Thermal History of the Early Universe
From Hot Plasma to Neutral Atoms
Why the Universe was hot
As we extrapolate the expansion backward in time, densities and temperatures increase. The early Universe was therefore a hot, dense plasma of particles and radiation in approximate thermal equilibrium. As the Universe expanded and cooled, reactions froze out, species annihilated, nuclei formed, and eventually atoms became stable.
Milestones in the early Universe
Why does radiation dominate at very early times?
Radiation density scales as , which grows fastest backward in time.
Correct answer: Because its density scales as , making it larger at small
What event made the cosmic microwave background possible?
When neutral atoms formed, photons scattered far less and began traveling freely through space.
Correct answer: Photon decoupling at recombination
Common misconception
The Big Bang was not an explosion at a point in preexisting space; it was the expansion of space everywhere.
A useful scaling law
For a fluid with equation-of-state parameter , the density evolves approximately as . This explains why matter, radiation, and vacuum energy behave so differently as the Universe expands.