Galaxy Movement

What is the relation between the redshift and the Doppler shift when observing very distant objects?

As Vesto Slipher, Edwin Hubble, and other pioneering astronomers showed nearly a century ago, Doppler shifts in astronomy indicate the motions of objects toward or away from the observer. “Blueshift” is Doppler shift of objects moving toward an observer, while “redshift” is Doppler shift of objects moving away. Since the expanding universe carries galaxies faster and faster away as distances increase, the redshift gets higher and higher as well. Beyond a distance of about one billion light-years, the redshift gets so large that Einstein’s special theory of relativity becomes a factor in the motion, and the usual formula converting redshift into Doppler shift no longer holds. In those cases, a more complicated equation called the relativistic Doppler formula must be used.


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