212 Dark Matter
ward Earth or away from it. This frequency shift is called a Doppler shift. The same kind of
shift happens with sound waves as a car passes and the sound of its engine seems to change
to a lower frequency. The greater that shift, the greater the object’s speed. Rubin wanted to
see if she could use Doppler shifts and Kent’s new spectrograph to measure the speed of
stars in distant galaxies.
She found that the stars near the outer edge of Andromeda moved just as fast as the
stars near the galaxy’s center. That wasn’t the way it was supposed to be.
Over a period of two months she completed 200 spectrographs. For every galaxy it was
the same. The velocities of stars she measured were all wrong. According to every known
law of physics, some of those stars were moving too fast for gravity to hold them in their
galaxies, and they should fly off into space. But they didn’t.
Rubin was left with two possible explanations. Either Newton’s equations were wrong
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