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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What is the Einstein Shift?

The 'Einstein Shift' is probably what astronomers customarily call the 'cosmological redshift'. It is a phenomenon which we think accounts for the progressive increase in the shifting of spectral lines in more and more distant objects in the universe as a consequence of the expansion of space. This is to be distinguished from the Doppler Effect or 'shift' which is caused by the motion of an object relative to an observer as in the change of pitch of a siren on a fire truck as it approaches and passes you on the street. The cosmological redshift occurs because, evidently, as the universe is expanding, galaxies are dragged along with the expansion as space is being stretched. As for many other phenomena in physics, humans have very little direct experience with the 'stretching of space' but we think that this is what is going on on a vast, 'cosmological' scale.

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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.