Various other possible interpretations of the red shift have been put forward from time to time, one of the most popular being some version of a ‘tired light’ proposal, according to which the photons simply ‘lose energy’ as they travel towards us. Another version proposes that time progressed more slowly in the past. Such schemes turn out to be either inconsistent with other well-established observations or principles, or ‘unhelpful’, in the sense that they can be re-phrased as being equivalent to the standard expanding-universe picture, but with unusual definitions of the measures of space
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