There is no perfect standard candle. Each method has limitations and uncertainties which can only be determined by thorough testing. We cannot emphasize this approach too strongly, as it is fundamental to the scientific method. Every standard candle candidate must be subjected to direct observational tests to measure its sensitivity to various parameters such as galaxy luminosity, color, metallicity, and Hubble type. Some of the candles discussed here have not yet been examined so completely, and thus further work is implied.
In some cases, complete testing is simply impossible. For example, all the techniques we discuss are applicable to giant elliptical galaxies (except, of course, for the Cepheids and the HI line width - luminosity relation). Unfortunately, nature has not provided us with even one giant elliptical galaxy which is close enough to be used as an absolute calibrator. Consequently, it is impossible to test these techniques directly in galaxies with well-determined distances. Indirect tests, however, are possible for certain methods, and these are discussed in the sections that follow.
An issue which some feel quite strongly about is that a distance indicator must have a solid physical basis if it is to be given any weight (Aaronson and Mould 1986; Hodge 1981). Each of the methods discussed here rests on a physical prescription to some degree, and although none of the methods can be said to be completely understood, our ability to model the relevant physics in these objects is improving all the time. Thus, the apparent robustness of a computer model should have only limited influence on our estimate of the quality of a real physical system as a distance indicator. In fact, an empirical candle that has survived a rigorous testing program should carry far more weight than an untested candle that has a superb physical basis. Conversely, we should welcome those occasions when a candle is shown to be excellent in the empirical sense, but was predicted to be poor from first principles. Such a direct confrontation between observation and theory provides us the best opportunity to learn something new about the universe.
While most of the foregoing may seem trivially obvious, the concepts are absolutely critical. For example, two new methods for deriving the Hubble constant have been proposed recently: the time delay in a gravitational lens system (Rhee 1991; Roberts et al. 1991; Kochanek 1991), and the application of the Sunyaev-Zeldovich effect (Birkenshaw et al. 1991). These are extremely alluring methods because they bypass the entire chain of local calibrators and secondary indicators to give a direct measure of the Hubble constant. There are, however, some model dependencies which must be tested before the systematics of the techniques can be properly evaluated, and following the precepts of the above paragraphs, work is progressing toward that end (e.g., Press et al. 1992).