Immediately upon publishing this results, Willem de Sitter who was working with Einstein's Theory of General Relativity recognized that these observations fit with a particular solution to the Einstein equations now known as the FRW metric. This solution explains that distant objects are observed with greater wavelengths than were emitted by the objects because they are moving with an expansion of the universe. In this formulation, there was still an analogous effect to the Doppler Effect, though relative velocities are not really well-defined for such situations.
Not everyone accepted this interpretation immediately. General relativity was notoriously difficult to understand and many of the astronomers were skeptical that this was the only possible explanation. Some, like Edward Milne proposed that there was a giant explosion that could explain redshifts (see Milne universe). Others thought that there might be systematic effects that made the observation suspect. Along this line, Fritz Zwicky proposed a "tired light" mechanism in 1929. Zwicky suggested that photons might slowly lose energy as they travel vast distances through a static universe by interaction with matter or other photons, or by some novel physical mechanism. Since a decrease in energy corresponds to an increase in light's wavelength, this effect would produce a redshift in spectral lines that increase proportionally with the distance of the source. The term "tired light" was coined by Richard Tolman in the early 1930s as a way to refer to this idea
Tired light mechanisms were among the proposed alternatives to the Big Bang and the Steady State cosmologies, both of which relied on the general relativistic expansion of the universe of the FRW metric. Through the middle of the twentieth century, most cosmologists supported one of these two paradigms, but there were a few scientists who worked with the tired light alternative. As the discipline of observational cosmology developed in the late twentieth century and the associated data became more numerous and accurate, the Big Bang emerged as the cosmological theory most supported by the observation evidence, and it remains the accepted consensus model in the current parametrization of the state and evolution of the universe. Additionally, a number of studies have shown that the "tired light" hypothesis is not a viable explanation for cosmological redshifts.
Redshift is directly observable and used by cosmologists as a direct measure of time, often referring to age in terms of redshift rather than years. The Big Bang is the end of this scale of time corresponding to a redshift of infinity. The relation between universal expansion and redshift is observationally confirmed through a great variety of tests, though alternative hypotheses such as tired light remain historical interest. So-called alternative theories of gravity that do not have an expanding universe need an alternative explanation to explain the correspondence between redshift and distance that is sui generis to expanding metrics of general relativity. Such cosmologies are sometimes referred to as "tired-light cosmologies", though not all authors are necessarily aware of the historical antecedents.
Zwicky investigated a number of redshift explanations, ruling out some himself (Zwicky, 1929):
Zwicky also notes, in the same paper, that according to a tired light model a distance-redshift relationship would necessarily be present in the light from sources within our own galaxy (even if the redshift would be so small that it would be hard to measure), that do not appear under a recessional-velocity based theory. He writes, referring to sources of light within our galaxy: "It is especially desirable to determine the redshift independent of the proper velocities of the objects observed".
The simplest form of a tired light theory assumes an exponential decrease in photon energy with distance traveled:
As part of a broader alternative cosmology, other observations that need explanation include:
To date, no established mechanism to produce such a drop in energy has been proposed that reproduces all the observations associated with the redshift-distance relation. Scattering by known mechanisms from gas or dust does not reproduce the observations. For example, scattering by any mechanism would blur an object more than observed. In general, cosmologists consider classical tired light models to have too many problems to be worth serious consideration. Tired light alone does not provide a full cosmological explanation and so cannot reproduce all the successes of the standard big bang cosmology. No tired light theory is known that by itself correctly accounts for the observed time dilation of distant supernovae light curves , the black body spectrum or anisotropy of the cosmic microwave background, and the observed change in the morphology, number count, and surface brightness of high redshift galaxies and quasars. Furthermore, the fact that the age of the oldest stars is roughly equal to the inverse of the Hubble constant emerges naturally from a Big Bang cosmology, but is an unexplained coincidence with most tired light models.
In 2008, Blondin et al. showed that even processes that take macroscopic amounts of time such as various elapsed times are slowed down by the same redshift factor as the light frequency.