At t = 0, the universe was a singularity with infinite matter/energy density and zero volume. Then the Big Bang occurred.
The time s is called the heavy particle era,and is characterized by high-energy photons colliding and producing protons and neutron.

so

The time s s is called the light particle era, and is characterized by photon colliding and producing electron,

at t = 100 s, T dropped below 10 GK and He was formed.
The period is called the radiation-dominated era. It satisfies




The time t > 3000 y, is called the matter-dominated era.