5.3.6 Cosmic Evolution If radio galaxies and quasars are related
strictly through aspect, then the relative numbers and
luminosities of each class should be broadly comparable at all
cosmological epochs. Indeed, the cosmological evolution of
steep-spectrum radio quasars, flat-spectrum radio quasars, and
radio galaxies can be described in each case by luminosity
increasing with redshift roughly as (1 + z)3 out to z ~ 2,
followed by a comparable decline in co-moving density at higher
redshifts (Dunlop and Peacock 1990). This means current samples
are broadly consistent with unification independent of redshift.
Over cosmic time, changes in the relative strengths of beamed
and unbeamed components in individual sources, or in the opening
angle of an obscuring torus (Lawrence 1991), might be expected.
In either of those cases, the observed evolution would be
different depending on orientation angle. The magnitude of this
effect is clearly model-dependent but if the unification of
radio galaxies and quasars is corre