Tired light

Tired light is a class of hypothetical redshift mechanisms that were proposed as an alternative explanation for the redshift-distance relationship. Tired light was first proposed in 1929 by Fritz Zwicky who 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 [1].

Tired light mechanisms were among the proposed alternatives to the Big Bang and the Steady State cosmologies, both of which proposed that Hubble's law was associated with a metric expansion of space. 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 predominant cosmological theory and is accepted in the current parametrization of the state and evolution of the universe. The vast majority of physicists and astronomers accept the conclusions of various studies that such an effect either does not or cannot account for cosmological redshifts.

Tired light models

A number of tired light mechanisms have been suggested over the years:

Zwicky's models

Zwicky investigated a number of redshift explanations, ruling out some himself (Zwicky, 1929):

• The Compton Effect:

"... light coming from distant nebulae would undergo a shift to the red by Compton effect on those free electrons [in interstellar spaces] [...] But then the light scattered in all directions would make the interstellar space intolerably opaque which disposes of the above explanation. [...] it is evident that any explanation based on a scattering process like the Compton effect or the Raman effect, etc., will be in a hopeless position regarding the good definition of the images"

• Gravitational potential:

"One might expect a shift of spectral lines due to the difference of the static gravitational potential at different distances from the center of a galaxy. This effect, of course, has no relation to the distance of the observed galaxy from our own system and, therefore, cannot provide any explanation of the phenomenon discussed in this paper."

• The Gravitational "Drag" of Light:

"... [a] gravitational analogue of the Compton effect [...] It is easy to see that the above redshift should broaden these absorption lines asymmetrically toward the red. If these lines can be photographed with a high enough dispersion, the displacement of the center of gravity of the line will give the redshift independent of the velocity of the system from which the light is emitted.

Hubble and Tolman's "energy loss" treatment

Following after Fritz Zwicky in 1935, Edwin Hubble and Richard Tolman compare recessional redshift with a non-recessional one, writing that they:

"... both incline to the opinion, however, that if the red-shift is not due to recessional motion, its explanation will probably involve some quite new physical priciples [.. and] use of a static Einstein model of the universe, combined with the assumption that the photons emitted by a nebula lose energy on their journey to the observer by some unknown effect, which is linear with distance, and which leads to a decrease in frequency, without appreciable transverse deflection"

Finlay-Freundlich Red Shift Hypothesis

In the early 1950s, Erwin Finlay-Freundlich proposed a redshift as "the result of loss of energy by observed photons traversing a radiation field.". R.A. Alpher noted "No generally accepted physical mechanism has been proposed for this loss" , though P.F. Brown ".. proposed that the energy lost reappears as neutrino pairs resulting from the exchange of a graviton between two photons".

General features of tired light models

The simplest form of a tired light theory assumes an exponential decrease in photon energy with distance traveled:

$E(x)=E(0)e^{-x/R},$

where $E(x)$ is the energy of the photon at distance x from the source of light, $E(0)$ is the energy of the photon at the source of light, and R is a large constant characterizing the "resistance of the space". To correspond to Hubble's law, the constant R must be several gigaparsecs.

Criticisms

Any "tired light" mechanism must solve some basic problems, in that the observed redshift must:

• admit the same measurement in any wavelength-band
• not exhibit blurring
• follow the detailed Hubble-relation observed with Supernova data (see accelerating universe)
• explain associated time dilation of cosmologically distant events.

As part of a broader alternative cosmology, other observations that need explanation include:

• the detail observations of the cosmic microwave background radiation
• the abundance of light elements
• large-scale structure statistics

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.