The universe has been expanding since the big bang, thus early on it was hot and dense. To trace the history of the universe we must understand the dynamics that operates when the universe was hot and particles were very energetic. Before the standard model we could not go back further than 200,000 years after the big bang. Today, especially since QCD simplifies at high energy, we can extrapolate to very eary times, where nucleons melt and quarks and gluons are liberated to form a quark-gluon plasma.