Editing Redshift (section)

=== 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" />