(1) Despite our failure to find (or recognize) "standard candles" or
"standard rods" relating L and S or d and
, they would allow
potentially definitive tests of world
models and should not be forgotten. Every advance in instrumentation, every
increase in astrophysical knowledge of radio sources should be
considered an opportunity for determining world-model parameters. For
example, recent high-sensitivity VLBI maps have found milliarcsecond
jets with bulk relativistic velocities
nearly equal to the speed of light, even in extended steep-spectrum
sources whose orientations relative to the line of sight should be
random. The angular separations between cores and jet knots in a
complete sample of such sources might be combined
with their nearly "standard velocities" v
c to measure
directly the angular-size distances corresponding to the source redshifts.
(2) The source counts n(S |
) are generally better
determined than most of the data needed for their interpretation. Little
progress can be made at high flux densities
because essentially the whole sky has been covered with accurate
measurements. The sensitivity limits of the current generation of
telescopes has nearly been reached. Source counts below
S
10µJy at = 1.4
GHz will be
difficult to obtain directly, although some limits to the source counts
at fainter levels might be obtained from background
measurements. Increasing the sky area covered by deep surveys is still
needed to reduce the statistical errors, especially at
= 5 GHz,
where each map only covers about 10-5 steradians. Overlapping
surveys at 1.4 and 5 GHz are badly
needed to determine the spectral-index distributions of faint sources
selected at these two frequencies.
(3) The weighted local luminosity function at
= 1.4 GHz has larger statistical
errors than the weighted source count, particularly near the peaks of
the spiral-galaxy and elliptical-galaxy contributions, so it contributes
significantly to the uncertainty in the evolution function
E(L, z). Improvements are especially needed
near L
1023 W Hz-1 at
= 1.4 GHz to interpret the
source count near S
1 mJy
and decide whether a "new population" of sources exists. This will
require an optically complete sample with known redshifts filling a
large volume of space; the UGC sample
(Nilson 1973)
for example. Radio observations of the same sample
should be made at = 5 GHz as
well as at a lower frequency so that the
local luminosity functions of steep- and flat-spectrum sources can be
determined independently.
(4) Redshifts of complete samples of flat-spectrum sources stronger than
S 0.1
Jy should be measured to confirm suggestions that evolution peaks at
z 2. This may
not be too difficult because most flat-spectrum sources stronger than
S 0.1
Jy can be identified with fairly bright quasars with strong emission
lines. If very-high-redshift quasars are common, they will surely be
found in such samples. If they are not, only radio-selected samples may
be sufficiently immune to selection effects to prove it.
(5) Optical programs like the Leiden-Berkeley Deep Survey should be
extended to sources as faint as
S 0.1 mJy at
= 1.4 GHz
to determine the redshift
distribution and host galaxy population of the faint-source population.
(6) High-resolution (
1") maps of sources fainter
than S 1 mJy
should also be made to determine their
angular-size distributions and improve their radio positions for
reliable optical identifications.
(7) Isotropy tests at somewhat higher effective resolutions should be made to detect density fluctuations on scales less than 50 Mpc.