220 The cosmic microwave background
in the early universe, have the potential to provide the most precise constraints
on the overall properties of the universe of any data source. The reasons are
that (1) the universe was very simple at the time imaged by the microwave
background and is extremely well described by linear perturbation theory around
a completely homogeneous and isotropic cosmological spacetime; and (2) the
physical processes relevant at that time are all simple and very well understood.
The microwave background is essentially unique among astrophysical systems in
these regards.
The goal behind this chapter is to provide a qualitative description of
the physics of the microwave background, an appreciation for the microwave
background’s cosmological importance, and an understanding of what kinds of
constraints may be placed on cosmological models. It is not intended to be a
definitive technical reference to the microwave background. Unfortunately, such
a reference does not really exist at this time, but I have attempted to provide
pedagogically useful references to other literature. I have also not attempted to
give a complete bibliography; please do not consider this article to give definitive
references to any topics mentioned. A recent review of the microwave background
with a focus on potential particle physics constraints is Kamionkowski and
Kosowsky (1999). A more general review of the microwave background and
large-scale structure with references to many early microwave background articles
is Whiteet al(1994).
7.1 A brief historical perspective
The story of the serendipidous discovery of the microwave background in 1965
is widely known, so I will only briefly summarize it here. A recent book by the
historian of science Helge Kragh (1996) is a careful and authoritative reference
on the history of cosmology, from which much of the information in this section
was obtained. Arno Penzias and Robert Wilson, two radio astronomers at Bell
Laboratories in Crawford, New Jersey, were using a sensitive microwave horn
radiometer originally intended for talking to the early Telstar telecommunications
satellites. When Bell Laboratories decided to get out of the communications
satellite business in 1963, Penzias and Wilson began to use the radiometer to
measure radio emission from the Cassiopeia A supernova remnant. They detected
a uniform noise source, which was assumed to come from the apparatus. But after
many months of checking the antenna and the electronics (including removal of a
bird’s nest from the horn), they gradually concluded that the signal might actually
be coming from the sky. When they heard about a talk given by P J E Peebles
of Princeton predicting a 10 K blackbody cosmological background, they got
in touch with the group at Princeton and realized that they had detected the
cosmological radiation. At the time, Peebles was collaborating with Dicke,
Roll and Wilkinson in a concerted effort to detect the microwave background.
The Princeton group wound up confirming the Bell Laboratories discovery a