222 The cosmic microwave background
the Brans–Dicke theory of gravitation. They were soon scooped by Penzias and
Wilson’s discovery.
As soon as the microwave background was discovered, theorists quickly
realized that fluctuations in its temperature would have fundamental significance
as a reflection of the initial perturbations which grew into galaxies and clusters.
Initial estimates of the amplitude of temperature fluctuations were a part in
a hundred; this level of sensitivity was attained by experimenters after a few
years with no observed fluctuations. Thus began a quarter-century chase
after temperature anisotropies in which the theorists continually revised their
estimates of the fluctuation amplitude downwards, staying one step ahead of
the experimenters’ increasingly stringent upper limits. Once the temperature
fluctuations were shown to be less than a part in a thousand, baryonic density
fluctuations did not have time to evolve freely into the nonlinear structures visible
today, so theorists invoked a gravitationally dominant DM component (structure
formation remains one of the strongest arguments in favour of non-baryonic DM).
By the end of the 1980s, limits on temperature fluctuations were well below a part
in 10^4 and theorists scrambled to reconcile standard cosmology with this small
level of primordial fluctuations. Ideas like late-time phase transitions at redshifts
less thanz=1000 were taken seriously as a possible way to evade the microwave
background limits (see, e.g., Jaffeet al1990). Finally, the COBE satellite detected
fluctuations at the level of a few parts in 10^5 (Smootet al1990), just consistent
with structure formation in inflation-motivated Cold Dark Matter cosmological
models. The COBE results were soon confirmed by numerous ground-based and
balloon measurements, sparking the intense theoretical and experimental interest
in the microwave background over the past decade.
7.2 Physics of temperature fluctuations
The minute temperature fluctuations present in the microwave background
contain a wealth of information about the fundamental properties of the universe.
In order to understand the reasons for this and the kinds of information available,
an appreciation of the underlying physical processes generating temperature
and polarization fluctuations is required. This section and the following one
give a general description of all basic physics processes involved in producing
microwave background fluctuations.
First, one practical matter. Throughout this chapter, common cosmological
units will be employed in whichh ̄ =c=kb=1. All dimensionful quantities
can then be expressed as powers of an energy scale, commonly taken as GeV.
In particular, length and time both have units of [GeV]−^1 , while Newton’s
constantGhas units of [GeV]−^2 since it is defined as equal to the square of
the inverse Planck mass. These units are very convenient for cosmology, because
many problems deal with widely varying scales simultaneously. For example,
any computation of relic particle abundances (e.g. primordial nucleosynthesis)