Redshift: Difference between revisions

In [[physics]], a '''redshift''' is an increase in the [[wavelength]], and corresponding decrease in the [[frequency]] and [[photon energy]], of [[electromagnetic radiation]] (such as [[light]]). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a [[#Blueshift|blueshift]], or negative redshift. The terms derive from the colours [[red]] and [[blue]] which form the extremes of the [[Visible spectrum|visible light spectrum]]. The main causes of electromagnetic redshift in [[astronomy]] and [[cosmology]] are the relative motions of radiation sources, which give rise to the [[relativistic Doppler effect]], and gravitational potentials, which [[gravitational redshift|gravitationally redshift]] escaping radiation. All sufficiently distant light sources show [[cosmological redshift]] corresponding to recession speeds proportional to their distances from Earth, a fact known as [[Hubble's law]] that implies the [[expansion of the universe|universe is expanding]].

All redshifts can be understood under the umbrella of [[Frame of reference|frame transformation laws]]. [[Gravitational wave]]s, which also travel at [[Speed of light|the speed of light]], are subject to the same redshift phenomena.<ref>{{cite journal | title=Detectability of primordial black hole binaries at high redshift | last=Ding | first=Qianhang | journal=Physical Review D | volume=104 | issue=4 | at=id. 043527 | date=August 2021 | doi=10.1103/PhysRevD.104.043527 | arxiv=2011.13643 | bibcode=2021PhRvD.104d3527D }}</ref> The value of a redshift is often denoted by the letter {{math|''z''}}, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio {{math|1 + ''z''}} (which is greater than 1 for redshifts and less than 1 for blueshifts).

The history of the subject began with the development 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>

}}</ref> TheIn 1845, the hypothesis was tested and confirmed for [[sound wave]]s by the [[Netherlands|Dutch]] scientist [[C. H. D. Buys Ballot|Christophorus Buys Ballot]] in 1845.<ref>

}}</ref> Doppler correctly predicted that the phenomenon shouldwould 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>

}}</ref> Before this was verified, however, it was found that stellar colors were primarily due to a star's [[color temperature|temperature]], not motion. Only later was Doppler vindicated by verified redshift observations.{{cn|date=March 2023}}

The first Doppler redshift was first described by French physicist [[Hippolyte Fizeau]] in 1848, who pointed tonoted the shift in [[spectral line]]s seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer [[William Huggins]] was the first to determine the velocity of a star moving away from the Earth by thisthe method.<ref name=Huggins>

}}</ref> 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>

}}</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>

[[Arthur Eddington]] used the term ''"red -shift''" as early as 1923.,<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> Thealthough the word does not appear unhyphenated until about 1934, bywhen [[Willem de Sitter]] used it.<ref>

Beginning with observations in 1912, [[Vesto Slipher]] discovered that most [[Spiralspiral galaxy|spiral galaxies]], then mostly thought to be [[Spiral galaxy#Spiral nebula|spiral nebulae]], had considerable redshifts. Slipher first reportsreported on his measurement in the inaugural volume of the ''[[Lowell Observatory]] Bulletin''.<ref>

}}</ref> In it he statesstated 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. Subsequently, [[Edwin Hubble]] discovered an approximate relationship between the redshifts of such "nebulae", and the [[distance]]s to them, with the formulation of his eponymous [[Hubble's law]].<ref>

}}</ref> [[Milton Humason]] worked on thesethose observations with Hubble.<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}}</ref> These observations corroborated [[Alexander Friedmann]]'s 1922 work, in which he derived the [[Friedmann equations|Friedmann–Lemaître equations]].<ref>{{cite journal

}} 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> InThey theare presentnow dayconsidered theyto are consideredbe strong evidence for an [[expanding universe]] and the [[Big Bang]] theory.<ref name=Eddington>This was recognized early on by physicists and astronomers working in cosmology in the 1930s. The earliest layman publication describing the details of this correspondence is {{cite book

