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1.3 The nature of redshift

The quasars 3C 48 and 3C 273 had large redshifts in the context of the galactic redshifts known in the early 1960s. Subsequently, however, quasars with even higher redshifts began to be discovered. The source 3C 9 was the first very large redshift object to be found, with redshift z = 2. In fact, a large number of quasars with redshifts in this neighbourhood became known during the first decade of the discovery of quasars. These discoveries were also accompanied by theoretical discussions on their nature.

In the 1960s the astronomer was familiar with three kinds of redshift: (1) the Doppler shift arising from the relative motion between the source of light and the observer, (2) the gravitational redshift of light travelling from a strong to a weak gravitational field and (3) the redshift arising from the expansion of the universe. In the early days of quasar astronomy, all three interpretations were tried, before the consensus settled in favour of the last of these. It may be worth looking at the pros and cons of these different hypotheses briefly. We will return to this issue in a more detailed fashion in Chapter 15.