6 Thermal History of the Universe
This history should start at the Big Bang which we defined in the previous chapter as
time zero. General relativity is a classical theory in which the coordinates푡, 푥, 푦, 푧can
take any real values. However, Equations (5.41) and (5.42) showed that at time zero
no meaningful description of the Universe exists.
Actually there are problems already for small but nonzero times, at the energy scale
where gravitational and quantum effects are of equal importance. Then we can not
do particle physics neglecting gravitation nor can we describe the Universe with the
Einstein equation neglecting quantum mechanics.
In Section 6.1 we begin with a definition of the Planck time, leaving the cosmic
inflation for a later chapter. In Section 6.2 we continue with the primordial hot plasma
which deals with particle physics symmetries, photons in thermal equilibrium, energy
densities, relativistic versus nonrelativistic particles, spin and statistics, temperature
and time scales.
We follow the cooling plasma until, in Section 6.3, photon and lepton decoupling
occurs. Next comes photon reheating, neutrino decoupling and the recombination
era. We end with a brief discussion on equilibrium theory.
In Section 6.4 we follow the thermal history of the nucleons for the momentous
fusion processes in Big Bang nucleosynthesis (BBN) which has left us very important
clues in the form of relic abundances of helium and other light nuclei. The nucleosyn-
thesis is really a very narrow bottleneck for all cosmological models, and one which
has amply confirmed the standard Big Bang model. We find that the baryonic matter
present since nucleosynthesis is completely insufficient to close the Universe.
In Section 6.5 we try to understand baryosynthesis and the absence of antimatter.
Introduction to Cosmology, Fourth Edition. Matts Roos
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.