152 Chapter 3 The Universe Was Born With a Big Bang
Well, then Slipher, who gave the data to Hubble, seems to be the person
who noticed that the universe is expanding—not Hubble, right?
But Slipher isn't very famous in the United States.
He believed that the redshift meant that many galaxies were receding
from us, but he thought it was due to the basic motion of heavenly
bodies. Hubble, however, conducted a thorough investigation of the
correlation between distance and redshift. He discovered that the
farther away a galaxy is, the faster it is receding—a fact that sup-
ports the theory of an expanding universe.
You'll understand it if we write three
letters on a balloon and then inﬂate it.
If they were receding, table salt would become more
orange, and copper would become yellowish green.
But why does that mean the
universe itself is expanding?
Very hot stars emit spectra that have prominent lines for helium and the
ionized heavy elements (that is, atoms that have gained or lost an electron
or electrons). On the other hand, very cool stars emit spectra with no visible
helium lines but with lines for neutral atoms and molecules. Nevertheless,
if temperature differences cause differences in spectra, then similar stars
(that is, stars with similar mass and temperature) should have practically
identical spectra. However, Slipher (see page 146) discovered that a shift
toward red occurred in the wavelengths.