William Duncan MacMillan was an American mathematician and astronomer on the faculty of the University of Chicago. He published research on the applications of classical mechanics to astronomy, and is noted for pioneering speculations on physical cosmology. For the latter, Helge Kragh noted, "the cosmological model proposed by MacMillan was designed to lend support to a cosmic optimism, which he felt was threatened by the world view of modern physics."
W. D. MacMillan, University of Chicago, 1920s
Tired light is a class of hypothetical redshift mechanisms that was proposed as an alternative explanation for the redshift-distance relationship. These models have been proposed as alternatives to the models that involve the expansion of the universe. The concept was first proposed in 1929 by Fritz Zwicky, who suggested that if photons lost energy over time through collisions with other particles in a regular way, the more distant objects would appear redder than more nearby ones.
The Hubble Ultra Deep Field is an image of galaxies that are in excess of 10 billion light years away. If tired light was a correct explanation, these galaxies would appear blurred in comparison to closer galaxies. That they do not rules out the suggestion that scattering processes are causing the redshift-distance relation.