tired light

The phrase tired light was coined for a type of theory that electromagnetic radiation from very distant objects appears more red (wavelengths lengthened or filtered) for reasons other than a Doppler shift due to the Hubble expansion, our current notion of the cosmological redshift. When evidence of cosmological expansion, a completely new and radical 20th-century notion, was first uncovered, it was natural and necessary to explore alternative hypotheses. Typically what is meant today by the phrase tired light is the best-known alternative hypothesis, which was pursued by Fritz Zwicky: that scattering along the way resulted in photons of reduced energy, i.e., longer wavelength. However, Zwicky also pointed out that such scattering could be expected to cause a blurring of distant objects, a blurring that was not evident in observations. Cosmological expansion has since been supported by a number of phenomena independent of the apparent lengthening of EMR wavelength, one of which is cosmological time dilation such as time dilation of distant supernova light curves.