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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 appeq 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 Deltat from the present to a galaxy at redshift z is given by

Equation 1 (1)

where OmegaM and OmegaLambda are the contributions to the energy density from the matter and cosmological constant respectively. Figure 1 shows Deltat as a function of z for three cosmological models which span the range of accecptable values for OmegaM and OmegaLambda. Despite the continuing debate in cosmology regarding the precise value of the quantities OmegaM, OmegaLambda and H0, observations of galaxies at z appeq 1-5 always probe back to a time when the Universe was appeq 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: OmegaM = 1.0, OmegaLambda = 0 (dashed line); OmegaM = 0.2, OmegaLambda = 0 (solid line); OmegaM = 0.1, OmegaLambda = 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 appeq 3.1 x 1016 m.

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