Freedman and Kaufmann
Remarkably, Hubble found that the
distance to a galaxy (which he found independently,
essentially by measuring how bright certain
kinds of stars were) was very well correlated
with its recessional velocity. Also: all
galaxies are moving away from us!
Freedman and Kaufmann
From this discovery, we derive Hubble's law,
or the Hubble law:
where
This is very strange -- the farther away something
is, the faster it is moving away from us! How
can that be?
is called the Hubble constant.