Tired light

Tired light is a term coined by Richard Tolman to describe a theory proposed by physicist Fritz Zwicky in 1929, that the progressive red shift of photons coming from distant galaxies is not due to the ongoing expansion of space resulting from the creation of the universe, but is instead due to something that drains photons of their energy as they travel over vast distances. Zwicky believed in tired light despite the fact that he himself admitted that the red shift could not result from any Compton scattering due to dust. This hypothesis has been soundly rejected by the scientific community.