Searches for PGs are
really attempts to detect the furthest recognisable galaxies and thereby
determine the epoch of galaxy formation. Let us begin by examining the
relationship between the cosmological distance to a galaxy and the
light-travel time over that distance; this will illustrate the general power
of PG searches to probe the early Universe. Astronomers measure the
distance to a galaxy using the property known as the redshift (z). This
is the term
given to the increase in wavelength of light as it propagates through
space, caused by the expansion of the Universe. The size of the redshift is
a measure of how fast
distant objects are moving away from the Earth and for distances not
large compared to the size of the Universe (z < 0.1), the
redshift can be
related to the distance (D) by Hubble's law, D 
cz/H0, where c is
the speed of light, and H0 is Hubble's constant. At larger
redshifts the look-back time to a galaxy starts to become significant
compared to the age of the Universe and the dynamics of the expansion,
determined by its energy density content, becomes progressively
more important. The basic relationship between look-back time
t from
the present to a galaxy at redshift z is given by
where
M and 
are the contributions to
the energy density from the matter and cosmological constant respectively.
Figure 1 shows t as a function of z for three
cosmological models which span the range of accecptable values for
M and .
Despite the continuing debate in cosmology regarding the precise value
of the quantities M,
and H0,
observations of galaxies at z 1-5 always probe back to a time when the
Universe was 50%-10% of its
current age respectively, in any plausible cosmology. This makes observations
of galaxies at z > 1 a very powerful tool for understanding the
conditions in the early Universe and, not surprisingly, observations of
distant objects, including searches for PGs, have over the
years been extensive and wide-ranging: most, if not all, of the world's
principal ground-based and space-born astronomical facilities, have, at some
stage, devoted significant amounts of telescope time to primeval galaxy
search programmes, covering the electromagnetic spectrum from the
optical to the radio.
Figure 1. Look-back time as a function of redshift for the
cosmological models:
M = 1.0, = 0 (dashed line);
M = 0.2, = 0 (solid line);
M = 0.1, = 0.9 (dotted line). These
models have been calculated for H0 = 100h
km s-1 Mpc-1, where
h expresses the uncertainty in the value of H0 taking
values h = 0.5-1 and 1 pc 3.1 x 1016 m.