Preface to Third Edition
This preface can start just like the previous one: in the seven years since the second
edition was finalized, the field of cosmology has seen many important developments,
mainly due to new observations with superior instruments. In the past, cosmology
often relied on philosophical or aesthetic arguments; now it is maturing to become
an exact science. For example, the Einstein–de Sitter universe, which has zero cos-
mological constant (훺휆=0), used to be favored for esthetical reasons, but today it is
known to be very different from zero (훺휆= 0. 73 ± 0 .04).
In the first edition I quoted훺 0 = 0. 8 ± 0 .3 (daring to believe in errors that many oth-
ers did not), which gave room for all possible spatial geometries: spherical, flat and
hyperbolic. Since then the value has converged to훺 0 = 1. 02 ± 0 .02, and everybody is
now willing to concede that the geometry of the Universe is flat,훺 0 =1. This result
is one of the cornerstones of what we now can call the ‘Concordance Model of Cos-
mology’. Still, deep problems remain, so deep that even Einstein’s general relativity is
occasionally put in doubt.
A consequence of the successful march towards a ‘concordance model’ is that many
alternative models can be discarded. An introductory text of limited length like the cur-
rent one cannot be a historical record of failed models. Thus I no longer discuss, or
discuss only briefly,푘≠0 geometries, the Einstein–de Sitter universe, hot and warm
dark matter, cold dark matter models with휆=0, isocurvature fluctuations, topologi-
cal defects (except monopoles), Bianchi universes, and formulae which only work in
discarded or idealized models, like Mattig’s relation and the Saha equation.
Instead, this edition contains many new or considerably expanded subjects:
Section 2.3 on Relativistic Distance Measures, Section 3.3 on Gravitational
Lensing, Section 3.5 on Gravitational Waves, Section 4.3 on Dark Energy and
Quintessence, Section 5.1 on Photon Polarization, Section 7.4 on The Inflaton
as Quintessence, Section 7.5 on Cyclic Models, Section 8.3 on CMB Polarization
Anisotropies, Section 8.4 on model testing and parameter estimation using mainly the
first-year CMB results of the Wilkinson Microwave Anisotropy Probe, and Section 9.