Hubble’s Law or the Hubble-Lemaître law shows that there is a direct correlation between the distance to a galaxy and its recessional velocity ie. the velocity of a galaxy is directly proportional to its distance away from the Earth.
v = H₀ r
where v is the recessional velocity, H0 is the Hubble constant and r is the distance.
The value of Hubble’s constant (or Hubble’s parameter) is challenging as it is dependent on the accurate measurement of astronomical distances. The Particle Data Group quote a ‘best modern value’ of 72 km/s per megaparsec (+/- 10%), whereas the more recent Planck mission led to a lower value of 67.66 +/- 0.42 (2018).
One frequently mentioned problem for Hubble’s law is Stephan’s Quintet. Four of these five galaxies have similar redshifts but that of the fifth is quite different, and yet they appear to be interacting.
Unless a galaxy is gravitationally bound to another, all galaxies are moving away from each other, as evidenced by their red shifts. This is because the Universe is expanding.
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