Editing Redshift (section)

==Observations in astronomy==
[[File:Look-back time by redshift.png|thumb|The [[lookback time]] of extragalactic observations by their redshift up to z=20.<ref name="Pilipenko">{{cite arXiv |eprint=1303.5961 |last1=Pilipenko |first1=Sergey V. |title=Paper-and-pencil cosmological calculator |date=2013 |class=astro-ph.CO }} Including [https://code.google.com/archive/p/cosmonom/downloads Fortran-90 code] upon which the citing charts and formulae are based.</ref> There are websites for calculating many such physical measures from redshift.<ref name="UCLA-2018"/><ref name="ICRAR-2022"/>]]

The redshift observed in astronomy can be measured because the [[emission spectrum|emission]] and [[Absorption spectroscopy|absorption]] spectra for [[atom]]s are distinctive and well known, calibrated from [[spectroscopic]] experiments in [[laboratories]] on Earth. When the redshifts of various absorption and emission lines from a single astronomical object are measured, {{math|''z''}} is found to be remarkably constant. Although distant objects may be slightly blurred and lines broadened, it is by no more than can be explained by [[Kinetic theory of gases|thermal]] or mechanical [[motion]] of the source. For these reasons and others, the consensus among astronomers is that the redshifts they observe are due to some combination of the three established forms of Doppler-like redshifts. Alternative hypotheses and explanations for redshift such as [[tired light]] are not generally considered plausible.<ref name=reboul>When cosmological redshifts were first discovered, [[Fritz Zwicky]] proposed an effect known as tired light. While usually considered for historical interests, it is sometimes, along with [[intrinsic redshift]] suggestions, utilized by [[nonstandard cosmologies]]. In 1981, H. J. Reboul summarised many [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1981A%26AS...45..129R&db_key=AST&data_type=HTML&format=&high=42ca922c9c23806 alternative redshift mechanisms] that had been discussed in the literature since the 1930s. In 2001, [[Geoffrey Burbidge]] remarked in a [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001PASP..113..899B&db_key=AST&data_type=HTML review] that the wider astronomical community has marginalized such discussions since the 1960s. Burbidge and [[Halton Arp]], while investigating the mystery of [[Quasar#History of observation and interpretation|the nature of quasars]], tried to develop alternative redshift mechanisms, and very few of their fellow scientists acknowledged let alone accepted their work. Moreover, {{cite journal | title=Timescale Stretch Parameterization of Type Ia Supernova B-Band Lightcurves | first1=G. | last1=Goldhaber | first2=D. E. | last2=Groom | first3=A. | last3=Kim | first4=G. | last4=Aldering | first5=P. | last5=Astier | first6=A. | last6=Conley | first7=S. E. | last7=Deustua | first8=R. | last8=Ellis | first9=S. | last9=Fabbro | first10=A. S. | last10=Fruchter | first11=A. | last11=Goobar | first12=I. | last12=Hook | first13=M. | last13=Irwin | first14=M. | last14=Kim | first15=R. A. | last15=Knop | first16=C. | last16=Lidman | first17=R. | last17=McMahon | first18=P. E. | last18=Nugent | first19=R. | last19=Pain | first20=N. | last20=Panagia | first21=C. R. | last21=Pennypacker | first22=S. | last22=Perlmutter | first23=P. | last23=Ruiz-Lapuente | first24=B. | last24=Schaefe | first25=N. A. | last25=Walton | first26=T. | last26=York | display-authors=1 | year=2001 | journal=Astrophysical Journal | volume=558 | issue=1 | pages=359–386 | doi=10.1086/322460 | arxiv=astro-ph/0104382 | bibcode=2001ApJ...558..359G | s2cid=17237531| doi-access=free }} pointed out that alternative theories are unable to account for timescale stretch observed in [[type Ia supernovae]]</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>

Determining the redshift of an object with spectroscopy requires the wavelength of the emitted light in the rest frame of the source. Astronomical applications rely on distinct spectral lines. Redshifts cannot be calculated by looking at unidentified features whose rest-frame frequency is unknown, or with a spectrum that is featureless or [[white noise]] (random fluctuations in a spectrum). Thus [[gamma-ray burst]]s themselves cannot be used for reliable redshift measurements, but optical afterglow associated with the burst can be analyzed for redshifts.<ref>{{Cite web |title=Swift: About Swift |url=https://swift.gsfc.nasa.gov/about_swift/redshift.html |access-date=2025-04-07 |website=swift.gsfc.nasa.gov}}</ref>

===Local observations===
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 enable astronomers to measure [[velocity|velocities]] and parametrize the [[mass]]es of the [[orbit]]ing [[star]]s in [[spectroscopic binaries]]. 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. Some approaches are able to track the redshift variations in multiple objects at once.<ref>{{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-Generation High-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> The [[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.

