4 The very early universe
The laws of particle interactions are well established only below the energy cur-
rently reached by accelerators, which is about a few hundred GeV.The next gen-
eration of accelerators will allow us to go a couple of orders of magnitude fur-
ther, but even in the remote future it will be impossible to overcome the existing
gap of about seventeen orders of magnitude to reach the Planckian scale. There-
fore, the only “laboratories” for testing particle theories at very high energies are
the very early universe and astrophysical sources of highly energetic particles.
The quality of cosmological information is much worse than that gained from
accelerators. However, given the lack of choice, we can still hope to learn es-
sential features of high-energy physics based on cosmological and astrophysical
observations.
The particle theory describing interactions below the TeV scale is called the
Standard Model and it comprises the unified electroweak theory and quantum
chromodynamics, both based on the idea of local gauge symmetry. Attempts to
incorporate the electroweak and strong interactions in some larger symmetry group
and thus unify them have not yet met with success. Unfortunately, there are too
many ways to extend the theory beyond the Standard Model while remaining in
agreement with available experimental data. Only further experiments can help us
in selecting the “correct theory of nature.”
This situation determines our selection of topics for this chapter. First, we con-
sider the Standard Model, and explore the most interesting consequences of this
theory for cosmology. In particular, the quark–gluon transition, restoration of elec-
troweak symmetry and nonconservation of the fermion number will be discussed
in great detail.
Two important cosmological issues beyond the Standard Model are the gener-
ation of baryon asymmetry in the universe and the nature of weakly interacting
massive particles, a possible component of cold dark matter. In the following chap-
ter we will see that any initial baryon asymmetry is washed out during inflation and
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