132 The very early universe
its generation is a crucial element of inflationary cosmology. The general conditions
under which this asymmetry occurs are rather simple and model-independent. How-
ever, the particular realization of these conditions depends on the particle theory
involved. At present, there exists no preferable scenario for the origin of baryon
asymmetry. There are many possibilities and the problem is, as always, to select
the correct one. For these reasons, we will only demonstrate that the importantsin-
glenumber, characterizing baryon excess, can be easily “explained.” The situation
for the origin of cold dark matter is very similar, and we likewise concentrate on
general ideas here.
Almost all plausible extensions of the Standard Model have a number of fea-
tures in common, which are rather insensitive to the details of any particular
theory. Among these features is a nontrivial vacuum structure, potentially re-
sponsible for phase transitions in the very early universe. As a result, topologi-
cal defects, such as domain walls, strings, or monopoles, could also have been
formed. There is no doubt that such good physics belongs to a primary course on
cosmology.
We begin with a brief overview of theelementsof the Standard Model, which
should by no means be considered a substitute for standard textbooks in particle
physics. It serves as a reminder of the basic ideas we need in cosmological appli-
cations. To shorten the presentation, we follow an “antihistorical” approach: the
theory is formulated in its “final” form, and then its consequences for cosmology
are explored. However, the reader should not forget that the numerous building
blocks of the Standard Model were discovered as a result of concerted−and
rarely straightforward−efforts to understand and interpret an enormous amount
of experimental data.
4.1 Basics
Elementary particles are the fundamental indivisible components of matter. They are
completely characterized by their masses, spins and charges. Different charges are
responsible for different interactions and the interaction strength is proportional to
the corresponding charge. There are four known forces:gravitational, electromag-
netic, weak, and strong. The first two are long-range forces whose strength decays
following an inverse square law. The weak and strong interactions are short-range
forces. They are effective only over short distances and then decay exponentially
quickly outside this range. Gravity is described by Einstein’s theory of General
Relativity, and the other three interactions by the Standard Model, based on the
idea of local gauge invariance.