Editing Galaxy cluster (section)

== Galaxy clusters as measuring instruments ==

=== Gravitational redshift ===
Galaxy clusters have been used by Radek Wojtak from the Niels Bohr Institute at the University of Copenhagen to test predictions of [[general relativity]]: energy loss from light escaping a gravitational field. Photons emitted from the center of a galaxy cluster should lose more energy than photons coming from the edge of the cluster because gravity is stronger in the center. Light emitted from the center of a cluster has a longer wavelength than light coming from the edge. This effect is known as [[gravitational redshift]]. Using the data collected from 8000 galaxy clusters, Wojtak was able to study the properties of gravitational redshift for the distribution of galaxies in clusters. He found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster as predicted by general relativity. The result also strongly supports the [[Lambda-CDM model|Lambda-Cold Dark Matter]] model of the Universe, according to which most of the cosmos is made up of [[Dark matter|Dark Matter]] that does not interact with matter.<ref>{{cite magazine |last1=Yudhijit |first1=Bhattacharjee |title=Galaxy Clusters Back Up Einstein's Theory of Relativity |url=https://www.wired.com/2011/09/galaxies-einstein-relativity/ |magazine=Wired |access-date=2022-04-04}}</ref>

=== Gravitational lensing ===
Galaxy clusters are also used for their strong gravitational potential as [[gravitational lens]]es to boost the reach of telescopes.<ref>{{Cite journal |last=Walker |first=Stephen |last2=Simionescu |first2=Aurora |last3=Nagai |first3=Daisuke |last4=Okabe |first4=Nobuhiro |last5=Eckert |first5=Dominique |last6=Mroczkowski |first6=Tony |last7=Akamatsu |first7=Hiroki |last8=Ettori |first8=Stefano |last9=Ghirardini |first9=Vittorio |date=2019-01-02 |title=The Physics of Galaxy Cluster Outskirts |url=https://link.springer.com/article/10.1007/s11214-018-0572-8 |journal=Space Science Reviews |language=en |volume=215 |issue=1 |page=7 |doi=10.1007/s11214-018-0572-8 |issn=1572-9672|arxiv=1810.00890 }}</ref> The gravitational distortion of space-time occurs near massive galaxy clusters and bends the path of photons to create a cosmic magnifying glass. This can be done with photons of any wavelength from the optical to the X-ray band. The latter is more difficult, because galaxy clusters emit a lot of X-rays.<ref>{{Cite journal |last=Reiprich |first=Thomas H. |last2=Basu |first2=Kaustuv |last3=Ettori |first3=Stefano |last4=Israel |first4=Holger |last5=Lovisari |first5=Lorenzo |last6=Molendi |first6=Silvano |last7=Pointecouteau |first7=Etienne |last8=Roncarelli |first8=Mauro |date=2013-08-01 |title=Outskirts of Galaxy Clusters |url=https://link.springer.com/article/10.1007/s11214-013-9983-8 |journal=Space Science Reviews |language=en |volume=177 |issue=1 |pages=195–245 |doi=10.1007/s11214-013-9983-8 |issn=1572-9672|arxiv=1303.3286 }}</ref> However, X-ray emission may still be detected when combining X-ray data to optical data. One particular case is the use of the Phoenix galaxy cluster to observe a dwarf galaxy in its early high energy stages of star formation.<ref>{{cite web |last1=Chu |first1=Jennifer |title=Astronomers use giant galaxy cluster as X-ray magnifying lens |url=https://news.uchicago.edu/story/astronomers-use-giant-galaxy-cluster-x-ray-magnifying-lens |website=MIT News |date=15 October 2019 |access-date=2022-04-04}}</ref>