| When quasars were first discovered, their large luminosities and small sizes troubled many astronomers. In the 1960s and 1970s no known mechanism could account for the generation of 1000 Milky Ways' worth of energy within a region comparable in size to the solar system. The idea that supermassive black holes may be quite common in the cores of galaxies was unknown.
In response to these problems some astronomers, notably the respected observer Halton Arp and the equally reputable theorist Geoffrey Burbidge, sought an alternative explanation for quasars. Instead of believing that these objects were at cosmological distances and so very luminous, these researchers argued that perhaps there was an alternative, noncosmological, explanation for their great redshifts. Quasars could then be relatively nearby and hence much less bright. For example, if 3C 273 were 100 times closer—only 6.6 Mpc, not 660 Mpc, away—the inverse-square law says that the luminosity required to account for its observed apparent brightness would be reduced by a factor of 10,000.  (Sec. 17.4) In that case, 3C 273 would radiate "only" 1036 W (1/10 the energy output of our Galaxy). This is still a lot of energy, but it is perhaps more easily explainable in terms of "familiar" stellar events: the formation of high-mass stars, supernovae, and so on.
Arp has reported many examples of instances where galaxies and quasars are found close together on the sky but have very different, conflicting redshifts. Figure 25.18 is an example of such an alignment. He argues that there are simply too many of these "coincidences," where a foreground galaxy lies in nearly the same direction as a supposed background quasar, for the distant-quasar hypothesis to be correct. Instead, he claims, the quasars must be physically close to the galaxies, and the redshifts have some other, noncosmological, explanation.
Arp and coworkers have gone further, citing instances where neighboring galaxies have conflicting redshifts, such as the system in the first image here, where one of the five galaxies (Stephen's Quintet) has a redshift very different from those of the others. Similarly, in the second image (which is shown as a photographic negative to bring out the faint structure), a small galaxy appears to be connected to a larger galaxy (NGC 7603) having a very different redshift by a faint "bridge" of gas, reinforcing the view that the two really are close together in space. Such findings call into question both the cosmological interpretation of all galactic redshifts and Hubble's law itself.
Most astronomers would argue that the claims of conflicting redshifts are not statistically significant. In other words, given the numbers of |
known galaxies and quasars, accidental superpositions on the sky should be quite commonplace, and the observed quasar—galaxy and galaxy—galaxy alignments are quite consistent with pure chance. The apparent bridges may simply be photographic or image-processing defects. Usually, a lot of computer enhancement is needed to bring out the images, and there is ample opportunity for "features" to appear where none really exist. Finally, Hubble's law is well established for galaxies within a few hundred megaparsecs, and some quasars have been found in galaxy clusters, sharing the redshift of their neighbors. Thus, at least some quasar redshifts are known to be cosmological, so the violations of Hubble's law that Arp claims exist appear only quite selectively—if they really do exist at all.
If quasars are actually local, no convincing explanation has ever been advanced for their redshifts. If they are simply moving at high speeds, then we are led to the new "energy problem" of explaining how they were accelerated to such speeds. If quasars were fired out of galaxies—one alternative explanation for their great speed—we would expect some to be moving toward us, and hence blueshifted. So why are they all redshifted? If quasars are all "local" and come from our own Galaxy, then the Milky Way is special—a major violation of the principle of mediocrity and current scientific dogma—or the quasars must be so close (and thus quite faint) that we should have observed the motion of some of them across the sky. Arp's own supposed correlations between other galaxies and quasars would also argue against a Milky Way origin. In short, no matter where we try to put the quasars, their redshifts pose problems. An alternative attempt to explain the redshift as a gravitational redshift suffered by light as it climbs out from the vicinity of a black hole (see Chapter 22) also fails because it cannot account for the observed widths of quasar spectral lines. (Sec. 22.6)
There is no clear observational evidence for conflicting quasar redshifts, and no satisfactory mechanism for producing the redshifts by noncosmological means. Furthermore, as we have seen, there really is no "quasar luminosity problem" anymore. Quasar luminosities can be explained by the same mechanism that powers active galaxies, with only a modest increase in fuel consumption and certainly without posing a serious challenge to the laws of physics. Consequently, the overwhelming majority of astronomers hold that quasar redshifts are cosmological in origin and that quasars really are the most distant objects known in the universe. Of course, there are many instances in the history of astronomy where the majority has later been proved totally wrong! |
