Editing Redshift (section)

==Blueshift==
{{redirect|Blueshift|the term as used in photochemistry|hypsochromic shift|the political phenomenon|blue shift (politics)|other uses of "blueshift" or "blue shift"}}
The opposite of a redshift is a '''blueshift'''. A blueshift is any decrease in [[wavelength]] (increase in [[energy]]), with a corresponding increase in frequency, of an [[electromagnetic wave]]. In [[Light|visible light]], this shifts a color towards the blue end of the spectrum.

=== Doppler blueshift ===
[[File:Redshift blueshift.svg|thumb|Doppler redshift and blueshift]]
[[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>

Doppler blueshift is used in [[astronomy]] to determine relative motion:
* The [[Andromeda Galaxy]] is moving toward our own [[Milky Way]] [[galaxy]] within the [[Local Group]]; thus, when observed from Earth, its light is undergoing a blueshift.<ref>{{cite book |last=Woodhouse |first=Chris |chapter=M31 (Andromeda Galaxy) |date=2017-12-04 |title=The Astrophotography Manual |pages=308–313 |edition=2nd |publisher=Routledge |language=en |doi=10.4324/9781315159225-42 |isbn=978-1-315-15922-5}}</ref>
* Components of a [[binary star]] system will be blueshifted when moving towards Earth
* When observing spiral galaxies, the side spinning toward us will have a slight blueshift ''relative to'' the side spinning away from us (see [[Tully–Fisher relation]]).
* [[Blazar]]s are known to propel [[relativistic jet]]s toward us, emitting [[synchrotron radiation]] and [[bremsstrahlung]] that appears blueshifted.<ref>{{Cite book |arxiv=2412.11565 |doi=10.69646/aob104p029 |chapter=Monitoring Blazar Variability to Understand Extragalactic Jets |title=Publications of the Astronomical Observatory of Belgrade |date=2024 |last1=Maria Raiteri |first1=Claudia |volume=104 |pages=29–38 |isbn=978-86-82296-11-9 }}</ref>
* Nearby stars such as [[Barnard's Star]] are moving toward us, resulting in a very small blueshift.
* Doppler blueshift of distant objects with a high ''z'' can be subtracted from the much larger [[Hubble's law|cosmological redshift]] to determine relative motion in the [[Metric expansion of space|expanding universe]].<ref name="Aoki2005">{{cite journal | title = The Largest Blueshifts of the [O III] Emission Line in Two Narrow-Line Quasars | journal = Astrophysical Journal | date = January 2005 | first1=Kentaro | last1=Aoki | first2=Toshihiro | last2=Kawaguchi | first3=Kouji | last3=Ohta | volume = 618 | issue = 2 | pages = 601–608 |arxiv = astro-ph/0409546 |bibcode = 2005ApJ...618..601A |doi = 10.1086/426075 | s2cid = 17680991 }}</ref>

=== Gravitational blueshift ===
[[Image:Gravitional well.jpg|thumb|[[Matter waves]] (protons, electrons, photons, etc.) falling into a [[gravity well]] become more energetic and undergo observer-independent blueshifting.]]
Unlike the ''relative'' Doppler blueshift, caused by movement of a source towards the observer and thus dependent on the received angle of the photon, gravitational blueshift is ''absolute'' and does not depend on the received angle of the photon:
{{Blockquote|Photons climbing out of a gravitating object become less energetic. This loss of energy is known as a "redshifting", as photons in the visible spectrum would appear more red. Similarly, photons falling into a gravitational field become more energetic and exhibit a blueshifting. ... Note that the magnitude of the redshifting (blueshifting) effect is not a function of the emitted angle or the received angle of the photon—it depends only on how far radially the photon had to climb out of (fall into) the potential well.<ref name=R.N_1>{{cite web| first=R. J. | last=Nemiroff| title=Gravitational Principles and Mathematics| url=http://antwrp.gsfc.nasa.gov/htmltest/gifcity/nslens_math.html| date=1993| publisher=[[NASA]]}}</ref><ref name=R.N_2>{{cite journal| first=R. J. | last=Nemiroff| title=Visual distortions near a neutron star and black hole| date=1993| journal=American Journal of Physics| volume=61| issue=7| pages=619–632| bibcode=1993AmJPh..61..619N| doi=10.1119/1.17224| arxiv=astro-ph/9312003v1| s2cid=16640860}}</ref>}}

It is a natural consequence of [[conservation of energy]] and [[mass–energy equivalence]], and was confirmed experimentally in 1959 with the [[Pound–Rebka experiment]]. Gravitational blueshift contributes to [[cosmic microwave background]] (CMB) anisotropy via the [[Sachs–Wolfe effect]]: when a gravitational well evolves while a photon is passing, the amount of blueshift on approach will differ from the amount of [[gravitational redshift]] as it leaves the region.<ref name="Bonometto2002">{{cite book | last1 = Bonometto | first1 = Silvio | last2 = Gorini | first2 = Vittorio | last3 = Moschella | first3 = Ugo | title = Modern Cosmology | publisher = [[CRC Press]] | date = 2002 | isbn = 978-0-7503-0810-6 }}</ref>

==== Blue outliers ====
There are faraway [[active galaxies]] that show a blueshift in their [[Doubly ionized 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" />

===Cosmological blueshift===
In a hypothetical universe undergoing a runaway [[Big Crunch]] contraction, a cosmological blueshift would be observed, with galaxies further away being increasingly blueshifted—the exact opposite of the actually observed [[cosmological redshift]] in the present [[expanding universe]].<ref>{{Cite web |last=Miller |first=Cole |title=Cosmology |url=https://www.astro.umd.edu/~miller/teaching/questions/cosmology.html |access-date=2025-03-06 |website=www.astro.umd.edu}}</ref>