===Extragalactic observations===

The most distant objects exhibit larger redshifts corresponding to the [[Hubble flow]] of the [[universe]]. The largest-observed redshift, corresponding to the greatest distance and furthest back in time, is that of the [[cosmic microwave background]] radiation; the [[Hubble's law#Redshift velocity|numerical value of its redshift]] is about {{math|''z'' {{=}} 1089}} ({{math|''z'' {{=}} 0}} corresponds to present time), and it shows the state of the universe about 13.8 billion years ago,<ref>{{cite web
 | title=Cosmic Detectives
 | url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
 | publisher=The European Space Agency (ESA)
 | date=2013-04-02
 | access-date=2013-04-25
}}</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>

The luminous point-like cores of [[quasar]]s were the first "high-redshift" ({{math|''z'' > 0.1}}) objects discovered before the improvement of telescopes allowed for the discovery of other high-redshift galaxies.<ref name="Kellermann">{{cite journal |last1=Kellermann |first1=K.I. |title=The Discovery of Quasars and its Aftermath |journal=Journal of Astronomical History and Heritage |date=2014 |volume=17 |issue=3 |pages=267–282 |doi=10.3724/SP.J.1440-2807.2014.03.03 |arxiv=1304.3627 }}</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 1912, while Hubble correlated Slipher's measurements with distances he [[cosmic distance ladder|measured by other means]] to formulate his law.{{sfn|Peebles|1993|pp=78–79}} 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.<ref>{{Cite web |last=Halstead |first=Evan |date=2021-08-16 |title=Introduction to General Relativity: 7.3: Redshift |url=https://phys.libretexts.org/Courses/Skidmore_College/Introduction_to_General_Relativity/07:_Cosmology/7.03:_Redshift |access-date=2025-03-06 |website=Physics LibreTexts |language=en}}</ref>

[[Gravitation]]al interactions of galaxies with each other and clusters cause a significant [[variance|scatter]] in the normal plot of the Hubble diagram. The [[peculiar velocity|peculiar velocities]] associated with galaxies superimpose a rough trace of the [[mass]] of [[virial theorem|virialized objects]] in the universe. This effect leads to such phenomena as nearby galaxies (such as the [[Andromeda Galaxy]]) exhibiting blueshifts as we fall towards a common [[barycenter]], and redshift maps of clusters showing a [[fingers of god]] effect due to the scatter of peculiar velocities in a roughly spherical distribution.{{sfn|Peebles|1993|p=34}} These "redshift-space distortions" can be used as a cosmological probe in their own right, providing information on how structure formed in the Universe,<ref>{{cite journal|last1=Percival|first1=Will J.|last2=White|first2=Martin|title=Testing cosmological structure formation using redshift-space distortions|journal=Monthly Notices of the Royal Astronomical Society|date=11 February 2009|volume=393|issue=1|pages=297–308|doi=10.1111/j.1365-2966.2008.14211.x|doi-access=free |arxiv = 0808.0003 |bibcode = 2009MNRAS.393..297P }}</ref> and how gravity behaves on large scales.<ref>{{cite journal|last1=Raccanelli|first1=A.|last2=Bertacca|first2=D.|last3=Pietrobon|first3=D.|last4=Schmidt|first4=F.|last5=Samushia|first5=L.|last6=Bartolo|first6=N.|last7=Dore|first7=O.|last8=Matarrese|first8=S.|last9=Percival|first9=W. J.|title=Testing gravity using large-scale redshift-space distortions|journal=Monthly Notices of the Royal Astronomical Society|date=25 September 2013|volume=436|issue=1|pages=89–100|doi=10.1093/mnras/stt1517|doi-access=free |arxiv = 1207.0500 |bibcode = 2013MNRAS.436...89R }}</ref>

