page_title: Hubble's law
section_title: Combining redshifts with distance measurements
hierarchical_section_title: Hubble's law / Discovery / Combining redshifts with distance measurements
caption_attribution_description: English: Scatter plot of fit of redshifts to Hubble's law. Patterned after https://books.google.com/books?id=BUgJGypUYF0C&amp;pg=PA7&amp;dq=hubble+constant+H0+date:2005-2010&amp;lr=&amp;as_brr=0&amp;as_pt=ALLTYPES#PPA8,M1
context_page_description: Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at velocities proportional to their distance. In other words, the further they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit to the red end of the spectrum.
  Hubble's law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.
  The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is often expressed by the equation v = H₀D, with H₀ the constant of proportionality—Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to cosmological time coordinate.
  Hubble constant is most frequently quoted in /Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec away, and its value is about 70 /Mpc.
context_section_description: The parameters that appear in Hubble's law, velocities and distances, are not directly measured. In reality we determine, say, a supernova brightness, which provides information about its distance, and the redshift z = ∆λ/λ of its spectrum of radiation. Hubble correlated brightness and parameter z.
  Combining his measurements of galaxy distances with Vesto Slipher and Milton Humason's measurements of the redshifts associated with the galaxies, Hubble discovered a rough proportionality between redshift of an object and its distance. Though there was considerable scatter (now known to be caused by peculiar velocities—the 'Hubble flow' is used to refer to the region of space far enough out that the recession velocity is larger than local peculiar velocities), Hubble was able to plot a trend line from the 46 galaxies he studied and obtain a value for the Hubble constant of 500 km/s/Mpc (much higher than the currently accepted value due to errors in his distance calibrations; see cosmic distance ladder for details).
  At the time of discovery and development of Hubble's law, it was acceptable to explain redshift phenomenon as a Doppler shift in the context of special relativity, and use the Doppler formula to associate redshift z with velocity. Today, in the context of general relativity, velocity between distant objects depends on the choice of coordinates used, and therefore, the redshift can be equally described as a Doppler shift or a cosmological shift (or gravitational) due to the expanding space, or some combination of the two.
caption: Fit of redshift velocities to Hubble's law.[24] Various estimates for the Hubble constant exist. The HST Key H0 Group fitted type Ia supernovae for redshifts between 0.01 and 0.1 to find that H0 = 71 ± 2 (statistical) ± 6 (systematic) km s−1Mpc−1,[25] while Sandage et al. find H0 = 62.3 ± 1.3 (statistical) ± 5 (systematic) km s−1Mpc−1.[26]
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