[[File:High-redshift galaxy candidates in the Hubble Ultra Deep Field 2012.jpg|thumb|High-redshift galaxy candidates in the [[Hubble Ultra Deep Field]], 2012<ref>{{cite news|title=Hubble census finds galaxies at redshifts 9 to 12|url=https://esahubble.org/news/heic1219/|access-date=13 December 2012|newspaper=ESA/Hubble Press Release}}</ref> ]]

The [[visible spectrum|spectrum]] of light that comes from a source (see idealized spectrum illustration top-right) can be measured. To determine the redshift, one searches for features in the spectrum such as [[spectral line|absorption lines]], [[spectral line|emission lines]], or other variations in light intensity<!--Don't link to disambiguation page-->. If found, these features can be compared with known features in the spectrum of various chemical compounds found in experiments where that compound is located on Earth. A very common [[chemical element|atomic element]] in space is [[hydrogen]]. The spectrum of originally featureless light shone through hydrogen will show a [[hydrogen spectrum|signature spectrum]] specific to hydrogen that has features at regular intervals. If restricted to absorption lines it would look similar to the illustration (top right). If the same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. If the same spectral line is identified in both spectra—but at different wavelengths—then the redshift can be calculated using the table below.

is obtained. In an expanding universe such as the one we inhabit, the scale factor is [[monotonic function|monotonically increasing]] as time passes, thus, {{math|''z''}} is positive and distant galaxies appear redshifted.

:<math> t(z) \approx \frac {2}{3 H_0 {\Omega_0}^{1/2} (1+} z )^{-3/2}} \ , </math>

where {{math|''H''₀}} is the present-day [[Hubble constant]], and {{math|''z''}} is the redshift.<ref name="Bergström">{{cite book |title=Cosmology and Particle Astrophysics |url=https://books.google.com/books?id=CQYu_sutWAoC&pg=PA77 |page=77, Eq.4.79 |isbn=978-3-540-32924-4 |publisher=Springer |edition=2nd|date=2006|first1 = Lars |last1=Bergström|first2 = Ariel |last2=Goobar|author-link1=Lars Bergström (physicist) |author-link2=Ariel Goobar }}</ref><ref name = Longair>{{cite book |title=Galaxy Formation |first=M. S. |last=Longair |url=https://books.google.com/books?id=2ARuLT-tk5EC&pg=PA161 |page=161 |isbn=978-3-540-63785-1 |publisher=Springer |date=1998}}</ref><ref name=Sanchez>{{cite book |editor-first=Norma |editor-last=Sanchez |page=223 |title=Current Topics in Astrofundamental Physics |chapter-url=https://books.google.com/books?id=GOJoas-Dg7QC&pg=PA223 |isbn=978-0-7923-6856-4 |date=2001 |publisher=Springer |chapter=The High Redshift Radio Universe |author=Yu N Parijskij}}</ref>

There are several websites for calculating light-travelvarious distancetimes and distances from redshift, as the precise calculations require numerical integrals for most values of the parameters.<ref name="UCLA-2015">{{cite web |author=Staff |title=UCLA Cosmological Calculator |url=http://www.astro.ucla.edu/~wright/ACC.html |date=2015 |work=[[UCLA]] |access-date=6 August 2022 }} Light travel distance was calculated from redshift value using the UCLA Cosmological Calculator, with parameters values as of 2015: H₀=67.74 and Omega_M=0.3089 (see Table/Planck2015 at "[[Lambda-CDM model#Parameters]]" )</ref><ref name="UCLA-2018">{{cite web |author=Staff |title=UCLA Cosmological Calculator |url=http://www.astro.ucla.edu/~wright/ACC.html |date=2018 |work=[[UCLA]] |access-date=6 August 2022 }} Light travel distance was calculated from redshift value using the UCLA Cosmological Calculator, with parameters values as of 2018: H₀=67.4 and Omega_M=0.315 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]" )</ref><ref name="ICRAR-2022">{{cite web |author=Staff |title=ICRAR Cosmology Calculator |url=https://cosmocalc.icrar.org/ |date=2022 |work=[[International Centre for Radio Astronomy Research]] |access-date=6 August 2022 }} ICRAR Cosmology Calculator - Set H₀=67.4 and Omega_M=0.315 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]")</ref><ref name="KEMP-2022">{{cite web |last=Kempner |first=Joshua |title=KEMPNER Cosmology Calculator |url=https://www.kempner.net/cosmic.php |date=2022 |work=Kempner.net |access-date=6 August 2022 }} KEMP Cosmology Calculator - Set H₀=67.4, Omega_M=0.315, and Omega_Λ=0.6847 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]")</ref>

