Because the highest-luminosity AGNs can be detected at very large distances (and hence ‘lookback’ times) they provide an important probe of the history of the Universe. Luminous quasars are detected at redshifts up to z ≈ 5; we see these objects as they were when the Universe was ∼ 10% its current age. Quasars are the most distant discrete objects that we have observed, and they are therefore of tremendous importance in understanding the formation of discrete structures from the primordial gas and as measures of the first appearance of metals. Furthermore, as w7e shall see in subsequent chapters, the luminosity function of quasars has varied over the observable history of the Universe, and this has something to tell us about the formation and evolution of galaxies and photoionization equilibrium of the intergalactic medium as a function of redshift, among other things. Finally, as we will see in Chapter 12, quasars can be used as background sources against which we can detect the absorption signatures of less luminous objects, thus providing us with a probe of otherwise unobservable gas at high redshifts.

Determination of the space density and luminosity function of any type of astronomical object is difficult, primarily because of the different volumes over which the most luminous and least luminous objects of a given class can be detected. In the case of quasars, the situation is further complicated by the fact that quasars are observed at sufficiently large distances that the curvature and expansion of the Universe must be taken into account.