What is Hubble's law?
[[File:Redshift.svg|thumb|upright|[[spectral line|Absorption lines]] in the visible [[spectrum]] of a [[supercluster]] of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).]] '''Hubble's law''', or the '''Hubble{{ndash}}Lemaître law''', is the name for an astronomical observation. The observation is that: #All objects observed in deep space have a [[doppler shift]]-measured [[velocity]] relative to Earth, and to each other; #That velocity is [[Proportionality|proportional]] to their distance from the Earth and all other interstellar bodies. The [[space-time]] volume of the observable [[universe]] is expanding. Hubble's law is the direct physical observation of this.<ref name=Coles>{{cite book |title=Routledge critical dictionary of the new cosmology |url=https://books.google.com/books?id=BgNGWVr5yhIC&pg=PA202 |page=202 |editor=Peter Coles |isbn=0-203-16457-1 |year=2001 |publisher=Routledge }}</ref> This is why we believe the [[expansion of the universe|Universe is expanding]]. The evidence supports the [[Big Bang]] model. Although widely attributed to [[Edwin Hubble]], the law was first derived from the [[General Relativity]] equations by [[Georges Lemaître]] in a 1927 article. There he proposed that the Universe is expanding, and suggested a value for the rate of expansion, now called the '''Hubble constant'''.<ref>{{Cite journal | last=Lemaître | first=Georges | author-link=Georges Lemaître | title=Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques | year=1927 | journal=Annales de la Société Scientifique de Bruxelles | volume=A47 | pages=49–56 | bibcode=1927ASSB...47...49L }} ([http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1927ASSB...47...49L&link_type=ARTICLE&db_key=AST&high= Full article], PDF). Partially translated (the translator remains unidentified) in {