Chapter 7

The cosmic microwave background

Arthur Kosowsky

Rutgers University, Piscataway, New Jersey, USA

It is widely accepted that the field of cosmology is entering an era dubbed
‘precision cosmology’. Data directly relevant to the properties and evolution of
the universe are flooding in by the terabyte (or soon will be). Such vast quantities
of data were the purview only of high-energy physics just a few years ago; now
expertise from this area is being coopted by some astronomers to help deal with
our wealth of information. In the past decade, cosmology has gone from a data-
starved science in which often highly speculative theories went unconstrained to
a data-driven pursuit where many models have been ruled out and the remaining
‘standard cosmology’ will be tested with stringent precision.
The cosmic microwave background (CMB) radiation is at the centre of this
revolution. The radiation present today as a 2.7 K thermal background originated
when the universe was denser by a factor of 10^9 and younger by a factor of
around 5× 104. The radiation provides the most distant direct image of the
universe we can hope to see, at least until gravitational radiation becomes a useful
astronomical data source. The microwave background radiation is extremely
uniform, varying in temperature by only a few parts in 10^5 over the sky (apart
from an overall dipole variation arising from our peculiar motion through the
microwave background’s rest frame); its departure from a perfect blackbody
spectrum has yet to be detected.
The very existence of the microwave background provides crucial support
for the hot big bang cosmological model: the universe began in a very hot, dense
state from which it expanded and cooled. The microwave background visible
today was once in thermal equilibrium with the primordial plasma of the universe,
and the universe at that time was highly uniform. Crucially, the universe could
not have been perfectly uniform at that time or no structures would have formed
subsequently. The study of small temperature and polarization fluctuations in
the microwave background, reflecting small variations in density and velocity

219