The principal concern of this book is the way in which recent particle physics, including electroweak theory, quantum chromodynamics, grand unified theory, supersymmetry, supergravity and superstring theory, has changed our standpoint on the history of the universe when its temperature was greater than 1015 K. This will be studied in the context of the Friedman–Robertson–Walker solution of the Einstein equations of general relativity. This chapter derives the field equations relating the scale factor that appears in the metric to the energy density and the pressure that characterize the energy–momentum tensor. It shows how energy–momentum conservation determines the scale dependence of the energy density and pressure. The standard solutions for the time dependence of the scale factor in a radiation-dominated universe, in a matter-dominated universe, and in a cosmological constant-dominated universe are presented. The chapter discusses briefly the ‘recombination’ of protons and electrons that left the presently observed cosmic microwave background radiation.