Editing Tired light (section)

==History and reception==
{{See also|Redshift|Non-standard cosmology#Tired light}}
Tired [[light]] was an idea that came about due to the observation made by [[Edwin Hubble]] that [[galaxy|distant galaxies]] have [[redshift]]s proportional to their [[distance measures (cosmology)|distance]]. Redshift is a shift in the [[astronomical spectroscopy|spectrum]] of the emitted [[electromagnetic radiation]] from an object toward lower energies and frequencies, associated with the phenomenon of the [[Doppler effect]]. Observers of [[spiral nebula]]e such as [[Vesto Slipher]] observed that these objects (now known to be separate [[galaxy|galaxies]]) 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 contribution was to show that the magnitude of the redshift correlated strongly with the distance to the galaxies.

Basing on Slipher's and Hubble's data, in 1927 [[Georges Lemaître]] 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 solution is attributable to the effect an expanding universe has on a photon traveling on a null [[spacetime interval]] (also known as a "light-like" [[geodesic]]). In this formulation, there was still an analogous effect to the [[Doppler effect]], though relative velocities need to be handled with more care since [[comoving distance|distances]] can be defined in different ways in [[expansion of the universe|an expanding universe]].

At the same time, other explanations were proposed that did not concord with general relativity. [[Edward Arthur Milne|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 [[systematics|systematic effects]] could explain the redshift-distance correlation. Along this line, [[Fritz Zwicky]] proposed a "tired light" mechanism in 1929.<ref name="Zwicky">{{Cite journal |doi = 10.1073/pnas.15.10.773|pmid = 16577237|pmc = 522555|bibcode = 1929PNAS...15..773Z|title = On the Redshift of Spectral Lines Through Interstellar Space|year = 1929|last1 = Zwicky|first1 = F.|journal = Proceedings of the National Academy of Sciences|volume = 15|issue = 10|pages = 773–779|doi-access = free}}</ref> Zwicky suggested that [[photon]]s might slowly lose [[energy]] as they travel vast [[distance]]s 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 line]]s that increase [[Proportionality (mathematics)|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.<ref>{{cite book |last1=Evans |first1=Myron W. |last2=Vigier |first2=Jean-Pierre |title=The Enigmatic Photon: Theory and Practice of the B3 Field |url=https://books.google.com/books?id=2-0p1eOCYeIC&q=%22tired+light%22+coined+tolman&pg=PA29 |page=29 |publisher=Springer |date=1996 |isbn=978-0-7923-4044-7}}</ref> [[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]].<ref>{{cite book |editor1-first=Helge |editor1-last=Kragh |editor2-first=Malcolm S. |editor2-last=Longair |title=The Oxford Handbook of the History of Modern Cosmology |date=2019 |chapter=Alternative Cosmological Theories |first=Helge |last=Kragh |doi=10.1093/oxfordhb/9780198817666.013.4 |page=29|isbn=978-0-19-881766-6 }}</ref>

Tired light mechanisms were among the proposed alternatives to the [[Big Bang]] and the [[Steady State Theory|Steady State]] [[physical cosmology|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 [[paradigm]]s, but there were a few scientists, especially those who were working on alternatives to general relativity, who worked with the tired light alternative.<ref>{{Cite journal |bibcode = 1939ApJ....90..634W|title = Possible Applications of Supernovae to the Study of the Nebular Red Shifts|last1 = Wilson|first1 = O. C.|journal = The Astrophysical Journal|volume = 90|pages = 634|year = 1939|doi = 10.1086/144134}}</ref> 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 observational evidence, and it remains the accepted [[scientific consensus|consensus model]] with a current [[Lambda-CDM model|parametrization]] that precisely specifies the state and evolution of the universe. Although the proposals of "tired light cosmologies" are now more-or-less relegated to the dustbin of history, as a completely alternative proposal tired-light cosmologies were considered a remote possibility worthy of some consideration in cosmology texts well into the 1980s, though it was dismissed as an unlikely and ''ad hoc'' proposal by mainstream astrophysicists.<ref>See, for example, p. 397 of [[Joseph Silk]]'s book, ''The Big Bang''. (1980) W. H. Freeman and Company. {{ISBN|0-7167-1812-X}}.</ref>

