Part II: Thermal History of the Universe

From the Planck Epoch to Today

The early universe was a hot, dense plasma in thermal equilibrium. As it expanded and cooled, a sequence of phase transitions and decoupling events shaped the matter and radiation we observe today. Big Bang nucleosynthesis forged the lightest elements in the first few minutes, while recombination at T ≈ 3000 K released the photons of the cosmic microwave background. Understanding this thermal history is essential for interpreting all cosmological observations.