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\subsection{Hubble's Constant}  Observations of distant galaxies made by Edwin Hubble in the 1920s turned up an interesting fact:  that every galaxy is moving away from every other galaxy, and the further apart two galaxies are, the faster they are receding from each other.   This observation is explained by realising that the velocity is due to the expansion of the space   between galaxies, rather than relative motion between the galaxies, i.e. i.e.,  the Universe is expanding. A common analogy is how the distance between two points drawn on an uninflated balloon increases  as the balloon is blown up.  This is known as Hubble's Law, and it can be expressed as