Preface to Fourth Edition
Just like the previous times I can state that the field of cosmology has seen so many
important developments in the 11 years since the third edition that a fourth edition
has become necessary.
In the first Chapter there is a new section presenting briefly various forms of bary-
onic matter: in supernovae and neutron stars and in noncollapsed objects such as
interstellar dust, hot gas in the intergalactic medium, the cosmic rays, neutrinos and
antiparticles. Active galactic nuclei, gamma ray bursts and quasars are also men-
tioned. The Hubble parameter and the age of the Universe are updated.
Chapter 2 is reorganized to contain only special relativity whereas all of general
relativity forms Chapter 3. Chapter 2 ends with a brief new section on tests of special
relativity and variable speed of light.
In the previous editions the Einstein equation was “derived” in the weak field limit
with some hand-waving arguments. That derivation is still in Chapter 3, but the Ein-
stein equation is now properly derived from the Hilbert–Einstein action. To some
readers the derivation will then be more difficult, but one can of course skip it and be
satisfied with the weak field limit. How to incorporate the energy of a gravitational
field as a pseudotensor is briefly mentioned.
Chapter 4 addresses tests of general relativity with the exception for black holes
which are now in Chapter 5. A new binary pulsar has been added and the section on
detectors of gravitational radiation has been updated.
Chapter 5 on cosmological models contains the Schwarzschild model (previously
in Chapter 2), a considerably expanded and modernized section on black holes, some-
what speculative perhaps, because it is based on Hawking’s recent ideas. The last
section is entirely new, it discusses extended gravity models starting from a gener-
alization of the Einstein–Hilbert action, and it lays the basis for dark energy models
in Chapter 11.
Chapter 6 is now a very much abridged version of the previous chapters 5 on the
thermal history of the Universe and 6 on particles and symmetries. This reduction was