Editing Redshift (section)

===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>