The current Universe has evolved since its early phase of thermal equilibrium until the present state. The departure from thermal equilibrium in the early Universe made a fossil record we can observe at current time!

There are some easy rules for thermal equilibrium. The easiest one, is that coming from the “interaction rate” . It can be expressed in the following way:

and then, at a given temperature T, we get

and where

**Remark:** If , then

and it implies that a particle interacts less than once after the time .

Moreover, we can understand roughly the so-called decoupling era:

**1st. Any interaction mediated by a massless gauge boson provides**

with and

and this implies that

and

so the equilibrium temperature is found whenever !

**2nd. Interactions mediated by any massive gauge boson provides**

with

and this implies that

and

and then

Moreover,

As a consequence, we can realize that the out-of-equilibrium phenomena in the early and current Universe are very important processes! In particular:

1) They provide the formation of light elements during the * Big Bang Nucleosynthesis (BBN)*, also known as

**, i.e., the formation of the first light elements after the Big Bang (circa 300000 years after the Universe “birth”).**

*primordial nucleosynthesis*2) They provide the path of * recombination* of electrons and protons into hydrogen atoms.

3) They imply the (likely) production of dark matter (or equivalently the presence of some kind of “modified gravity” or/and modified newtonian dynamics).

**Boltzmann’s equation for annihilation of particles in equilibrium**

There is a beautiful equation that condenses the previous physical process of equilibrium at a given temperature and the particle production it yields. Conceptually speaking, we have

Consider a process like

and where the particle 1 is the one we are interested in. Then, we deduce that

where A is certain complicated facter involving “delta functions” of the energies and momenta of the particles 1,2,3,4 and an additional term depending on the statistics of the particle. Explicitly, it takes the form

with

and where

is the Fermi-Dirac (FD, -)/Bose-Einstein (BE,+) distribution. In fact, the above FD/BE factors provide the so-called Pauli blocking/”Bose-Einstein” enhancement effects for the particle production in the processes and . Indeed, particle physics enter into the game here (see above formulae again) and we assume

Do you recognize the *principle* of *detailed balance *in this equation?

We can simplify the assumptions a little bit:

1st. The kinetic equilibrium is taken to be a rapid elastic scattering and we input the FD/BE statistics without loss of generality.

2nd. The annihilation in thermal equilibrium will be calculated from the sum of the chemical potential in any balanced equation.

3rd. Low temperature approximation. Suppose that

then we obtain the Maxwell-Boltzmann approximation to the FD/BE statistics

and , so, since , we get

What is ? After a change of variable

is the “equilibrium number density” deducen from the expression

and it yields

and then we finally get that

equals

Now, we can define the thermally averaged cross section

The Boltzmann equation becomes with these conventions

Remark (I): LHS is similar to and the RHS is similar to

Remark (II): If the reaction rate is , then it provides the chemical equilibrium condition well known in the nuclear statistical equilibrium as the Saha equation, i.e.,