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/* Fritz Zwicky and Ten Bruggencate */
Of course, that was a nice idea, but it was just a hypotheses. So here's where Ten Bruggencate comes into the scene. He wanted to test Zwicky's hypotheses, so he decided to test the redshifts of globular clusters. These globular clusters were a good testing sample since their distances would be somewhat accurate, and he could just deal with redshifts. If Hubble's idea was right, then they should all have the same redshifts, but if Zwicky's idea was right, then their redshifts should relate to the intervening matter around them. And that's just what he found - that the redshifts of the globular clusters were definitely related to the intervening matter surrounding them. Zwicky's theory was proven!
So to sum up, redshift is caused by light travelling through gravitational fields. The longer the light has traveled, the more gravitational fields it has gone through, and the redder it appears. When light does not go through many gravitational fields, it does not appear as redshifted. Hubble's constant actually varies up to 30% (you can even see that by having a look at any redshift/distance diagram).
Below are the two articles I'm talking about.