}}</ref> The resulting situation can be illustrated by the [[Expansion of the universe#Other conceptual models of expansion|Expanding Rubber Sheet Universe]], a common cosmological analogy used to describe the expansion of space. If two objects are represented by ball bearings and spacetime by a stretching rubber sheet, the Doppler effect is caused by rolling the balls across the sheet to create peculiar motion. The cosmological redshift occurs when the ball bearings are stuck to the sheet and the sheet is stretched.<ref name=Kuhn>{{cite book |title=In Quest of the Universe | first1=Theo | last1=Koupelis | first2=Karl F. | last2=Kuhn |edition=5th |url=https://archive.org/details/inquestofunivers00koup |url-access=registration |page=[https://archive.org/details/inquestofunivers00koup/page/557 557] |publisher=Jones & Bartlett Publishers |date=2007 |isbn=978-0-7637-4387-1}}</ref><ref name=Lewis>{{cite journal | quote=It is perfectly valid to interpret the equations of relativity in terms of an expanding space. The mistake is to push analogies too far and imbue space with physical properties that are not consistent with the equations of relativity. |title=Cosmological Radar Ranging in an Expanding Universe |arxiv=0805.2197 |journal=[[Monthly Notices of the Royal Astronomical Society]] | first1=Geraint F. | last1=Lewis |date=2008 |pages=960–964 |issue=3 |volume=388 |doi=10.1111/j.1365-2966.2008.13477.x |bibcode=2008MNRAS.388..960L|display-authors=4|last2=Francis |first2=Matthew J. |last3=Barnes |first3=Luke A. |last4=Kwan |first4=Juliana |last5=James |first5=J. Berian |doi-access=free |s2cid=15147382 }}</ref><ref name=Chodorowski>{{Cite journal | first=Michal | last=Chodorowski |title=Is space really expanding? A counterexample |date=2007 |arxiv=astro-ph/0601171 |journal=Concepts Phys |volume=4 |issue=1 |pages=17–34|bibcode = 2007ONCP....4...15C |doi = 10.2478/v10005-007-0002-2 |s2cid=15931627 }}</ref>

}}</ref> The redshift due to expansion of the universe depends upon the recessional velocity in a fashion determined by the cosmological model chosen to describe the expansion of the universe, which is very different from how Doppler redshift depends upon local velocity.<ref name="Harrison2">{{cite journal |last=Harrison |first=Edward |date=1992 |title=The redshift-distance and velocity-distance laws |journal=Astrophysical Journal, Part 1 |volume=403 |pages=28–31 |bibcode=1993ApJ...403...28H |doi=10.1086/172179 |doi-access=free }}. A pdf file can be found here [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ApJ...403...28H&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf].</ref> Describing the cosmological expansion origin of redshift, cosmologist [[Edward Robert Harrison]] said, "Light leaves a galaxy, which is stationary in its local region of space, and is eventually received by observers who are stationary in their own local region of space. Between the galaxy and the observer, light travels through vast regions of expanding space. As a result, all wavelengths of the light are stretched by the expansion of space. It is as simple as that..."<ref>{{Harvnb|Harrison|2000|p=302}}.</ref> [[Steven Weinberg]] clarified, "The increase of wavelength from emission to absorption of light does not depend on the rate of change of {{math|''a''(''t'')}} [here {{math|''a''(''t'')}} is the [[Scale factor (cosmology)|Robertson–Walker scale factor]]] at the times of emission or absorption, but on the increase of {{math|''a''(''t'')}} in the whole period from emission to absorption."<ref name=Weinberg_Cosmology>{{cite book |url=https://books.google.com/books?id=48C-ym2EmZkC&pg=PA11 |first=Steven | last=Weinberg |title=Cosmology |publisher=Oxford University Press |page=11 |date=2008 |isbn=978-0-19-852682-7}}</ref>