The Hubble law's linear relationship between distance and redshift assumes that the rate of expansion of the universe is constant. However, when the universe was much younger, the expansion rate, and thus the Hubble "constant", was larger than it is today. For more distant galaxies, then, whose light has been travelling to us for much longer times, the approximation of constant expansion rate fails, and the Hubble law becomes a non-linear integral relationship and dependent on the history of the expansion rate since the emission of the light from the galaxy in question. Observations of the redshift-distance relationship can be used, then, to determine the expansion history of the universe and thus the matter and energy content.<ref>{{Cite web |last=Knox |first=Lloyd |date=2016-12-22 |title=Physics 156: A Cosmology Workbook: 1.7: The Distance-Redshift Relation |url=https://phys.libretexts.org/Courses/University_of_California_Davis/Physics_156:_A_Cosmology_Workbook/01:_Workbook/1.07:_The_Distance-Redshift_Relation |access-date=2025-03-06 |website=Physics LibreTexts |language=en}}</ref>

While it was long believed that the expansion rate has been continuously decreasing since the Big Bang, observations beginning in 1988 of the redshift-distance relationship using [[Type Ia supernova]]e have suggested that in comparatively recent times the expansion rate of the universe has [[Accelerating expansion of the universe|begun to accelerate]].<ref>{{cite web|url=https://www.nobelprize.org/uploads/2019/05/popular-physicsprize2011.pdf |title=The Nobel Prize in Physics 2011: Information for the Public |website=nobelprize.org |access-date=2023-06-13}}</ref>

===Highest redshifts===
{{see also|List of the most distant astronomical objects#List of most distant objects by type{{!}}List of most distant objects by type}}
[[File:Comoving distance and lookback time (Planck 2018).png|thumb|upright=1.8|[[Comoving and proper distances|Comoving distance]] and [[lookback time]] for the Planck 2018 cosmology parameters, from redshift 0 to 15, with distance (blue solid line) on the left axis, and time (orange dashed line) on the right. Note that the time that has passed (in giga years) from a given redshift until now is not the same as the distance (in giga light years) light would have traveled from that redshift, due to the expansion of the universe over the intervening period.]]

