amount the astronomical community.