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## Hubble's law
The law was first formulated by Edwin Hubble in 1929.
Hubble compared the distances to nearby galaxies to their redshift, found a linear relationship, and interpreted the redshift as caused by the receding velocity. His estimate of the proportionality constant, now known as The law can be stated as follows: *v*=*H*_{0}*D*
v is the receding velocity of a galaxy due to the expansion of the universe (typically measured in km/sec), H_{0} is Hubble's constant, and D is the current distance to the galaxy (measured in mega parsec Mpc).
One can derive Hubble's law mathematically if one assumes that the universe expands (or shrinks) and that the universe is
For most of the second half of the 20th century the value of
Hubble's constant is "constant" in the sense that it is believed to work for all velocities and distances
Based on recent observations, it is now believed that galaxies accelerate away from us, which means that There are several additional notes to be made:
- The distance
*D*to nearby galaxies can be estimated for example by comparing their apparent luminosity to their postulated absolute luminosity.
D the distance to the galaxy right now, not when the light from it was emitted.
This distance is extremely hard to determine.
- The velocity
*v*is defined to be the time rate of change of*D*.
v can be determined from the galaxy's redshift z using the formula v ≈ zc where c is the speed of light.
However, only the velocity due to the expansion of the universe should be used: all galaxies move relative to each other independent of the expansion of the universe, and these relative velocities, called peculiar velocities are not accounted for by Hubble's law. For far away galaxies, v cannot easily be determined from the redshift z and can be larger than c.
- Systems that are gravitationally bound, such as galaxies or our planetary system, are not subject to Hubble's law and do not expand.
## External links and references:- The Hubble Key Project: http://www.ipac.caltech.edu/H0kp/H0KeyProj.html
- Freedman et. al.:
*Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant*. The Astrophysical Journal, Volume 553, Issue 1, pp. 47-72. Online at http://adsabs.harvard.edu/cgi-bin/nph-bib_query?2001ApJ...553...47F
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