The most reliable redshifts are from [[spectroscopic]] data,<ref>{{Cite web |title=Redshift |url=https://lco.global/spacebook/light/redshift/ |access-date=2025-03-06 |website=lco.global |publisher=[[Las Cumbres Observatory]] |language=en}}</ref> and the highest-confirmed spectroscopic redshift of a galaxy is that of [[JADES-GS-z14-0]] with a redshift of {{math|''z'' {{=}} 14.32}}, corresponding to 290 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 |arxiv=2405.18485 |bibcode=2024Natur.633..318C }}</ref> The previous record was held by [[GN-z11]],<ref>{{cite journal
 | title=A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy
 | last1=Oesch | first1=P. A. | last2=Brammer | first2=G.
 | last3=van Dokkum | first3=P. G. | last4=Illingworth | first4=G. D.
 | last5=Bouwens | first5=R. J. | last6=Labbé | first6=I.
 | last7=Franx | first7=M. | last8=Momcheva | first8=I.
 | last9=Ashby | first9=M. L. N. | last10=Fazio | first10=G. G.
 | last11=Gonzalez | first11=V. | last12=Holden | first12=B.
 | last13=Magee | first13=D. | last14=Skelton | first14=R. E.
 | last15=Smit | first15=R. | last16=Spitler | first16=L. R.
 | last17=Trenti | first17=M. | last18=Willner | first18=S. P.
 | display-authors=1 | journal=The Astrophysical Journal
 | date=March 1, 2016 | volume=819 | issue=2 | page=129
 | arxiv=1603.00461 | doi=10.3847/0004-637X/819/2/129
 | bibcode=2016ApJ...819..129O | s2cid=119262750
| doi-access=free }}</ref> with a redshift of {{math|''z'' {{=}} 11.1}}, corresponding to 400 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
 | title=Discovery of a Very Bright Strongly Lensed Galaxy Candidate at z ~ 7.6
 | first1=L. D. | last1=Bradley | first2=R. J. | last2=Bouwens
 | first3=H. C. | last3=Ford | first4=G. D. | last4=Illingworth
 | first5=M. J. | last5=Jee | first6=N. | last6=Benítez
 | first7=T. J. | last7=Broadhurst | first8=M. | last8=Franx
 | first9=B. L. | last9=Frye | first10=L. | last10=Infante
 | display-authors=1 | journal=[[The Astrophysical Journal]]
 | volume=678 | issue=2 | pages=647–654 | year=2008
 | bibcode=2008ApJ...678..647B | s2cid=15574239
 | doi=10.1086/533519 | arxiv=0802.2506
}}</ref> and the next highest being {{math|''z'' {{=}} 7.0}}.<ref>{{cite journal
 | display-authors=1 | first1=E. | last1=Egami
 | first2=J.-P. | last2=Kneib | first3=G. H. | last3=Rieke
 | first4=R. S. | last4=Ellis | first5=J. | last5=Richard
 | first6=J. | last6=Rigby | first7=C. | last7=Papovich
 | first8=D. | last8=Stark | first9=M. R. | last9=Santos
 | first10=J.-S. | last10=Huang | first11=H. | last11=Dole
 | first12=E. Le | last12=Floc'H | first13=P. G. | last13=Pérez-González
 | title=Spitzer and Hubble Space Telescope Constraints on the Physical Properties of the z~7 Galaxy Strongly Lensed by A2218
 | journal=[[The Astrophysical Journal]]
 | volume=618 | issue=1 | pages=L5–L8 | year=2005
 | bibcode=2005ApJ...618L...5E | doi=10.1086/427550
 | arxiv=astro-ph/0411117 | s2cid=15920310 }}</ref> The most distant-observed [[gamma-ray burst]] with a spectroscopic redshift measurement was [[GRB 090423]], which had a redshift of {{math|''z'' {{=}} 8.2}}.<ref>{{cite journal
 | title=GRB 090423 reveals an exploding star at the epoch of re-ionization
 | last1=Salvaterra | first1=R. | first2=M. Della | last2=Valle
 | last3=Campana | first3=S. |author-link3=Sergio Campana (astrophysicist)| last4=Chincarini | first4=G.
 | last5=Covino | first5=S. | last6=d'Avanzo | first6=P.
 | last7=Fernández-Soto | first7=A. | last8=Guidorzi | first8=C.
 | last9=Mannucci | first9=F. | last10=Margutti | first10=R.
 | last11=Thöne | first11=C. C. | last12=Antonelli | first12=L. A.
 | last13=Barthelmy | first13=S. D. | last14=De Pasquale | first14=M.
 | last15=d'Elia | first15=V. | last16=Fiore | first16=F.
 | last17=Fugazza | first17=D. | last18=Hunt | first18=L. K.
 | last19=Maiorano | first19=E. | last20=Marinoni | first20=S.
 | last21=Marshall | first21=F. E. | last22=Molinari | first22=E.
 | last23=Nousek | first23=J. | last24=Pian | first24=E.
 | last25=Racusin | first25=J. L. | last26=Stella | first26=L.
 | last27=Amati | first27=L. | last28=Andreuzzi | first28=G.
 | last29=Cusumano | first29=G. | last30=Fenimore | first30=E. E.
 | display-authors=4 | journal=[[Nature (journal)|Nature]]
 | volume=461 | issue=7268 | pages=1258–60
 | doi=10.1038/nature08445 | date=2009 | pmid=19865166
 | s2cid=205218263 | bibcode=2009Natur.461.1258S |arxiv=0906.1578
 }}</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>

''Extremely red objects'' (EROs) are [[Radio astronomy#Astronomical sources|astronomical sources]] of radiation that radiate energy in the red and near infrared part of the electromagnetic spectrum. These may be starburst galaxies that have a high redshift accompanied by reddening from intervening dust, or they could be highly redshifted elliptical galaxies with an older (and therefore redder) stellar population.<ref>
{{cite journal
 | display-authors=4
 | author=Smail, Ian
 | author2=Owen, F. N.
 | author3=Morrison, G. E.
 | author4=Keel, W. C.
 | author5=Ivison, R. J.
 | author6=Ledlow, M. J.
 | journal=The Astrophysical Journal | volume=581 | issue=2
 | pages=844–864 | doi=10.1086/344440 | bibcode=2002ApJ...581..844S
 | title=The Diversity of Extremely Red Objects
 | date=2002
|arxiv = astro-ph/0208434 | s2cid=51737034
 }}</ref> Objects that are even redder than EROs are termed ''hyper extremely red objects'' (HEROs).<ref>
{{cite journal
 | display-authors=4
 | author=Totani, Tomonori
 | author2=Yoshii, Yuzuru
 | author3=Iwamuro, Fumihide
 | author4=Maihara, Toshinori
 | author5=Motohara, Kentaro
 | title=Hyper Extremely Red Objects in the Subaru Deep Field: Evidence for Primordial Elliptical Galaxies in the Dusty Starburst Phase
 | journal=The Astrophysical Journal | volume=558 | issue=2
 | date=2001 | pages=L87–L91 | doi=10.1086/323619
 | bibcode=2001ApJ...558L..87T
|arxiv = astro-ph/0108145 | s2cid=119511017
 }}</ref>