Spectroscopy, as a measurement, is considerably more difficult than simple [[photometry (astronomy)|photometry]], which measures the [[brightness]] of astronomical objects through certain [[Optical filter|filters]].<ref>For a review of the subject of photometry, consider: {{cite book | last=Budding | first=E. | title=Introduction to Astronomical Photometry | publisher=Cambridge University Press | date=September 24, 1993 | isbn=0-521-41867-4 }}</ref> When photometric data is all that is available (for example, the [[Hubble Deep Field]] and the [[Hubble Ultra Deep Field]]), astronomers rely on a technique for measuring [[photometric redshift]]s.<ref>The technique was first described by: {{cite conference | last=Baum | first=W. A. | year=1962 | editor-first=G. C. | editor-last=McVittie | title=Problems of extra-galactic research | page=390 | conference=IAU Symposium No. 15 }}</ref> Due to the broad wavelength ranges in photometric filters and the necessary assumptions about the nature of the spectrum at the light-source, [[observational error|errors]] for these sorts of measurements can range up to {{math|δ''z'' {{=}} 0.5}}, and are much less reliable than spectroscopic determinations.<ref>{{cite journal | last1=Bolzonella | first1=M. | last2=Miralles | first2=J.-M. | last3=Pelló | first3=R. | title=Photometric redshifts based on standard SED fitting procedures | journal=Astronomy and Astrophysics | volume=363 | pages=476–492 | year=2000 | arxiv=astro-ph/0003380 | bibcode=2000A&A...363..476B }}</ref>

However, photometry does at least allow a qualitative characterization of a redshift. For example, if a Sun-like spectrum had a redshift of {{math|''z'' {{=}} 1}}, it would be brightest in the [[infrared]](1000nm) rather than at the blue-green(500nm) color associated with the peak of its [[Black body|blackbody]] spectrum, and the light intensity will be reduced in the filter by a factor of four, {{math|(1 + ''z''){{sup|2}}}}. Both the photon count rate and the photon energy are redshifted. (See [[K correction]] for more details on the photometric consequences of redshift.)<ref>A pedagogical overview of the K-correction by David Hogg and other members of the [[Sloan Digital Sky Survey|SDSS]] collaboration can be found at: {{cite arXiv | title=The K correction | last1=Hogg | first1=David W. | last2=Baldry | first2=Ivan K. | last3=Blanton | first3=Michael R. | last4=Eisenstein | first4=Daniel J. | display-authors=1 | date=October 2002 | eprint=astro-ph/0210394}}</ref>

