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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Do astronomers take into account gravitational redshifts when they compute the cosmological redshift of a distant galaxy?

We don't need to. The only way that gravitational redshifts can be important relative to the cosmological redshift is if galaxy masses were thousand times greater than that of normal, local galaxies, AND if this mass were concentrated into regions of space no larger than conventional galaxies. When we look at progressively more and more distant galaxies, we see no dynamical effects among paris or groups of galaxies that imply their masses are that much greater than typical galaxies nearby. If an observed redshift for a distant galaxy is seen to be z = 0.33 or 100,000 kilometers/sec, then the ordinary Doppler shift accounts for about 1,000 kilometers/sec of z = 0.033 and the gravitational redshift probably accounts for only a few kilometers/sec or z = 0.000033.

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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.