In the first decade following Hubble's discovery of the red shift law, there was some disagreement concerning its meaning and cosmological significance. There were researchers such as Fritz Zwicky and Hubble himself who believed that the nebular spectral shifts occurred as the result of some physical process acting on light as it travels the long distances in space from source to observer. It was natural to assume that the effect of this process would be proportional to the distance traveled by the light. According to this hypothesis, the universe is static or at least is not subject to any large-scale systematic motions. Zwicky and other proponents of the "tired light" hypothesis were unable to work out a theory to explain the physical process that would lead to the observed pattern of red shifts. Although there have up to the present been occasional proponents of a static interpretation of Hubble's law, the majority of scientists have concluded that the universe is expanding, and this conviction has become the fundamental tenet of modern cosmology.

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