Hubble's Law has two more important consequences.  First, it is a powerful way to measure distances to distant objects.  Look again at the bottom plot on the Clusters of Galaxies page.  How were the distances to those galaxies determined?  Most are too far away for us to identify Cepheids in them.  But once we know Hubble's constant by comparing recessional velocities and distances (from Cepheids or other techniques) to relatively nearby galaxies and clusters, we can use it to determine distances to much more distant objects: if we can measure a galaxy's recessional velocity and if we know Hubble's constant, then Hubble's Law can be used to solve for the distance.  This is a very powerful distance technique and we will see it illustrated in this and the next lab.

We can easily estimate the age of the universe if we assume that the universe has always been expanding at the current rate. Since the galaxies are currently flying away from each other, we can run the expansion back in time, and ask how long it would take for all the galaxies in the universe to come back together again. First of all, remind yourself of the basic relationship between velocity, distance and time: time = distance / velocity.

If we know the distance between M100 and the Milky Way (using Cepheids), and their relative velocity, we can calculate how long it took them to travel their distance of separation. age of universe = [ (distance to M100) / (recessional velocity of M100) ] In other words,

That seems simple enough. There's only one little hitch...we used units of (km/sec/Mpc) for H0 in the previous section. Inverting these gives units of (sec Mpc / km) for the age of the universe. How long is a (sec Mpc / km)?!!  We'll have to use some conversion factors to transform these units into a comprehensible age.

Note that the units of sec, Mpc and km all cancel, leaving an age in years. It is unlikely the Hubble constant has been constant over the lifetime of the universe. In fact, in recent years, the expansion of the universe has been discovered to be accelerating.  That means the Hubble constant was smaller in the past.   In the early universe, it was thought to be larger.  The net result is that our best estimate of the age of the universe does turn out to be very close to the above estimate based on the current value of the Hubble constant.