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motion: TOC for Knowledge Concepts, Exercises, and Solutions

Edwin Hubble's discovery of cosmic expansion proved that motion was truly 
universal in its scope.  Mathematically, that expansion can be expression in an 
equation known as Hubble's Law:

v = Hr       (recessional velocity = the Hubble constant * distance)

"H" is the constant of proportionality, which astronomers have named
the Hubble constant in Edwin's honor. 

As a simple exercise, if H = 70 km/s per Mpc, and you measured a galaxy moving 
away from Earth at v = 350 km/s, then the distance
to the galaxy would be r, where  

350 km/s = (70 km/s/Mpc) * r   and thus r = 5 Mpc.

Now, if we realize that both km and Mpc are measurements of length, this actually 
gives us a way to compute the approximate age of the universe!  The quantity
1/H - the multiplicative inverse of the Hubble constant - can be expressed in 
units of time, and is roughly the amount of time the universe has been expanding.