Editing Gravitational redshift (section)

=== Astronomical observations ===
{{see also|Tests of general relativity}}

A number of experimenters initially claimed to have identified the effect using astronomical measurements, and the effect was considered to have been finally identified in the spectral lines of the star [[Sirius B]] by [[Walter Sydney Adams|W.S. Adams]] in 1925.<ref name= "Hetherington1980">Hetherington, N. S., [http://adsabs.harvard.edu/full/1980QJRAS..21..246H "Sirius B and the gravitational redshift - an historical review"], ''Quarterly Journal Royal Astronomical Society'', vol. 21, Sept. 1980, pp. 246–252. Accessed 6 April 2017.</ref> However, measurements by Adams have been criticized as being too low<ref name= "Hetherington1980"/><ref name= "Holberg2010">Holberg, J. B., [http://articles.adsabs.harvard.edu//full/2010JHA....41...41H/0000041.000.html "Sirius B and the Measurement of the Gravitational Redshift"], ''Journal for the History of Astronomy'', vol. 41, 1, 2010, pp. 41–64. Accessed 6 April 2017.</ref> and these observations are now considered to be measurements of spectra that are unusable because of scattered light from the primary, Sirius A.<ref name= "Holberg2010" /> The first accurate measurement of the gravitational redshift of a white dwarf was done by Popper in 1954, measuring a 21&nbsp;km/s gravitational redshift of [[40 Eridani]] B.<ref name= "Holberg2010" /> The redshift of [[Sirius|Sirius B]] was finally measured by Greenstein ''et al.'' in 1971, obtaining the value for the gravitational redshift of 89±16&nbsp;km/s, with more accurate measurements by the Hubble Space Telescope, showing 80.4±4.8&nbsp;km/s.<ref>[https://ui.adsabs.harvard.edu/abs/1971ApJ...169..563G/abstract Effective Temperature, Radius, and Gravitational Redshift of Sirius B], J. L. Greenstein, J.B. Oke, H. L. Shipman, ''Astrophysical Journal'' '''169''' (Nov. 1, 1971), pp. 563&ndash;566.</ref>{{Citation needed|date=January 2021}}

[[James W. Brault]], a graduate student of [[Robert Dicke]] at [[Princeton University]], measured the gravitational redshift of the sun using optical methods in 1962.<ref>{{cite thesis |type=PhD |last=Brault |first=James W. |date=1962 |title=The Gravitational Redshift in the Solar Spectrum |url=https://www.proquest.com/docview/302083560|via=ProQuest |id={{ProQuest|302083560}} }}</ref> In 2020, a team of scientists published the most accurate measurement of the solar gravitational redshift so far, made by analyzing [[Iron|Fe]] spectral lines in sunlight reflected by the Moon; their measurement of a mean global 638 ± 6&nbsp;m/s lineshift is in agreement with the theoretical value of 633.1&nbsp;m/s.<ref>{{Cite journal|last1=Hernández|first1=J. I. González|last2=Rebolo|first2=R.|last3=Pasquini|first3=L.|last4=Curto|first4=G. Lo|last5=Molaro|first5=P.|last6=Caffau|first6=E.|last7=Ludwig|first7=H.-G.|last8=Steffen|first8=M.|last9=Esposito|first9=M.|last10=Mascareño|first10=A. Suárez|last11=Toledo-Padrón|first11=B.|date=2020-11-01|title=The solar gravitational redshift from HARPS-LFC Moon spectra - A test of the general theory of relativity|url=https://www.aanda.org/articles/aa/abs/2020/11/aa38937-20/aa38937-20.html|journal=Astronomy & Astrophysics|language=en|volume=643|pages=A146|doi=10.1051/0004-6361/202038937|arxiv=2009.10558|bibcode=2020A&A...643A.146G |s2cid=221836649|issn=0004-6361}}</ref><ref name=":3">{{Cite journal|last=Smith|first=Keith T.|date=2020-12-18|title=Editors' Choice|quote=Gravitational redshift of the Sun|journal=Science|language=en|volume=370|issue=6523|pages=1429–1430|doi=10.1126/science.2020.370.6523.twil|bibcode=2020Sci...370Q1429S|issn=0036-8075|doi-access=free}}</ref> Measuring the solar redshift is complicated by the Doppler shift caused by the motion of the Sun's surface, which is of similar magnitude as the gravitational effect.<ref name=":3" />

In 2011, the group of Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen collected data from 8000 galaxy clusters and found that the light coming from the cluster centers tended to be red-shifted compared to the cluster edges, confirming the energy loss due to gravity.<ref>{{cite web|last=Bhattacharjee|first=Yudhijit|year=2011|title=Galaxy Clusters Validate Einstein's Theory|url=https://www.science.org/content/article/galaxy-clusters-validate-einsteins-theory|access-date=2013-07-23|publisher=News.sciencemag.org}}</ref>

