The subject of the cosmological constant has had a long and checkered history in cosmology. The reasons for skepticism regarding a non-zero value of the cosmological constant are many. First, there is a discrepancy of 120 orders of magnitude between current observational limits and estimates of the vacuum energy density based on current standard particle theory (e.g. Carroll, Press and Turner 1992). Second, it would require that we are now living at a special epoch when the cosmological constant has begun to affect the dynamics of the Universe (other than during a time of inflation). In addition, it is difficult to ignore the fact that historically a non-zero has been dragged out prematurely many times to explain a number of other apparent crises, and moreover, adding additional free parameters to a problem always makes it easier to fit data. Certainly the oft-repeated quote from Einstein to Gamov about his ``biggest blunder" continues to undermine the credibility of a non-zero value for .
However, despite the strong arguments that can be made for = 0, there are compelling reasons to keep an open mind on the issue. First, at present there is no known physical principle that demands = 0. Although supersymmetry can provide a mechanism, it is known that supersymmetry is broken (e.g., Weinberg 1989). Second, unlike the case of Einstein's original arbitrary constant term, standard particle theory and inflation now provide a physical interpretation of : it is the energy density of the vacuum (e.g., Weinberg 1989). Third, if theory demands total = 1, then a number of observational results can be explained with a low m and m + = 1: a) for instance, the observed large scale distribution of galaxies, clusters, large voids, and walls is in conflict with that predicted by the (standard) cold dark matter model for the origin of structure (e.g. Davis et al. 1992; Peacock & Dodds 1994); and b) the low values of the matter density based on a number of methods as described in Section 2. In addition, the discrepancy between the ages of the oldest stars and the expansion age can be resolved. Perhaps the most important reason to keep an open mind is that this is an issue that ultimately must be resolved by experiment.
The importance of empirically establishing whether there is a non-zero value of cannot be overemphasized. However, it underscores the need for high-accuracy experiments: aspects of the standard model of particle theory have been tested in the laboratory to precisions unheard of in most measurements in observational cosmology. Nevertheless, cosmology offers an opportunity to test the standard model over larger scales and higher energies than can ever be achieved by other means. It scarcely needs to be said that overthrowing the Standard Model (i.e., claiming a measurement of a non-zero value for ) will require considerably higher accuracy than is currently available.
What are the current observational limits on ? In the next sections, limits based on both the observed numbers of quasars multiply imaged by galaxy ``lenses'' and limits from a sample of strongly lensed galaxies are briefly discussed.