Editing Redshift (section)

==History==
The history of the subject began in the 19th century, with the development of classical [[wave]] mechanics and the exploration of phenomena which are associated with the [[Doppler effect]]. The effect is named after the [[Austria|Austrian]] mathematician, [[Christian Doppler]], who offered the first known physical explanation for the phenomenon in 1842.<ref>
{{cite book
 |last=Doppler | first=Christian
 |date=1846
 |title=Beiträge zur fixsternenkunde
 |location=Prague |publisher=G. Haase Söhne
 |bibcode=1846befi.book.....D
 |volume=69
}}</ref><ref name=Becker-2011>{{Cite book |last=Becker |first=Barbara J. |url=https://www.cambridge.org/core/product/identifier/9780511751417/type/book |title=Unravelling Starlight: William and Margaret Huggins and the Rise of the New Astronomy |date=2011-02-17 |publisher=Cambridge University Press |isbn=978-1-107-00229-6 |edition=1 |doi=10.1017/cbo9780511751417}}</ref>{{rp|107}} In 1845, the hypothesis was tested and confirmed for [[sound wave]]s by the [[Netherlands|Dutch]] scientist [[C. H. D. Buys Ballot|Christophorus Buys Ballot]].<ref>
{{cite book
 |last=Maulik | first=Dev
 |chapter=Doppler Sonography: A Brief History
 |chapter-url=https://books.google.com/books?id=HedeGJms0n4C&q=%22Ballot%22&pg=PA3
 |editor1-last=Maulik | editor1-first=Dev
 |editor2-last=Zalud | editor2-first=Ivica
 |date=2005
 |title=Doppler Ultrasound in Obstetrics And Gynecology
 |url= https://www.springer.com/west/home/medicine/gynecology?SGWID=4-10066-22-46625046-0
 |isbn=978-3-540-23088-5
 |publisher=Springer
 }}</ref> Doppler correctly predicted that the phenomenon would apply to all waves and, in particular, suggested that the varying [[color]]s of [[star]]s could be attributed to their motion with respect to the Earth.<ref>
{{cite web
 |last1=O'Connor | first1=John J.
 |last2=Robertson | first2=Edmund F.
 |date=1998
 |url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Doppler.html
 |title=Christian Andreas Doppler
 |work=[[MacTutor History of Mathematics archive]]
 |publisher=[[University of St Andrews]]
}}</ref> 

Unaware of Doppler's work, French physicist [[Hippolyte Fizeau]] in 1848, suggested that a shift in [[spectral line]]s from stars might be used to measure their motion relative to Earth.<ref name=Becker-2011/>{{rp|109}} In 1850 [[François-Napoléon-Marie Moigno]] analyzed about both Doppler's and Fizeau's ideas in a publication read by both [[James Clerk Maxwell]] and [[William Huggins]], who initially stuck to the idea that the color of stars related to their chemistry, however by 1868, Huggins was the first to determine the velocity of a star moving away from the Earth by the analysis of spectral shifts.<ref name=Huggins>
{{cite journal
 |last=Huggins | first=William
 |date=1868
 |title=Further Observations on the Spectra of Some of the Stars and Nebulae, with an Attempt to Determine Therefrom Whether These Bodies are Moving towards or from the Earth, Also Observations on the Spectra of the Sun and of Comet II
 |journal=[[Philosophical Transactions of the Royal Society of London]]
 |volume= 158 |pages=529–564
 |bibcode=1868RSPT..158..529H
 |doi=10.1098/rstl.1868.0022
}}</ref><ref name=Becker-2011/>{{rp|111}} 

In 1871, optical redshift was confirmed when the phenomenon was observed in [[Fraunhofer lines]], using solar rotation, about 0.1 Å in the red.<ref name="Nolte">{{cite journal |last1=Nolte |first1=David D. |title=The fall and rise of the Doppler effect |journal=Physics Today |date=1 March 2020 |volume=73 |issue=3 |pages=30–35 |doi=10.1063/PT.3.4429 |doi-access=free |bibcode=2020PhT....73c..30N }}</ref> In 1887, Vogel and Scheiner discovered the "annual Doppler effect", the yearly change in the Doppler shift of stars located near the ecliptic, due to the orbital velocity of the Earth.<ref>{{cite book|last=Pannekoek|first=A.|title=A History of Astronomy |date=1961|publisher=Dover|page=451|isbn=978-0-486-65994-7}}</ref> In 1901, [[Aristarkh Belopolsky]] verified optical redshift in the laboratory using a system of rotating mirrors.<ref>
{{cite journal
 |last=Bélopolsky | first=A.
 |date=1901
 |bibcode=1901ApJ....13...15B
 |title=On an Apparatus for the Laboratory Demonstration of the Doppler-Fizeau Principle
 |journal=[[Astrophysical Journal]]
 |volume=13 |page=15
 |doi=10.1086/140786
|doi-access=free
 }}</ref><ref name="Nolte"/>

