Run the "Hubble Movie" backwards:

- some time in the past, all galaxies would have been on top of
each other!

- if the universe has been expanding at a constant
rate for all time, then all galaxies would have been on top of
each other at a time equal to 1 / H
_{0}

Distance between any 2 galaxies at the present day:

distance = speed x time (the standard formula)

speed = H

Substitute Hubble's Law in to the standard formula:

distance = H

Cancel factors of distance and rearrange:

time = 1 / H

The time above is the time it would have taken the galaxies to reach their current separation, having started from being on top of each other and is an estimate of the age of the universe that is known as the Hubble Age.

Current best measurements of H

On to The Cosmological Redshift or back to start.