[[File:Tolman_surface_brightness_test.png|thumb|upright=1.5|The Tolman surface brightness test rules out the tired light explanation for the cosmological redshift.]]
By the 1990s and on into the twenty-first century, a number of falsifying observations have shown that "tired light" hypotheses are not viable explanations for cosmological redshifts.<ref name="treu">Tommaso Treu, Lecture slides for [[University of California at Santa Barbara]] Astrophysics course. [http://www.physics.ucsb.edu/~tt/ASTRO2/lecture16.pdf p. 16]. {{Webarchive|url=https://web.archive.org/web/20100623125705/http://www.physics.ucsb.edu/~tt/ASTRO2/lecture16.pdf|date=2010-06-23}}.</ref> For example, 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. In an expanding universe, 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.<ref name = "Geller">{{Cite journal |bibcode = 1972ApJ...174....1G|title = Test of the Expanding Universe Postulate|last1 = Geller|first1 = M. J.|last2 = Peebles|first2 = P. J. E.|journal = The Astrophysical Journal|volume = 174|pages = 1|year = 1972|doi = 10.1086/151462|doi-access = free}}</ref><ref>{{cite journal|last1=Goldhaber|first1=G.|last2=Groom|first2=D. E.|last3=Kim|first3=A.|last4=Aldering|first4=G.|last5=Astier|first5=P.|last6=Conley|first6=A.|last7=Deustua|first7=S. E.|last8=Ellis|first8=R.|last9=Fabbro|first9=S.|last10=Fruchter|first10=A. S.|last11=Goobar|first11=A.|last12=Hook|first12=I.|last13=Irwin|first13=M.|last14=Kim|first14=M.|last15=Knop|first15=R. A.|last16=Lidman|first16=C.|last17=McMahon|first17=R.|last18=Nugent|first18=P. E.|last19=Pain|first19=R.|last20=Panagia|first20=N.|last21=Pennypacker|first21=C. R.|last22=Perlmutter|first22=S.|last23=Ruiz-Lapuente|first23=P.|last24=Schaefer|first24=B.|last25=Walton|first25=N. A.|last26=York|first26=T.|author27=The Supernova Cosmology Project|title=Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves|doi=10.1086/322460|date=2001|journal=The Astrophysical Journal|volume=558|issue=1|pages=359–368|arxiv=astro-ph/0104382|bibcode = 2001ApJ...558..359G |s2cid=17237531}}</ref><ref>{{Cite journal |arxiv = astro-ph/0106566|doi = 10.1086/322134|bibcode = 2001AJ....122.1084L|title = The Tolman Surface Brightness Test for the Reality of the Expansion. IV. A Measurement of the Tolman Signal and the Luminosity Evolution of Early-Type Galaxies|year = 2001|last1 = Lubin|first1 = Lori M.|last2 = Sandage|first2 = Allan|journal = The Astronomical Journal|volume = 122|issue = 3|pages = 1084–1103|s2cid = 118897528}}</ref>

Redshift is directly observable and used by cosmologists as a direct measure of [[cosmological time|lookback time]]. They often refer to age and distance to objects in terms of redshift rather than years or light-years. In such a scale, the [[Big Bang]] corresponds to a redshift of infinity.<ref name="Geller" /> [[Alternative theories of gravity]] that do not have an expanding universe in them need an alternative to explain the correspondence between redshift and distance that is ''[[sui generis]]'' to the [[Metric expansion of space|expanding metrics]] of general relativity. Such theories are sometimes referred to as "tired-light cosmologies", though not all authors are necessarily aware of the historical antecedents.<ref>{{cite book |last=Barrow |first=John D. |title=The Routledge Companion to the New Cosmology |editor=Peter Coles |publisher=Routledge |date=2001 |isbn=978-0-415-24312-4 |page=308|bibcode=2001rcnc.book.....C }}</ref>