The Energy Problem

The quasars lie at great distances. The fact that they are visible at such distances implies that they emit enormous amounts of energy, and the variability argues that this energy is produced in a region that can be no larger than the Solar System (and could be smaller). When this was first realized, it raised serious issues about whether there were known physical processes that could account for the energy characteristics of quasars.

Since the discovery of quasars, a likely mechanism to produce energy on this scale in such a compact region has emerged. As we shall see, there is relatively uniform agreement that rotating black holes containing of order a billion solar masses (which would be smaller in size than the Solar System) are the most plausible candidates for such energy production. In addition, we shall see that this picture of a quasar implies that both quasars and other active galaxies can be described by a unified model with a central black hole engine.

Non-Cosmological Interpretations of Quasars The primary difficulty in explaining the energy production of quasars comes from the interpretation through the Hubble law that they lie at such great distance, which means they must pack enormous luminosity. Some early attempts were made to explain the redshifts as not being cosmological, that is, as being due to something other than the Hubble Law. Then, they could perhaps be nearby objects and thus not so luminous. Doppler Shifts from Peculiar Velocities For example, one proposal was that quasars were more local objects that had been ejected from nearby galaxies at high velocity away from us. Then the red shift would be due to the normal Doppler effect for a local (peculiar) velocity, not the Hubble Law. This explanation was unable to overcome two fundamental objections. First, no plausible mechanism was proposed to explain how quasars could be ejected at such high velocities from more nearby objects. Second, even if there were such a mechanism, why do we see only redshifts for quasars and not blueshifts too? If quasars are being ejected from nearby galaxies by some unknown mechanism, we would expect that some quasars would be ejected toward us as well as away from us, but no blueshifted quasar has ever been recorded among the 8000 or so that have been discovered. Redshift Discrepancies? As another example, some instances were found where quasars were adjacent to other objects on the celestial sphere having much smaller redshifts, and there was some evidence that the quasar and other object were connected to each other by bridges of matter. This would call into question the redshift interpretation of the quasars, since objects connected to each other obviously must lie at about the same distance from us. However, just because objects lie next to each other on the celestial sphere does not mean that they are connected to each other physically. Even apparent bridges of matter may be an optical illusion created by part of an object in the foreground overlapping part of an object in the far background. Most astronomers now believe that these examples are just chance alignments of two objects that have no physical connection. Gravitational Redshifts Another possibility is that the quasar redshifts could be because of a strong gravitational field rather than expansion of the Universe. As we saw in discussing the general theory of relativity in Chapter 4, strong gravity can produce redshifts that have nothing to do with either expansion of the Universe or Doppler shifts. However, a gravitational redshift interpretation for quasars can be ruled out rather quickly because the intense gravitational field that would be required implies significant effects on the spectrum that are not observed. The Verdict: Redshifts are Cosmological Thus, the evidence is very strong that the redshifts are cosmological. Almost all astronomers now agree that quasars really are at the great distances implied by their redshifts and the Hubble law.