The [[cosmic microwave background]] has a redshift of {{math|z {{=}} 1089}}, corresponding to an age of approximately 379,000 years after the Big Bang and a [[Comoving and proper distances|proper distance]] of more than 46 billion light-years.<ref name="ly93">
{{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> This redshift corresponds to a shift in average temperature from 3000K down to 3K.<ref>{{cite journal|last1=Gawiser|first1=E.|last2=Silk|first2=J.|date=2000|title=The cosmic microwave background radiation|journal=[[Physics Reports]]|volume=333–334|issue=2000|pages=245–267|doi=10.1016/S0370-1573(00)00025-9|arxiv=astro-ph/0002044|bibcode = 2000PhR...333..245G |citeseerx=10.1.1.588.3349|s2cid=15398837}}</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>

In June 2015, astronomers reported evidence for [[Stellar population#Population III stars|Population III stars]] in the [[Cosmos Redshift 7]] [[galaxy]] at {{math|''z'' {{=}} 6.60}}. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of [[chemical element]]s heavier than [[hydrogen]] that are needed for the later formation of [[planet]]s and [[life]] as we know it.<ref name="AJ-20150604">{{cite journal |last1=Sobral |first1=David |last2=Matthee |first2=Jorryt |last3=Darvish |first3=Behnam |last4=Schaerer |first4=Daniel |last5=Mobasher |first5=Bahram |last6=Röttgering |first6=Huub J. A. |last7=Santos |first7=Sérgio |last8=Hemmati |first8=Shoubaneh |title=Evidence For POPIII-Like Stellar Populations In The Most Luminous LYMAN-α Emitters At The Epoch Of Re-Ionisation: Spectroscopic Confirmation |date=4 June 2015 |journal=[[The Astrophysical Journal]] |doi=10.1088/0004-637x/808/2/139 |bibcode=2015ApJ...808..139S |volume=808 |issue=2 |page=139|arxiv=1504.01734|s2cid=18471887 }}</ref><ref name="NYT-20150617">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Astronomers Report Finding Earliest Stars That Enriched Cosmos |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |date=17 June 2015 |work=[[The New York Times]] |access-date=17 June 2015 }}</ref>

===Redshift surveys===
{{Main|Redshift survey}}
[[File:2dfgrs.png|thumb|Rendering of the 2dFGRS data]]
With advent of automated [[telescope]]s and improvements in [[astronomical spectroscopy|spectroscopes]], a number of collaborations have been made to map the universe in redshift space. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the [[Observable universe|large-scale structure]] of the universe. The [[CfA2 Great Wall|Great Wall]], a vast [[supercluster]] of galaxies over 500 million [[light-year]]s wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.<ref>{{cite journal | title=Mapping the Universe | first1=M. J. | last1=Geller | first2=J. P. | last2=Huchra | journal=Science | volume=246 | issue=4932 | pages=897–903 | year=1989 | doi=10.1126/science.246.4932.897 | pmid=17812575 | bibcode=1989Sci...246..897G | s2cid=31328798 }}</ref>