In nearby objects (within our [[Milky Way]] galaxy) observed redshifts are almost always related to the [[Line-of-sight propagation|line-of-sight]] velocities associated with the objects being observed. Observations of such redshifts and blueshifts have enabled astronomers to measure [[velocity|velocities]] and parametrize the [[mass]]es of the [[orbit]]ing [[star]]s in [[spectroscopic binaries]], a method first employed in 1868 by British astronomer [[William Huggins]].<ref name=Huggins/> Similarly, small redshifts and blueshifts detected in the spectroscopic measurements of individual stars are one way astronomers have been able to [[Methods of detecting exoplanets#Radial velocity|diagnose and measure]] the presence and characteristics of [[Exoplanet|planetary systems]] around other stars and have even made very [[Rossiter–McLaughlin effect|detailed differential measurements]] of redshifts during [[Methods of detecting exoplanets|planetary transits]] to determine precise orbital parameters.<ref>The [[Exoplanet Tracker]] is the newest observing project to use this technique, able to track the redshift variations in multiple objects at once, as reported in {{cite journal |last1=Ge |first1=Jian |last2=Van Eyken |first2=Julian |last3=Mahadevan |first3=Suvrath |author3-link=Suvrath Mahadevan |last4=Dewitt |first4=Curtis |last5=Kane |first5=Stephen R. |last6=Cohen |first6=Roger |last7=Vanden Heuvel |first7=Andrew |last8=Fleming |first8=Scott W. |last9=Guo |first9=Pengcheng |last10=Henry |first10=Gregory W. |last11=Schneider |first11=Donald P. |last12=Ramsey |first12=Lawrence W. |last13=Wittenmyer |first13=Robert A. |last14=Endl |first14=Michael |last15=Cochran |first15=William D. |display-authors=4 |date=2006 |title=The First Extrasolar Planet Discovered with a New‐GenerationNew-Generation High‐ThroughputHigh-Throughput Doppler Instrument |journal=The Astrophysical Journal |volume=648 |issue=1 |pages=683–695 |arxiv=astro-ph/0605247 |bibcode=2006ApJ...648..683G |doi=10.1086/505699 |s2cid=13879217 |last16=Ford |first16=Eric B. |last17=Martin |first17=Eduardo L. |last18=Israelian |first18=Garik |last19=Valenti |first19=Jeff |last20=Montes |first20=David}}</ref>

Finely detailed measurements of redshifts are used in [[helioseismology]] to determine the precise movements of the [[photosphere]] of the [[Sun]].<ref>{{cite journal | doi = 10.1007/BF00243557 | title = Solar and stellar seismology | date = 1988 | last1 = Libbrecht | first1 = Keng | journal = Space Science Reviews | volume = 47 | issue = 3–4 |bibcode=1988SSRv...47..275L | pages=275–301| s2cid = 120897051 | url = https://authors.library.caltech.edu/104214/1/1988SSRv___47__275L.pdf }}</ref> Redshifts have also been used to make the first measurements of the [[rotation]] rates of [[planet]]s,<ref>In 1871 [[Hermann Carl Vogel]] measured the rotation rate of [[Venus]]. [[Vesto Slipher]] was working on such measurements when he turned his attention to spiral nebulae.</ref> velocities of [[interstellar cloud]]s,<ref>An early review by [[Jan Hendrik Oort|Oort, J. H.]] on the subject: {{cite journal | title=The formation of galaxies and the origin of the high-velocity hydrogen | journal=[[Astronomy and Astrophysics]] | volume=7 | page=381 | date=1970 | bibcode=1970A&A.....7..381O | last= Oort | first= J. H. }}</ref> the [[Galaxy rotation curve|rotation of galaxies]],<ref name="basicastronomy" /> and the [[dynamics (mechanics)|dynamics]] of [[Accretion disk|accretion]] onto [[neutron star]]s and [[black hole]]s which exhibit both Doppler and gravitational redshifts.<ref>{{cite journal| last=Asaoka | first=Ikuko | bibcode=1989PASJ...41..763A | title=X-ray spectra at infinity from a relativistic accretion disk around a Kerr black hole | journal=Publications of the Astronomical Society of Japan | volume=41 | issue=4 | date=1989 | pages=763–778 }}</ref> Additionally, theThe [[temperature]]s of various emitting and absorbing objects can be obtained by measuring [[Doppler broadening]]—effectively redshifts and blueshifts over a single emission or absorption line.<ref>{{cite book | last1=Rybicki | first1=G. B. | first2=A. R. | last2=Lightman | title=Radiative Processes in Astrophysics | publisher=John Wiley & Sons | year=1979 | page=288 | isbn=0-471-82759-2 }}</ref> By measuring the broadening and shifts of the 21-centimeter [[hydrogen line]] in different directions, astronomers have been able to measure the [[Recessional velocity|recessional velocities]] of [[interstellar gas]], which in turn reveals the [[rotation curve]] of our Milky Way.<ref name=basicastronomy/> Similar measurements have been performed on other galaxies, such as [[Andromeda Galaxy|Andromeda]].<ref name=basicastronomy/> As a diagnostic tool, redshift measurements are one of the most important [[astronomical spectroscopy|spectroscopic measurements]] made in astronomy.

