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Above we described Optical Redshift in astronomy that occurs when an object (e.g. Barnard’s star) has a radial velocity with respect to us as observers. In that case it actually is a blue shift because the star moves towards us. It will be a Redshift for any star that is moving away.

Another cause of Redshift is that space itself expands. Objects in it may move relatively or not, but the fact that space expands, means that far away objects will always show a Redshift in their spectra. Generally this is called Cosmological Redshift and is denoted by z. Objects need to be far away enough to exceed any optical redshift due to their relative motion and make the expansion observable.


Before we go into detail, let us review the history of the discovery of the expanding Universe and what this has meant for the development of our understanding of the Universe during only the last hundred years.


Heber CurtisHarlowShapely crop
It is less than one hundred years ago that astronomers were uncertain whether our galaxy the Milky Way was all there is or that the actual Universe contains many more galaxies and must be “inconceivably large”.
Of historic significance is the “Great Debate” between astronomers Heber Curtis (left)
and Harlow Shapley in 1920.





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