The first redshift survey was the [[CfA Redshift Survey]], started in 1977 with the initial data collection completed in 1982.<ref>See the CfA website for more details: {{cite web
 | title=The CfA Redshift Survey
 | first=John P. | last=Huchra | author-link=John Huchra
 | publisher=Harvard & Smithsonian Center for Astrophysics
 | url=https://lweb.cfa.harvard.edu/~dfabricant/huchra/zcat/
 | access-date=2023-03-20
}}</ref> More recently, the [[2dF Galaxy Redshift Survey]] determined the large-scale structure of one section of the universe, measuring redshifts for over 220,000 galaxies; data collection was completed in 2002, and the final [[data set]] was released 30 June 2003.<ref>{{cite journal
 |title=The 2dF galaxy redshift survey: Power-spectrum analysis of the final dataset and cosmological implications
 | first1=Shaun | last1=Cole | author-link=Shaun Cole
 | last2=Percival | first2=Will J. | last3=Peacock | first3=John A.
 | last4=Norberg | first4=Peder | last5=Baugh | first5=Carlton M.
 | last6=Frenk | first6=Carlos S. | last7=Baldry | first7=Ivan
 | last8=Bland-Hawthorn | first8=Joss | last9=Bridges | first9=Terry
 | last10=Cannon | first10=Russell | last11=Colless | first11=Matthew
 | last12=Collins | first12=Chris | last13=Couch | first13=Warrick
 | last14=Cross | first14=Nicholas J. G. | last15=Dalton | first15=Gavin
 | last16=Eke | first16=Vincent R. | last17=De Propris | first17=Roberto
 | last18=Driver | first18=Simon P. | last19=Efstathiou | first19=George
 | last20=Ellis | first20=Richard S. | last21=Glazebrook | first21=Karl
 | last22=Jackson | first22=Carole | last23=Jenkins | first23=Adrian
 | last24=Lahav | first24=Ofer | last25=Lewis | first25=Ian
 | last26=Lumsden | first26=Stuart | last27=Maddox | first27=Steve
 | last28=Madgwick | first28=Darren | last29=Peterson | first29=Bruce A.
 | last30=Sutherland | first30=Will | last31=Taylor | first31=Keith
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=362 | issue=2 | pages=505–34 | date=2005
 | bibcode=2005MNRAS.362..505C | arxiv=astro-ph/0501174
 | doi=10.1111/j.1365-2966.2005.09318.x
 | doi-access=free | s2cid=6906627| display-authors=4
}} [http://msowww.anu.edu.au/2dFGRS/ 2dF Galaxy Redshift Survey homepage] {{Webarchive|url=https://web.archive.org/web/20070205010241/http://msowww.anu.edu.au/2dFGRS/ |date=2007-02-05 }}</ref> The [[Sloan Digital Sky Survey]] (SDSS) began collecting data in 1998<ref>{{cite journal |last1=Gunn |first1=James E. |last2=Siegmund |first2=Walter A. |last3=Mannery |first3=Edward J. |last4=Owen |first4=Russell E. |last5=Hull |first5=Charles L. |last6=Leger |first6=R. French |display-authors=etal |date=April 2006 |title=The 2.5 m Telescope of the Sloan Digital Sky Survey |journal=The Astronomical Journal |volume=131 |issue=4 |pages=2332–2359 |doi=10.1086/500975 |doi-access=free |arxiv=astro-ph/0602326 |bibcode=2006AJ....131.2332G }}</ref> and published its eighteenth data release in 2023.<ref>{{cite journal|last1=Almeida |first1=Andrés |display-authors=etal |title=The Eighteenth Data Release of the Sloan Digital Sky Surveys: Targeting and First Spectra from SDSS-V |journal=The Astrophysical Journal Supplement Series |date=2023 |volume=267 |number=2 |page=44 |doi=10.3847/1538-4365/acda98 |doi-access=free |arxiv=2301.07688 |bibcode=2023ApJS..267...44A}}</ref> SSDS has measured redshifts for galaxies as high as 0.8, and has recorded over 100,000 [[quasar]]s at {{math|''z'' {{=}} 3}} and beyond.<ref>{{cite web|url=https://www.sdss4.org/science/ |title=Science Results |website=SSDS |access-date=2025-05-20 }}</ref> The [[DEEP2 Redshift Survey]] used the [[Keck telescopes]] with the "DEIMOS" [[spectrograph]]; a follow-up to the pilot program DEEP1, DEEP2 was designed to measure faint galaxies with redshifts 0.7 and above, and it recorded redshifts of over 38,000 objects by its conclusion in 2013.<ref>{{cite conference | title=Science objectives and early results of the DEEP2 redshift survey| first1=Marc | last1=Davis |collaboration=DEEP2 collaboration |date=2002 | conference=Conference on Astronomical Telescopes and Instrumentation, Waikoloa, Hawaii, 22–28 Aug 2002 | arxiv=astro-ph/0209419 | bibcode=2003SPIE.4834..161D | doi=10.1117/12.457897 }}</ref><ref>{{cite journal|first1=Jeffrey A. |last1=Newman |display-authors=etal |title=The DEEP2 Galaxy Redshift Survey: Design, Observations, Data Reduction, and Redshifts |journal=The Astrophysical Journal Supplement Series |year=2013 |volume=208 |number=1 |page=5 |doi=10.1088/0067-0049/208/1/5|arxiv=1203.3192 |bibcode=2013ApJS..208....5N }}</ref>