Editing Redshift (section)

====Distinguishing between cosmological and local effects====

The redshift of a galaxy includes both a component related to [[recessional velocity]] from expansion of the universe, and a component related to the [[peculiar motion]] of the galaxy with respect to its local universe.<ref>{{cite journal
 | title=A comparison between the Doppler and cosmological redshifts
 | last=Bedran | first=M. L. | year=2002
 | journal=American Journal of Physics
 | volume=70 | issue=4 | pages=406–408
 | doi=10.1119/1.1446856 | bibcode=2002AmJPh..70..406B
 | url=http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/cosmo/doppler_redshift.pdf
 | access-date=2023-03-16
}}</ref> The redshift due to expansion of the universe depends upon the recessional velocity in a fashion determined by the cosmological model chosen to describe the expansion of the universe, which is very different from how Doppler redshift depends upon local velocity.<ref name="Harrison2">{{cite journal |last=Harrison |first=Edward |date=1992 |title=The redshift-distance and velocity-distance laws |journal=Astrophysical Journal, Part 1 |volume=403 |pages=28–31 |bibcode=1993ApJ...403...28H |doi=10.1086/172179 |doi-access=free}}. A pdf file can be found here [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ApJ...403...28H&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf].</ref> Describing the cosmological expansion origin of redshift, cosmologist [[Edward Robert Harrison]] said, "Light leaves a galaxy, which is stationary in its local region of space, and is eventually received by observers who are stationary in their own local region of space. Between the galaxy and the observer, light travels through vast regions of expanding space. As a result, all wavelengths of the light are stretched by the expansion of space. It is as simple as that..."<ref>{{Harvnb|Harrison|2000|p=302}}.</ref> [[Steven Weinberg]] clarified, "The increase of wavelength from emission to absorption of light does not depend on the rate of change of {{math|''a''(''t'')}} [the [[Scale factor (cosmology)|scale factor]]] at the times of emission or absorption, but on the increase of {{math|''a''(''t'')}} in the whole period from emission to absorption."<ref name=Weinberg_Cosmology>{{cite book |url=https://books.google.com/books?id=48C-ym2EmZkC&pg=PA11 |first=Steven | last=Weinberg |title=Cosmology |publisher=Oxford University Press |page=11 |date=2008 |isbn=978-0-19-852682-7}}</ref>

If the universe were contracting instead of expanding, we would see distant galaxies blueshifted by an amount proportional to their distance instead of redshifted.<ref>This is only true in a universe where there are no [[peculiar velocity|peculiar velocities]]. Otherwise, redshifts combine as

:<math>1+z=(1+z_{\mathrm{Doppler}})(1+z_{\mathrm{expansion}})</math>
which yields solutions where certain objects that "recede" are blueshifted and other objects that "approach" are redshifted. For more on this bizarre result see: {{cite journal
 | last1=Davis | first1=T. M. | last2=Lineweaver | first2=C. H. | last3=Webb | first3=J. K.
 | title=Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects
 | journal=American Journal of Physics
 | volume=71 | issue=4 | pages=358–364
 | date=April 2003 | doi=10.1119/1.1528916 
 | arxiv=astro-ph/0104349 | bibcode=2003AmJPh..71..358D | s2cid=3219383 }}</ref>