The ‘Tired Photons’ Hypothesis
The ‘tired photons’ hypothesis is in direct contrast to the Doppler hypothesis. It states that as photons travel through space, they experience collisions with interstellar matter and eventually lose energy. If this were true, the loss of energy would be from collisions (inelastic collisions) with interstellar matter. This would produce a ‘smearing’ of light that could be observed. No such smearing due to inelastic collisions has been observed to date.
The Doppler Hypothesis
The prevalent view is in favor of Doppler effect for the shifting of the frequency. However, even though this is the dominant view, it does not apply to all red-shifted objects (for e.g. the Quasar redshift is still an iffy…)
If the ‘tired photons’ view was true, there would be a ‘smearing’ effect – from the inelastic collisions of photons with interstellar medium. As of now, no such effect is observed.