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Originally Posted by Pi Man
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Long ago I learned that the red shift of light from galaxies was a Doppler effect caused by the galaxies moving away from each other. Now everyone says, "No, the red shift of light from galaxies is caused by the expansion of space." Does that mean that the galaxies are not moving away from each other after all? Or does it mean that the galaxies are moving away from each other without causing a Doppler effect for their light?
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This is a fairly common misunderstanding. The galaxies are flying apart, causing the redshift. An alternative way of viewing this situation is to say that the space in between us and a distant galaxy is stretching, causing the light to stretch and lengthening its wavelength. The two are alternative ways of viewing the same situation. |
To add one little thing: whether you consider the galaxies themeselves to be moving or the space in between them increasing, the doppler shift works the same way. The second is the more "correct" way to model it however because of relativity and the problems caused by objects moving at faster than C relative to each other as is the case with objects that are VERY far apart in the universe.