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.




The time


(1)
so

(2)
The time



(3)
at t = 100 s, T dropped below 10 GK and He was formed.
The period


(4)

(5)

(6)

(7)
The time t > 3000 y, is called the matter-dominated era.

(8)

(9)

(10)

(11)
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