Introduction

To understand the features of the universe today, it is necessary to grasp the past history of the universe. We now tackle this issue and describe the physical processes that occur in the earlier phase of the universe. Section 2 develops the basic thermodynamics needed to understand these processes. In Sections 4.3 and 4.4, we consider the possible existence of a relic background of massless or massive fermions (like the neutrinos) in our universe today. In Section 4.5 we discuss the primordial nucleosynthesis and its observational relevance; we study the decoupling of matter from radiation in Section 4.6. In the last section the very early universe and inflationary models are described.

Distribution Functions in the Early Universe

The analysis in Chap. 3 showed that the universe was dominated by radiation at redshifts higher than zeq ≃ 3.9 × 104(ΩNRh2). In the radiation-dominated phase, the temperature of the radiation will be higher than Teq ≃ 9.2 (ΩNRh2)eV ≃ 1.07 × 105(ΩNRh2)K and will be increasing as T ∝ (1 + z).

The contents of the universe, at these early epochs, will be in a form very different from that in the present-day universe. Atomic and nuclear structures have binding energies of the order of a few tens of electron-volts and mega-electron-volts, respectively. When the temperature of the universe was higher than these values, such systems could not have existed as bound objects.