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 require metric expansion of space of which the Big Bang and the Steady State cosmologies are the most famous examples. 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. Zwicky himself acknowledged that any sort of scattering of light would blur the images of distant objects more than what is seen. Additionally, the surface brightness of galaxies evolving with time, time dilation of cosmological sources, and a thermal spectrum of the cosmic microwave background have been observed—these effects should not be present if the cosmological redshift was due to any tired light scattering mechanism. Despite periodic re-examination of the concept, tired light has not been supported by observational tests and remains a fringe topic in astrophysics. History and reception Tired light was an idea that came about due to the observation made by Edwin Hubble that distant galaxies have redshifts proportional to their distance. Redshift is a shift in the spectrum of the emitted electromagnetic radiation from an object toward lower energies and frequencies, associated with the phenomenon of the Doppler effect. Observers of spiral nebulae such as Vesto Slipher observed that these objects (now known to be separate galaxies) generally exhibited redshift rather than blueshifts independent of where they were located. Since the relation holds in all directions it cannot be attributed to normal movement with respect to a background which would show an assortment of redshifts and blueshifts. Everything is moving away from the Milky Way galaxy. Hubble's contribution was to show that the magnitude of the redshift correlated strongly with the distance to the galaxies. Basing on Slipher's and Hubble's data, in 1927 Georges Lemaître realized that this correlation fit non-static solutions to the equations of Einstein's theory of gravity, the Friedmann–Lemaître solutions. However Lemaître's article was appreciated only after Hubble's publication of 1929. The universal redshift-distance relation in this solution is attributable to the effect an expanding universe has on a photon traveling on a null spacetime interval (also known as a "light-like" geodesic). In this formulation, there was still an analogous effect to the