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This is a book about modern cosmology. Because this is a big subject – as big as
the Universe – we have had to choose one particular theme upon which to focus
our treatment. Current research in cosmology ranges over fields as diverse as
quantum gravity, general relativity, particle physics, statistical mechanics, nonlinear
hydrodynamics and observational astronomy in all wavelength regions, from
radio to gamma rays. We could not possibly do justice to all these areas in one
volume, especially in a book such as this which is intended for advanced undergraduates
or beginning postgraduates. Because we both have a strong research
interest in theories for the origin and evolution of cosmic structure – galaxies,
clusters and the like – and, in many respects, this is indeed the central problem
in this field, we decided to concentrate on those elements of modern cosmology
that pertain to this topic. We shall touch on many of the areas mentioned above,
but only insofar as an understanding of them is necessary background for our
analysis of structure formation.
Cosmology in general, and the field of structure formation in particular, has
been a ‘hot’ research topic for many years. Recent spectacular observational breakthroughs,
like the discovery by the COBE satellite in 1992 of fluctuations in the
temperature of the cosmic microwave background, have made newspaper headlines
all around the world. Both observational and theoretical sides of the subject
continue to engross not only the best undergraduate and postgraduate students
and more senior professional scientists, but also the general public. Part of the
fascination is that cosmology lies at the crossroads of many disciplines. An introduction
to this subject therefore involves an initiation into many seemingly disparate
branches of physics and astrophysics; this alone makes it an ideal area in
which to encourage young scientists to work.
Nevertheless, cosmology is a peculiar science. The Universe is, by definition,
unique. We cannot prepare an ensemble of universes with slightly different parameter
values and look for differences or correlations in their behaviour. In many
branches of physical science such experimentation often leads to the formulation
of empirical laws which give rise to models and subsequently theories. Cosmology
is different. We have only one Universe, and this must provide the empirical
laws we try to explain by theory, as well as the experimental evidence we use to
test the theories we have formulated. Though the distinction between them is, of
course, not completely sharp, it is fair to say that physics is predominantly characterised
by experiment and theory, and cosmology by observation and paradigm. |
