Chapter 28 Stars and the Universe

728 CHAPTER 28: STARS AND THE UNIVERSE
science supplier works well. Swing the noisemaker in a circle
around your head as people at a safe distance listen for changes
in the pitch of the sound. Can the person swinging the device also
hear the pitch of the sound change? For observers outside the circle,
what part of the swing best represents redshift, and what part
of the swing represents a “blueshift.”
Two other factors supported Hubble’s hypothesis of an expanding
universe. The redshift of light of distant galaxies
could be observed in all directions. In addition, the dimmer
galaxies, which are thought to be dim because they are farther
from Earth, showed greater redshift. Hubble explained
that the redshift is caused by motion of distant galaxies away
from Earth. He also reasoned that this kind of motion is a
characteristic of an explosion.
You might think that the motion of distant galaxies away
from Earth in all directions means that we are at the center
of the expansion. However, from any position within the expanding
matter of an explosion, matter is moving away in all
directions.
In the 1960s, Arno Penzias and Robert Wilson were working
on long-distance radio communications for the Bell Telephone
Company. They constructed a special outdoor receiver
to detect weak radio signals. However, the device picked up
annoying radio noise that they were not able to eliminate. As
they investigated the source of these radio waves, they realized
that the energy they were picking up was billions of
years old. They were actually listening to the origin of the
universe. These radio signals, known as cosmic background
radiation, are weak electromagnetic radiation left
over from the formation of the universe.
The Big Bang
The outward motions of distant galaxies and the cosmic background
radiation are evidence that the universe began as an
event now called the big bang. The name was first proposed
as a joke to make fun of the theory, but the name stuck. This