}}</ref> and 379,000 years after the initial moments of the [[Big Bang]].<ref>An accurate measurement of the cosmic microwave background was achieved by the [[Cosmic Background Explorer|COBE]] experiment. The final published temperature of 2.73 K was reported in this paper: {{cite journal | last1=Fixsen | first1=D. J. | last2=Cheng | first2=E. S. | last3=Cottingham | first3=D. A. | last4=Eplee | first4=R. E. Jr. | last5=Isaacman | first5=R. B. | last6=Mather | first6=J. C. | last7=Meyer | first7=S. S. | last8=Noerdlinger | first8=P. D. | last9=Shafer | first9=R. A. | last10=Weiss | first10=R. | last11=Wright | first11=E. L. | last12=Bennett | first12=C. L. | last13=Boggess | first13=N. W. | author-link13 = Nancy Boggess|last14=Kelsall | first14=T. | last15=Moseley | first15=S. H. | last16=Silverberg | first16=R. F. | last17=Smoot | first17=G. F. | last18=Wilkinson | first18=D. T. | date=January 1994 | title=Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument | journal=Astrophysical Journal | volume=420 | page=445 | doi=10.1086/173575 | bibcode=1994ApJ...420..445F }}. The most accurate measurement as of 2006 was achieved by the [[Wilkinson Microwave Anisotropy Probe|WMAP]] experiment.</ref>

For galaxies more distant than the [[Local Group]] and the nearby [[Virgo Cluster]], but within a thousand mega[[parsec]]s or so, the redshift is approximately proportional to the galaxy's distance. This correlation was first observed by [[Edwin Hubble]] and has come to be known as [[Hubble's law]]. [[Vesto Slipher]] was the first to discover galactic redshifts, in about the year 1912, while Hubble correlated Slipher's measurements with distances he [[cosmic distance ladder|measured by other means]] to formulate his Law.<ref name="Peebles-1993"/>

In the widely accepted cosmological model based on [[general relativity]], redshift is mainly a result of the expansion of space: this means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us. Hubble's law follows in part from the [[Copernican principle]].<ref name="Peebles-1993">Peebles (1993).</ref> Because it is usually not known how [[luminosity|luminous]] objects are, measuring the redshift is easier than more direct distance measurements, so redshift is sometimes in practice converted to a crude distance measurement using Hubble's law.{{cn|date=March 2023}}

Currently, the objects with the highest known redshifts are galaxies and the objects producing gamma ray bursts.{{cn|date=August 2024}} The most reliable redshifts are from [[spectroscopic]] data,{{cn|date=August 2024}} and the highest-confirmed spectroscopic redshift of a galaxy is that of [[JADES-GS-z13z14-0]] with a redshift of {{math|''z'' {{=}} 1314.232}}, corresponding to 300290 million years after the Big Bang.<ref>{{Cite journal |last1=Carniani |first1=Stefano |last2=Hainline |first2=Kevin |last3=D’Eugenio |first3=Francesco |last4=Eisenstein |first4=Daniel J. |last5=Jakobsen |first5=Peter |last6=Witstok |first6=Joris |last7=Johnson |first7=Benjamin D. |last8=Chevallard |first8=Jacopo |last9=Maiolino |first9=Roberto |last10=Helton |first10=Jakob M. |last11=Willott |first11=Chris |last12=Robertson |first12=Brant |last13=Alberts |first13=Stacey |last14=Arribas |first14=Santiago |last15=Baker |first15=William M. |date=2024-07-29 |title=Spectroscopic confirmation of two luminous galaxies at a redshift of 14 |journal=Nature |volume=633 |issue=8029 |language=en |pages=318–322 |doi=10.1038/s41586-024-07860-9 |issn=1476-4687|doi-access=free |pmid=39074505 |pmc=11390484 }}</ref> The previous record was held by [[GN-z11]],<ref>{{cite journal

}}</ref> at a redshift of {{math|''z'' {{=}} 8.6}}, corresponding to 600 million years after the Big Bang.

