Annu. Rev. Astron. Astrophys. 1992. 30: 499-542 Copyright © 1992 by Annual Reviews. All rights reserved

4.7 Astrophysics of Distant Objects

One possible test for the cosmological constant has been explored very little. A zero value of is almost invariably assumed by investigators whose interest is focused not on cosmology per se but on attempts to build detailed physical models of distant cosmic objects. Since the various cosmic distance measures depend on significantly (see Section 3.3), the physical properties (sizes, velocities, luminosities, etc) of distant objects are influenced by these choices. At least in principle, it is possible that physical models of some such object or class of objects might work (or at least be plausible) for certain values of and _M and not for others. Possible candidate types of objects include high-redshift radio source lobes, quasars, the gas clouds that produce quasar absorption lines, superluminal motion VLBI sources, and thermal x-ray sources in high-redshift galaxy clusters. One such possibility which has been explored slightly (Malhotra & Turner 1992) is the population properties of quasars which differ significantly in flat -dominated models from those normally considered (based on zero cosmological constant cosmologies). Of course, our astrophysical understanding of extragalactic objects is not generally so firm (nor scale dependent) that this approach offers hope of easy progress, but it may deserve at least selective exploration.