Hubble's law

HVelocity of recession (v) = Hubble constant (H) x distance (r)

At present Hubble's constant is thought to be about 70 kms^-1 Mpc^-1.

This means that the velocity of recession of a galaxy increases by 70 kms^-1 for every 1 Mpc increase in distance.

The value of H quoted is its value at the present time. This value will have varied over the lifetime of the universe and will continue to do so in the future. Hubble's law provides a very powerful way of determining not only the distance of the galaxies but also of the age of the Universe itself.

The velocity of recession of a galaxy can be found by measuring the Doppler shift of lines in its spectrum. Once this is found the distance of the galaxy can be calculated using the Hubble formula.