In Part II of this book, we consider the history of the Universe after it is roughly one second old. It is quite well understood, and is known to begin with an era of rapid expansion dominated by hot radiation. In contrast, we have no certain knowledge about the preceding era that is the subject of Part IV.

We assume general relativity, which seems to accord with observation. This chapter is devoted to the unperturbed Universe. We begin with a general discussion of the main possibilities for particle distribution functions in thermal equilibrium, applying whenever the Universe has a gaseous component. Then we discuss baryon and lepton number. After these preliminaries, we lay down initial conditions, that are assumed to hold when the Universe is slightly less than one second old. It is assumed that there exist, in thermal equilibrium, protons, neutrons, electrons, positrons and all three species of neutrino and antineutrino.

We then see how, as the Universe expands, the initial conditions determine the primordial abundance of the lightest nuclei, in apparent agreement with observation. The success of this Big Bang Nucleosynthesis (BBN) calculation is one of the cornerstones of modern cosmology.

Next we follow the subsequent evolution of the unperturbed Universe. For this evolution to agree with observation, two more components have to be added to the cosmic fluid, leading to what is called the ∧CDM model.