focusing effects of the curvature and the more rapid deceleration.Astronomers endeavor to identify and employ classes of bright sourcesof known or calculable luminosity as “standard candles.” By measuring the apparentluminosity at various redshifts of these sources, cosmologists candetermine whether the universe is closed or open. Supernova explosionsprovide sources of this kind because their intrinsic brightness isgoverned by the physics of the explosion, of which there is goodtheoretical understanding.
Quasars are star-like objects with large redshifts and inferred luminositiesthat are often hundreds of times those of normal galaxies. They arethought to occur in the nuclei of galaxies, but the conditions requiredfor a galaxy to harbor a quasar are not known. Quasars show strongevolution in the sense that they emitted much more energy at earliercosmic epochs, but why this is so is also unknown. Moreover, theobserved rapid variation in brightness of individual quasars is notunderstood. The task of understanding the nature and origin of quasarsis still at the forefront of cosmological research.
Quasars can serve as background lamps against which absorption fromintervening material can be detected spectroscopically. The materialmay be in the form of gas in galaxies (the galaxy itself may or maynot be visible) or in the form of intergalactic clouds of gas thathave never been processed through stars. The technique has exceptionallyhigh sensitivity to small amounts of material, and so it providesa probe of the universe that is independent and complementary tothat provided by visible galaxies. The change in the average numberof absorbers as a function of redshift is an important diagnosticfor understanding the evolution of these objects. Spectroscopic attributesof the absorbers can tell us about their physical properties as wellas the intensity and spectrum of the intergalactic radiation fallingon them.
These absorption studies have shown that the absorbing gas occursin lumps, and that there is little neutral hydrogen in a smoothlydistributed intergalactic medium. One explanation of this lack ofneutral hydrogen is that, at some point, the entire universe wasreionized—heated so hot that hydrogen atoms were broken up into theirconstituent protons and electrons. But if the universe was reionized,what were the heating agents and when did the reionization occur?On the other hand, if there was no epoch of reionization, what conditionsyielded such high efficiency in clearing intergalactic space of neutralhydrogen? This field will be advanced with the further identificationof close pairs of quasars to provide nearly coincident lines of sight,as well as the further identification of galaxies likely to be responsiblefor individual absorption