Slightly less reliable are [[Lyman-break galaxy|Lyman-break]] redshifts, the highest of which is the lensed galaxy A1689-zD1 at a redshift {{math|''z'' {{=}} 7.5}}<ref>{{Cite journal|last1=Watson|first1=Darach|last2=Christensen|first2=Lise|last3=Knudsen|first3=Kirsten Kraiberg|last4=Richard|first4=Johan|last5=Gallazzi|first5=Anna|last6=Michałowski|first6=Michał Jerzy|title=A dusty, normal galaxy in the epoch of reionization|journal=Nature|volume=519|issue=7543|pages=327–330|doi=10.1038/nature14164|arxiv = 1503.00002 |bibcode = 2015Natur.519..327W|pmid=25731171|year=2015|s2cid=2514879}}</ref><ref>{{cite journal

| last3=Campana | first3=S. |author-link3=Sergio Campana (astrophysicist)| last4=Chincarini | first4=G.

}}</ref> The most distant-known quasar, [[ULAS J1342+0928]], is at {{math|''z'' {{=}} 7.54}}.<ref>{{cite web|url=https://news.mit.edu/2017/scientists-observe-supermassive-black-hole-infant-universe-1206|title=Scientists observe supermassive black hole in infant universe|website=MIT News |publisher=Massachusetts Institute of Technology |date=2017-12-06 |first=Jennifer |last=Chu}}</ref><ref name="Nature-2018-01">{{cite journal |last1=Bañados |first1=Eduardo |last2=Venemans |first2=Bram P. |last3=Mazzucchelli |first3=Chiara |last4=Farina |first4=Emanuele P. |last5=Walter |first5=Fabian |last6=Wang |first6=Feige |last7=Decarli |first7=Roberto |last8=Stern |first8=Daniel |last9=Fan |first9=Xiaohui |last10=Davies |first10=Frederick B. |last11=Hennawi |first11=Joseph F. |last12=Simcoe |first12=Robert A. |last13=Turner |first13=Monica L. |last14=Rix |first14=Hans-Walter |last15=Yang |first15=Jinyi |last16=Kelson |first16=Daniel D. |last17=Rudie |first17=Gwen C. |last18=Winters |first18=Jan Martin |title=An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5 |journal=Nature |date=January 2018 |volume=553 |issue=7689 |pages=473–476 |doi=10.1038/nature25180 |pmid=29211709 |arxiv=1712.01860 |bibcode=2018Natur.553..473B |s2cid=205263326 }}</ref> The highest-known redshift radio galaxy (TGSS1530) is at a redshift {{math|''z'' {{=}} 5.72}}<ref>{{cite journal|last1=Saxena|first1=A.|date=2018|title=Discovery of a radio galaxy at z = 5.72|journal=Monthly Notices of the Royal Astronomical Society|volume=480|issue=2|pages=2733–2742|arxiv=1806.01191|bibcode=2018MNRAS.480.2733S|doi=10.1093/mnras/sty1996|doi-access=free |s2cid=118830412}}</ref> and the highest-known redshift molecular material is the detection of emission from the CO molecule from the quasar SDSS J1148+5251 at {{math|''z'' {{=}} 6.42}}.<ref>{{cite journal | doi = 10.1038/nature01821 | title = Molecular gas in the host galaxy of a quasar at redshift z = 6.42 | date = 2003 | last1 = Walter | first1 = Fabian | last2 = Bertoldi | first2 = Frank | last3 = Carilli | first3 = Chris | last4 = Cox | first4 = Pierre | last5 = Lo | first5 = K. Y. | last6 = Neri | first6 = Roberto | last7 = Fan | first7 = Xiaohui | last8 = Omont | first8 = Alain | last9 = Strauss | first9 = Michael A. | last10 = Menten | first10 = Karl M. | journal = Nature | volume = 424 | issue = 6947 | pages = 406–8 | pmid = 12879063 |bibcode=2003Natur.424..406W|arxiv = astro-ph/0307410 |s2cid = 4419009| display-authors = 4 }}</ref>

