Observations made with great care and skill by Allan Sandage and collaborators at the Hale Observatories showed, in the mid-1970s, that in the case of the brightest galaxies in rich clusters, the Hubble law, linking redshift with cosmic distance, probably holds good to a redshift of at least 0.46. Beyond the distance correspond¬ing to that redshift there is no direct empirical evidence relating redshift and distance for any object. Recall that the radio galaxies and quasars present a formidable energy problem: if we use their redshifts to give distance via the Hubble law, then very large energy stores are required, and in the case of quasars they are packed into very tiny volumes. The magnitude of the problems involved has led some researchers to propose that the Hubble law breaks down in certain cases: at large redshifts and for quasars.

The arguments that have been advanced against the interpretation of quasar redshifts as distance indicators are several. Firstly there is the energy problem, which is a good deal less acute if quasars are nearer than their redshifts indicate. Secondly there is the formidable evidence brought forward by the distinguished observer Halton Arp. He has found several examples of apparently related objects with different redshifts, such as faint bridges between objects having different redshifts. He has also found that some quasars are remarkably close to bright galaxies Through Arp’s careful research there runs a consistent theme: not all the redshifts that we measure for extragalactic objects are hi agreement with Hubble’s law; how, therefore, can we be sure that unusual objects, such as quasars, obey it? Arp has certainly made interesting points and challenged the establishment view commendably, but it is hard to assess the significance a posteriori of the alleged relations between quasars and bright galaxies. Thirdly we may consider statistical evidence. Some researchers have tried to show that quasars are, on the average, in configurations relative to galaxies that are most unlikely to arise by chance. Therefore, the argument runs, the quasars must be somehow related to a set of nearby objects, galaxies, and cannot be at the cosmological distance derived from the redshift.

Several models have been advanced to explain how quasar light might be redshifted by a means other than the recession phenomenon associated with the expanding Universe. One idea, popular after the realization that black holes might really exist, is that quasars contain a very massive object which causes the emitted radiation to have a high gravitational redshift. This theory does not account for the very high redshifts. Another notion is that light from quasars might somehow get ‘tired’ on its long journey to MS, and, in using energy on its travels, become redshifted. However, nobody has got this light-fatigue model to work convincingly. Yet another concept that has-been tried is to alter the laws and constants of physics for large times and at large distances. Most physicists prefer to play the game by keeping the ‘constants’ constant under all conditions; once they are changed there are repercussions for all of physical theory, and it seems preferable to keep the present structure for the foreseeable future.

An important discovery in connection with the redshift controversy was the discovery of BL Lacertae objects, or LACERTIDS, named after the prototype. These are variable radio emitters that resemble quasars photographically but that have no emission or absorption lines in their optical spectra. The finding of quasar-like objects with featureless spectra showed that there might be a whole range of related extragalactic radio sources — active galaxies, lacertids, and quasars – differing mainly in the degree of activity in the nucleus.

Further important clues came with the realization that the nuclei of Seyfert galaxies and N-type galaxies are just as variable as quasars. Until variability in quasars was established, no careful observations had been made of changes in galactic nuclei. Once the variability had been found in other extragalactic objects, quasars seemed a little less unusual. Another factor was the detective work that led to the discovery of quasars in clusters of galaxies with a common redshift. This showed that some quasars at least behave as galaxies do rather than being a class apart.

In retrospect it seems that the redshift controversy grew up as an accident caused by the order in which key discoveries were made. If quasars had been found after Seyfert galaxies had been studied more closely, after the work on compact objects as energy sources had been started, after the discovery of lacertids, and after the exploration of a whole variety of galactic nuclei, they would have been accepted more or less immediately as a natural extension of phenomena already satisfactorily explained. We prefer to take the view that quasars do follow the Hubble law., and therefore that they are remote objects in the furthest reaches of our expanding Universe. This conclusion allows us to treat quasars as cosmological probes, that is, as observable test objects in the far-off universe , a universe that has properties rather different from prevailing at our time and in our locality.

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