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 was first uncovered, a completely new and radical 20th-century notion, it was natural and necessary to explore alternative hypotheses. The best-known theory of tired light (typically what is meant by the phrase today) was by Fritz Zwicky: that scattering along the way resulted in photons of reduced energy, i.e., longer wavelength. However Zwicky also pointed out that this could be expected to cause a blurring of distant objects that was not detected. Cosmological expansion has since been supported by a number of phenomena independent of the lengthening of EMR wavelength, one of which is cosmological time dilation such as time dilation of distant supernova light curves.