Editing Big Bang (section)

===Hubble's law and the expansion of the universe===
{{Main|Hubble's law|Expansion of the universe}}
{{See also|Distance measures (cosmology)|Scale factor (cosmology)}}
[[File:Redshifted.png|right|thumb|Redshift of absorption lines due to recessional velocity]]
Observations of distant galaxies and [[quasar]]s show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a [[Spectral density|frequency spectrum]] of an object and matching the [[spectroscopy|spectroscopic]] pattern of [[Spectral line|emission or absorption line]]s corresponding to atoms of the chemical elements interacting with the light. These redshifts are [[Homogeneity (physics)|uniformly]] isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the [[cosmic distance ladder]]. When the recessional velocities are plotted against these distances, a linear relationship known as [[Hubble's law]] is observed:<ref name="hubble" />
<math>v = H_0D</math>
where
* <math>v</math> is the recessional velocity of the galaxy or other distant object,
* <math>D</math> is the [[proper length|proper distance]] to the object, and
* <math>H_0</math> is [[Hubble's law|Hubble's constant]], measured to be {{val|70.4|+1.3|-1.4}} [[kilometres|km]]/[[second|s]]/[[Megaparsec|Mpc]] by the WMAP.<ref name="wmap7year" />

Hubble's law implies that the universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922<ref name=af1922 /> and Lemaître in 1927,<ref name=gl1927 /> well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker.

The theory requires the relation <math>v = HD</math> to hold at all times, where <math>D</math> is the proper distance, <math>v</math> is the recessional velocity, and <math>v</math>, <math>H</math>, and <math>D</math> vary as the universe expands (hence we write <math>H_0</math> to denote the present-day Hubble "constant"). For distances much smaller than the size of the [[observable universe]], the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity <math>v</math>. For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one.<ref name="Hogg">{{cite journal |author=Bunn |first1=E. F. |last2=Hogg |first2=D. W. |year=2009 |title=The kinematic origin of the cosmological redshift |journal=American Journal of Physics |volume=77 |issue=8 |pages=688–694 |arxiv=0808.1081 |bibcode=2009AmJPh..77..688B |doi=10.1119/1.3129103 |s2cid=1365918}}</ref>

An unexplained discrepancy with the determination of the Hubble constant is known as [[Hubble tension]]. Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder.<ref name="di Valentino 2021 153001">{{cite journal | last1=Di Valentino | first1=Eleonora | last2=Mena | first2=Olga | last3=Pan | first3=Supriya | last4=Visinelli | first4=Luca | last5=Yang | first5=Weiqiang | last6=Melchiorri | first6=Alessandro | last7=Mota | first7=David F. | last8=Riess | first8=Adam G. | last9=Silk | first9=Joseph | year=2021 | title=In the realm of the Hubble tension—a review of solutions | journal=Classical and Quantum Gravity | volume=38 | issue=15 | page=153001 | doi=10.1088/1361-6382/ac086d | arxiv=2103.01183|bibcode=2021CQGra..38o3001D | s2cid=232092525 }}</ref>