Tired-Light Hypothesis

Since the 1920s there has been a popular hypothesis trying to explain the cosmological redshift as a so called tired-light effect that is, the light loses energy as its photons race through space, getting more tired with distance, like a long-distance runner completing a race. What two specific pieces of evidence rule out this explanation for the cosmological redshift?

Answer

The only two specific pieces of evidence are the time dilation arising from the expansion of the universe, and the spectral shape of the cosmic microwave background. Astronomers see that exploding stars in distant galaxies brighten and fade more slowly than those nearby. If the star emits a light pulse on January 1 and a second pulse on February 1, these two pulses are separated by one light month. As they travel toward Earth, their separation distance increases, perhaps doubling, so that they are received two months apart. The tired light hypothesis cannot explain this extended time interval. In fact, distant supernovas are observed to wax and wane more slowly than nearby ones.

The observed spectrum of the microwave background radiation is a perfect blackbody shape, easily explained by the expansion of the universe from a thermodynamic equilibrium condition. For the tired-light hypothesis, an initial blackbody spectrum does not remain a blackbody spectrum as the light becomes redshifted.