In 2018, the star [[S2 (star)|S2]] made its closest approach to [[Sagittarius A*|Sgr A*]], the 4-million solar mass [[supermassive black hole]] at the centre of the [[Milky Way]], reaching 7650&nbsp;km/s or about 2.5% of [[the speed of light]] while passing the black hole at a distance of just 120 [[Astronomical unit|AU]], or 1400 [[Schwarzschild radius|Schwarzschild radii]]. Independent analyses by the GRAVITY collaboration<ref>{{Cite journal|last1=Abuter|first1=R.|last2=Amorim|first2=A.|last3=Anugu|first3=N.|last4=Bauböck|first4=M.|last5=Benisty|first5=M.|last6=Berger|first6=J. P.|last7=Blind|first7=N.|last8=Bonnet|first8=H.|last9=Brandner|first9=W.|last10=Buron|first10=A.|last11=Collin|first11=C.|date=2018-07-01|title=Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole|url=https://www.aanda.org/articles/aa/abs/2018/07/aa33718-18/aa33718-18.html|journal=Astronomy & Astrophysics|language=en|volume=615|pages=L15|doi=10.1051/0004-6361/201833718|arxiv=1807.09409|bibcode=2018A&A...615L..15G|s2cid=118891445|issn=0004-6361}}</ref><ref>{{Cite journal|last=Witze|first=Alexandra|date=2018-07-26|title=Milky Way's black hole provides long-sought test of Einstein's general relativity|journal=Nature|language=en|volume=560|issue=7716|pages=17|doi=10.1038/d41586-018-05825-3|pmid=30065325|bibcode=2018Natur.560...17W|s2cid=51888156|doi-access=free}}</ref><ref>{{Cite web|title=Tests of General Relativity|url=https://www.mpe.mpg.de/7260308/Tests-of-General-Relativity|access-date=2021-01-17|website=www.mpe.mpg.de|language=en}}</ref><ref>{{Cite web|last=|title=First Successful Test of Einstein's General Relativity Near Supermassive Black Hole - Culmination of 26 years of ESO observations of the heart of the Milky Way|url=https://www.eso.org/public/news/eso1825/|access-date=2021-01-17|website=www.eso.org|language=en}}</ref> (led by [[Reinhard Genzel]]) and the KECK/UCLA Galactic Center Group<ref>{{Cite journal|last1=Do|first1=Tuan|last2=Hees|first2=Aurelien|last3=Ghez|first3=Andrea|last4=Martinez|first4=Gregory D.|last5=Chu|first5=Devin S.|last6=Jia|first6=Siyao|last7=Sakai|first7=Shoko|last8=Lu|first8=Jessica R.|last9=Gautam|first9=Abhimat K.|last10=O’Neil|first10=Kelly Kosmo|last11=Becklin|first11=Eric E.|date=2019-08-16|title=Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole|url=https://www.science.org/doi/10.1126/science.aav8137|journal=Science|volume=365|issue=6454|pages=664–668|language=en|doi=10.1126/science.aav8137|issn=0036-8075|pmid=31346138|arxiv=1907.10731|bibcode=2019Sci...365..664D|s2cid=198901506}}</ref><ref>{{Cite web|last=Siegel|first=Ethan|date=2019-08-01|title=General Relativity Rules: Einstein Victorious In Unprecedented Gravitational Redshift Test|url=https://medium.com/starts-with-a-bang/general-relativity-rules-einstein-victorious-in-unprecedented-gravitational-redshift-test-7ab4076bcd61|access-date=2021-01-17|website=Medium|language=en}}</ref> (led by [[Andrea M. Ghez|Andrea Ghez]]) revealed a combined [[Transverse Doppler effect|transverse Doppler]] and gravitational redshift up to 200&nbsp;km/s/c, in agreement with general relativity predictions.

In 2021, Mediavilla ([[Instituto de Astrofísica de Canarias|IAC]], Spain) & Jiménez-Vicente ([[University of Granada|UGR]], Spain) were able to use measurements of the gravitational redshift in [[quasar]]s up to cosmological redshift of {{nowrap|''z'' ≈ 3}} to confirm the predictions of [[Einstein's equivalence principle]] and the lack of cosmological evolution within 13%.<ref>{{Cite journal|last1=Mediavilla|first1=E.|last2=Jiménez-Vicente|first2=J.|year=2021|title=Testing Einstein's Equivalence Principle and Its Cosmological Evolution from Quasar Gravitational Redshifts|journal=The Astrophysical Journal|volume=914|issue=2|pages=112|arxiv=2106.11699|doi=10.3847/1538-4357/abfb70|bibcode=2021ApJ...914..112M |s2cid=235593322 |doi-access=free }}</ref>

In 2024, Padilla et al. have estimated the gravitational redshifts of supermassive black holes (SMBH) in eight thousand quasars and one hundred Seyfert type 1 galaxies from the full width at half maximum (FWHM) of their emission lines, finding {{nowrap|log ''z'' ≈ −4}}, compatible with SMBHs of ~ 1 billion solar masses and broadline regions of ~ 1 parsec radius. This same gravitational redshift was directly measured by these authors in the SAMI sample of [[LINER]] galaxies, using the redshift differences between lines emitted in central and outer regions.<ref name="padillaetal2024">
{{cite journal
 |author1=N. D. Padilla |author2=S. Carneiro|author3=J. Chaves-Montero |author4= C. J. Donzelli|author5=C. Pigozzo|author6=P. Colazo|author7=J. S. Alcaniz| date=2024
 |title= Active galactic nuclei and gravitational redshifts
 |journal= Astronomy and Astrophysics
 |volume= 683 |pages=120–126
 |doi=10.1051/0004-6361/202348146 |bibcode=2024A&A...683A.120P
 |arxiv=2304.13036
}}</ref>