{{cite journal | last1 = Lineweaver | first1 = Charles | first2=Tamara M. | last2=Davis | date = 2005 | title = Misconceptions about the Big Bang | journal = Scientific American | volume = 292 | issue = 3 | pages = 36–45 | doi = 10.1038/scientificamerican0305-36 | bibcode = 2005SciAm.292c..36L }}</ref> The yet-to-be-observed first light from the oldest [[Population III stars]], not long after atoms first formed and the CMB ceased to be absorbed almost completely, may have redshifts in the range of {{math|20 < ''z'' < 100}}.<ref>{{cite journal|bibcode=2006MNRAS.373L..98N|arxiv = astro-ph/0604050 |doi = 10.1111/j.1745-3933.2006.00251.x|title=The first stars in the Universe|date=2006|last1=Naoz|first1=S.|last2=Noter|first2=S.|last3=Barkana|first3=R.|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=373|issue = 1 |pages=L98–L102 |doi-access = free |s2cid = 14454275 }}</ref> Other high-redshift events predicted by physics but not presently observable are the [[cosmic neutrino background]] from about two seconds after the Big Bang (and a redshift in excess of {{math|''z'' > 10{{sup|10}}}})<ref>{{cite journal|bibcode=2006PhR...429..307L|arxiv = astro-ph/0603494 |doi = 10.1016/j.physrep.2006.04.001|title=Massive neutrinos and cosmology|date=2006|last1=Lesgourgues|first1=J|last2=Pastor|first2=S|journal=Physics Reports|volume=429|issue=6|pages=307–379 |s2cid = 5955312 }}</ref> and the cosmic [[gravitational wave background]] emitted directly from [[inflation (cosmology)|inflation]] at a redshift in excess of {{math|''z'' > 10{{sup|25}}}}.<ref>{{cite journal|bibcode=2005PhyU...48.1235G|arxiv = gr-qc/0504018 |doi = 10.1070/PU2005v048n12ABEH005795|title=Relic gravitational waves and cosmology|date=2005|last1=Grishchuk|first1=Leonid P|journal=Physics-Uspekhi|volume=48|issue=12|pages=1235–1247 |s2cid = 11957123 }}</ref>

| doi-access=free | s2cid=6906627| display-authors=4

[[Doppler effect|Doppler]] blueshift is caused by movement of a source towards the observer. The term applies to any decrease in wavelength and increase in frequency caused by relative motion, even outside the [[visible spectrum]]. Only objects moving at near-[[Relativistic speed|relativistic speeds]] toward the observer are noticeably bluer to the [[naked eye]], but the wavelength of any reflected or emitted photon or other particle is shortened in the direction of travel.<ref>{{cite book|title=In Quest of the Universe | first1=Karl F. | last1=Kuhn | first2=Theo | last2=Koupelis |year= 2004|publisher=[[Jones & Bartlett Publishers]]|isbn=978-0-7637-0810-8|pages=122–3}}</ref>

There are faraway [[active galaxies]] that show a blueshift in their [[Oxygen|[O III]]] emission [[Emission spectrum|lines]]. One of the largest blueshifts is found in the narrow-line [[quasar]], [[PG 1543+489]], which has a relative velocity of -1150&nbsp;km/s.<ref name="Aoki2005" /> These types of galaxies are called "blue outliers".<ref name="Aoki2005" />

[[Category:Effects of gravitationgravity]]