        THE MOTION OF UNIVERSAL EXPANSION

 motion: TOC for Knowledge Concepts, Exercises, and Solutions

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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.

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