182 PART 2^ |^ THE STARS
from an explosion but rather are parts of a whole that fi lls all of the
volume of the universe and expands continuously as the total vol-
ume increases.
Th e big bang theory seems fantastic, but it has been tested
and confi rmed over and over, and modern astronomers have
great confi dence that there really was a big bang (How Do
We Know? P-3). Of course, there are more details to under-
stand. How did the fi rst galaxies form? Why did the galaxies
form in giant clusters? You live in an exciting age when astron-
omers are able to ask these questions and expect to discover
answers.
Th ese ideas strain human imagination and challenge the
most sophisticated mathematics, but the lesson is a simple one.
Th e universe had a beginning not so long ago, and the matter
you are made of had its birthday in that moment of cosmic
beginnings. You are small, but you are part of something
vast.The Story of Matter
Astronomers can tell the story of the matter in the universe.
Mathematical models of the big bang show that the fi rst few
minutes in the history of the universe were unimaginably hot.
Energy was so intense that it could form particles of matter, andP-6
from those galaxies has been traveling that long. If you used a big
enough telescope and looked at the most distant galaxies, you
would be looking back in time and seeing them as they were over
10 billion years ago when the universe was young.
What would you see if you looked at the empty places on
the sky between the most distant galaxies? You could detect a
glow that was emitted by the hot, dense clouds of gas of the big
bang, but you couldn’t see that glow with your unaided eyes
because the redshift is so great that the photons of light are
shifted into the long-wavelength infrared and radio parts of the
spectrum. Nevertheless, astronomers can “see” the radiation from
the big bang with infrared and radio detectors. Th is is called the
cosmic microwave background radiation, and it fi lls the uni-
verse, pouring in on Earth from all directions, and telling you
that you are part of a universe that was very hot and very dense
about 13.7 billion years ago (■ Figure P-11). Your atoms and
Earth’s atoms were part of the big bang.
Notice that the background radiation is visible in any direc-
tion in the sky. Th e big bang did not occur in a specifi c place, but
it fi lled the entire volume of the universe. As the universe
expanded, the total volume of space increased, and the hot gases
of the big bang cooled and formed galaxies. Except for relatively
small random motions, galaxies do not move as the universe
expands, but are carried away from each other as the volume of
space continues to increase. Galaxies are not fragments ejected
What is the difference between believing
in the big bang and understanding it? If
you ask a scientist, “Do you believe in the big
bang?” she or he may hesitate before respond-
ing. The question implies something incorrect
about the way science works.
Science is a system of knowledge and pro-
cess but does not operate as a system of belief.
Science is an attempt to understand logically
how nature works and is based on observa-
tions and experiments used to test and confi rm
hypotheses and theories. A scientist does not
really believe in even a well-confi rmed theory
in the way people normally use the word
believe. Rather, the scientist understands the
theory and recognizes how different pieces of
evidence support or contradict the theory.
There are other ways to know things, and
there are many systems of belief. Religions,
for example, are systems of belief that are not
entirely based on observation. In some cases,a political system is also a system of belief;
many people believe that democracy is the
best form of government and do not ask for,
or expect, evidence supporting that belief. A
system of belief can be powerful and lead to
deep insights, but it is different from science.
Scientists try to be careful with words,
so thoughtful scientists would not say they
believe in the big bang. They would say that
the evidence is overwhelming that the big bang
really did occur and that they are compelled,
by a logical analysis of both the observations
and the theory, to conclude that the theory is
very likely correct. In this way scientists try to
be objective and reason without distortion by
personal feelings and prejudices.
A scientist once referred to “the terrible
rule of evidence.” Sometimes the evidence
forces a scientist to a conclusion she or he
does not like, but science is not a system of
belief, so the personal preferences of eachScientifi c knowledge is based objectively on
evidence such as that gathered by spacecraft.
(NASA/WMAP Science Team)P-2 Science: A System of Knowledge
scientist must take second place to the rule of
evidence.
Do you believe in the big bang? Or, instead,
do you have confi dence that the theory is
right because of your analysis of the evidence?
There is a big difference.