An important feature of the standard cosmology is that the temperature of the Universe was arbitrarily high at the beginning of the Big Bang and has decreased continuously as the Universe expanded to its present state... the thermal history of the Universe follows from a simple application of thermodynamics to a small patch of the homogeneous and isotropic Universe. In this section we show that this thermal history, together with particle, nuclear and atomic physics, allows a detailed prediction of the matter content of the Universe at each epoch. The reason for this is simple: when the temperature of the Universe was higher than the rest mass of a kind of charged particles, the photon energy is high enough to create these particles and their antiparticles. This, in turn, could give rise to other kinds of particles. For example, when the temperature of the Universe was higher than the rest mass of an electron
of an electron, i.e. 
(corresponding to 
electrons and positrons could be generated via pair production, 
, and electronic neutrinos could be produced via neutral current reactions, such as 
. When the density of the Universe was sufficiently high, the creation and annihilation of (e, ē) pairs, and the Compton scattering between 
and photons, could establish a thermal equilibrium among these particles, while the neutrinos established such an equilibrium via their neutral current coupling to the electrons.
Consequently, the Universe was filled with a hot plasma that included 
and , all in thermal equilibrium at the same temperature.
In order to maintain thermodynamic equilibrium the frequency of interactions among the various particle species involved needs to be sufficiently high. The interaction rate is 
where n is the number density of particles, 
is their relative velocity, and σ is the interaction cross-section (which usually depends on ). As the Universe expands and the temperature drops, this rate in general decreases. When it becomes smaller than the expansion rate of the Universe, given by the Hubble parameter, 
, the particles ‘decouple’ from the photon fluid, and, as long as the particles are stable, their comoving number density ‘freezes-out’ at its current value. Except for possible particle species created out of thermal equilibrium (e.g. axions), and for particles that have been created more recently in high-energy processes, all elementary particles in the present-day Universe are thermal relics that have decoupled from the photon fluid at some time in the past.
In what follows we first present a brief outline of the chronology of the early Universe, and then discuss the production and survival of particles during a number of important epochs. Since the early Universe was dominated by relativistic particles, Eq. (3.81) can be used to relate temperature T (or energy 
) to the cosmic time. As this section is concerned with high energy physics, we will use the natural unit system in which the speed of light c, Boltzmann’s constant 
, and Planck’s constant 
are all set to 1. In cgs units [
. Therefore, making these constants dimensionless implies that
and all physical quantities can be expressed in one unit, usually mass or energy. However, they can also be expressed in one of the other units using the following conversion factors: 
. Whenever needed, the ‘missing’ powers of 
, and ℏ in equations can be reinserted straightforwardly from a simple dimensional analysis.
