The Abundance of Quasars over Time

Looking at large distance (redshift) is equivalent to looking back in time because of the finite speed of light. Thus, we may estimate how the density of quasars has changed over time by comparing the number of nearby quasars with the number that are far away.

The results are shown in the adjacent diagram, and indicate that quasars were much more abundant early in the history of the Universe than they are today. The decrease of quasar abundances at the very earliest times (redshifts larger than about 3) may represent partially the finite amount of time for quasars to begin forming after the formation of the Universe. It may also reflect some observational bias, since it is very difficult to detect objects at the largest distances and therefore the earliest times.