### 1. PARAMETRIZING THE UNIVERSE

Rapid advances in observational cosmology are leading to the
establishment of
the first precision cosmological model, with many of the key cosmological
parameters determined to one or two significant figure
accuracy. Particularly prominent are measurements of cosmic microwave
anisotropies, led by the first
results from the Wilkinson Microwave Anisotropy Probe (WMAP) announced in
February 2003
[1].
However the most accurate model of the Universe requires
consideration of a wide range of different types of observation, with
complementary probes providing consistency checks, lifting parameter
degeneracies, and enabling the strongest constraints to be placed.

The term `cosmological parameters' is forever increasing in its scope, and
nowadays includes the parametrization of some functions, as well as simple
numbers describing properties of the Universe. The original usage
referred to
the parameters describing the global dynamics of the Universe, such as its
expansion rate and curvature. Also now of great interest is how the matter
budget of the Universe is built up from its constituents: baryons, photons,
neutrinos, dark matter, and dark energy. We are interested in describing
the nature of perturbations in the Universe, through global statistical
descriptions such as the matter and radiation power spectra. There may
also be parameters
describing the physical state of the Universe, most prominent being the
ionization fraction as a function of time during the era since decoupling.
Typical comparisons of cosmological models with observational data now
feature about ten parameters.