Terminology/Tired light
Tired light
Tired light is a class of hypothetical redshift mechanisms that was proposed as an alternative explanation for the redshift-distance relationship. These models have been proposed as alternatives to the models that involve the expansion of the universe. The concept was first proposed in 1929 by Fritz Zwicky, who suggested that if photons lost energy over time through collisions with other particles in a regular way, the more distant objects would appear redder than more nearby ones.
Zwicky acknowledged that any sort of scattering of light would blur the images of distant objects more than what is seen. Additionally, the surface brightness of galaxies evolving with time, time dilation of cosmological sources, and a thermal spectrum of the cosmic microwave background have been observed—these effects should not be present if the cosmological redshift was due to any tired light scattering mechanism. Despite periodic re-examination of the concept, tired light has not been supported by observational tests and remains a fringe topic in astrophysics.
History
Redshift is said to be a shift in the spectrum of the emitted electromagnetic radiation from an object toward lower energies and frequencies, associated with the phenomenon of the Doppler effect.
The "Tired light" theory came about due to the observation made by Edwin Hubble that distant galaxies have redshifts proportional to their distance.
Observers of spiral nebulae such as Vesto Slipher observed that these objects generally exhibited redshift rather than blueshifts independent of where they were located. Since the relation holds in all directions it cannot be attributed to normal movement with respect to a background which would show an assortment of redshifts and blueshifts.
Hubble's theories are said to show that the magnitude of the redshift correlated strongly with the distance to the galaxies, as he is a Big-Bang proponent, believing everything is moving away from the Milky Way galaxy.
Basing on Slipher's and Hubble's data, in 1927 Georges Lemaître (A Jesuit) realized that this correlation could fit non-static solutions to the equations of Einstein's theory of gravity, the Friedmann–Lemaître solutions.
However Lemaître's article was appreciated only after Hubble's publication of 1929. The universal redshift-distance relation in this theory is based on the theorized effect an expanding universe has on a photon traveling on a null spacetime interval. In this formulation, there was still an analogous effect to the Doppler effect, though relative velocities need to be handled with more care since distances can be defined in different ways in a theorized expanding universe.
At the same time, other explanations were proposed that did not concord with general relativity. Edward Milne proposed an explanation compatible with special relativity but not general relativity that there was a giant explosion that could explain redshifts (see Milne universe). Others proposed that systematic effects could explain the redshift-distance correlation. 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.
Helge Kragh has noted "Zwicky’s hypothesis was the best known and most elaborate alternative to the expanding universe, but it was far from the only one. More than a dozen physicists, astronomers and amateur scientists proposed in the 1930s tired-light ideas having in common the assumption of nebular photons interacting with intergalactic matter to which they transferred part of their energy." Kragh noted in particular John Quincy Stewart, William Duncan MacMillan, and Walther Nernst.
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.
Tired light debunked?
It is theorized that in a static universe with tired light mechanisms, the surface brightness of stars and galaxies should be constant - that is, the farther an object is, the less light we receive, but its apparent area diminishes as well. So the light received divided by the apparent area should be constant; Of course this debunk relies heavily on theories as to what these lights in the heavens actually are.
In the expanding universe theory, the surface brightness diminishes with distance. As the observed object recedes, photons are emitted at a reduced rate because each photon has to travel a distance that is a little longer than the previous one, while its energy is reduced a little because of increasing redshift at a larger distance.
On the other hand, in an expanding universe, the object appears to be larger than it really is, because it was closer to us when the photons started their travel. This causes a difference in surface brilliance of objects between a static and an expanding Universe. This is known as the "Tolman surface brightness test" that in those studies favors the expanding universe hypothesis and rules out static tired light models.
Tolman's surface brightness test claims to compare the surface brightness of galaxies as a function of their redshift (measured as z). This test was used to determine whether the universe itself was expanding or static. The test is based on the concept of the Inverse-square law, where the light received from an object drops proportional to the square of its distance and the apparent area of the object also drops proportional to the square of the distance.
The assumption of a Static universe is that the surface brightness would be constant, independent of the distance.
In an expanding universe, however, there are two effects that change this relation. First, the rate at which photons are received is reduced because each photon has to travel a little farther than the one before. Second, the energy of each photon observed is reduced by the redshift. At the same time, distant objects appear larger than they really are because the photons observed were emitted at a time when the object was closer.