Beginning with observations in 1912, [[Vesto Slipher]] discovered that the [[Andromeda Galaxy]] had a blue shift, indicating that it was moving towards the Earth.<ref name="SmithInKragh-2019">{{Cite book |last=Robert |first=Smith |title=The Oxford handbook of the history of modern cosmology |date=2019 |publisher=Oxford University Press |isbn=978-0-19-881766-6 |editor-last=Kragh |editor-first=Helge |chapter=Observations and the universe |oclc=1052868704 |editor-last2=Longair |editor-first2=Malcolm S.}}</ref> Slipher first reported on his measurement in the inaugural volume of the ''[[Lowell Observatory]] Bulletin''.<ref>
{{cite journal
 |last=Slipher | first=Vesto
 |date=1912
 |title=The radial velocity of the Andromeda Nebula
 |journal=[[Lowell Observatory Bulletin]]
 |volume=1 |issue=8
 |pages=2.56–2.57
 |bibcode=1913LowOB...2...56S
 |quote=The magnitude of this velocity, which is the greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at present no other interpretation for it
}}</ref> Three years later, he wrote a review in the journal ''[[Popular Astronomy (US magazine)|Popular Astronomy]]''.<ref>
{{cite journal
 |last=Slipher | first=Vesto
 |title=Spectrographic Observations of Nebulae
 |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]]
 |volume=23 |pages=21–24 |date=1915
 |bibcode=1915PA.....23...21S
}}</ref> In it he stated that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km(/s) showed the means then available, capable of investigating not only the spectra of the spirals but their velocities as well."<ref>
{{cite journal |last=Slipher | first=Vesto |date=1915 |title=Spectrographic Observations of Nebulae |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=23 |page=22 |bibcode=1915PA.....23...21S}}</ref>  Slipher reported the velocities for 15 spiral nebulae spread across the entire [[celestial sphere]], all but three having observable "positive" (that is recessional) velocities.<ref name="SmithInKragh-2019"/> 

Until 1923 the nature of the nebulae was unclear. By that year 
[[Edwin Hubble]] had established that these were [[galaxies]] and worked out a procedure to measure distance based on the period-luminosity relation of variable [[Cepheids]] stars. This make it possible to test a prediction by [[Willem de Sitter]] in 1917 that redshift would be correlated with distance.
In 1929 Hubble combined his distance estimates with redshift data from Slipher's reports and measurements by [[Milton Humason]] to report an approximate relationship between the redshift and [[distance]], a result now called [[Hubble's law]].<ref name="SmithInKragh-2019"/>{{rp|64}}<ref>
{{cite journal
 |doi=10.1073/pnas.15.3.168
 |last=Hubble |first=Edwin
 |date=1929
 |bibcode=1929PNAS...15..168H
 |title=A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae
 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
 |volume=15 |issue=3 |pages=168–173
 |pmid=16577160
 |pmc=522427
 |doi-access=free
}}</ref><ref>{{Cite web|url=https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1929/expanding.html|title=Universe is Expanding|date=2017-12-08|access-date=2023-09-06 |publisher=Goddard Space Flight Center}}</ref> 

Theories relating to the redshift-distance relation also evolved during the decade of the 1920s.
The solution to the equations of general relativity described by de Sitter contained no matter, but in 1922 [[Alexander Friedmann]]'s derived dynamic solutions, now called the [[Friedmann equations|Friedmann–equations]], based on frictionless fluid models.<ref>{{cite journal
 |last=Friedman |first=A. A.
 |date=1922
 |title=Über die Krümmung des Raumes
 |journal=[[Zeitschrift für Physik]]
 |volume=10
 |issue=1 |pages=377–386
 |doi=10.1007/BF01332580
|bibcode = 1922ZPhy...10..377F |s2cid=125190902
 }} English translation in {{cite journal |title=On the Curvature of Space|doi=10.1023/A:1026751225741 |last=Friedman |first=A. |date=1999  |journal=[[General Relativity and Gravitation]] |volume=31 |issue=12 |pages=1991–2000 |bibcode=1999GReGr..31.1991F|s2cid=122950995 }})</ref> Independently [[Georges Lemaître]] derived similar equations in 1927 and his analysis became widely known around the time of Hubble's key publication.<ref name="SmithInKragh-2019"/>{{rp|77}}

By early 1930 the combination of the redshift measurements and theoretical models established a major breakthrough in the new science of cosmology: the universe had a history and its expansion could be investigated with physical models backed up with observational astronomy.<ref name="SmithInKragh-2019"/>{{rp|99}} 

[[Arthur Eddington]] used the term "red shift" as early as 1923, which is the oldest example of the term reported by the ''[[Oxford English Dictionary]].''<ref>{{Cite book |last=Eddington |first=Arthur Stanley |url=https://books.google.com/books?id=errkj2WXGzIC&pg=PA164 |title=The Mathematical Theory of Relativity |date=1923 |publisher=The University Press |page=164 |language=en |author-link=Arthur Eddington}}</ref><ref>{{Cite OED|term=redshift|id=160477|access-date=2023-03-17}}</ref> [[Willem de Sitter]] used the single-word version ''redshift'' in 1934.<ref>
{{cite journal
 |last=de Sitter | first=W.
 |date=1934
 |title=On distance, magnitude, and related quantities in an expanding universe
 |journal=[[Bulletin of the Astronomical Institutes of the Netherlands]]
 |volume=7 |page=205
 |bibcode=1934BAN.....7..205D
 |quote=It thus becomes urgent to investigate the effect of the redshift and of the metric of the universe on the apparent magnitude and observed numbers of nebulae of given magnitude
}}</ref>

In the 1960s the discovery of [[quasars]], which appear as very blue point sources and thus were initially thought to be unusual stars, lead to the idea that they were as bright as they were because they were closer than their redshift data indicated. A flurry of theoretical and observational work concluded that these objects were very powerful but distant astronomical objects.<ref name="SmithInKragh-2019"/>{{rp|261}}