The study of gravitational lenses is intimately tied to observational cosmology. When we observe a gravitationally lensed quasar, we are viewing a single object along two or more neighboring paths (null geodesics) of cosmological dimensions (Figure 1). What we see depends on bulk properties of the universe, such as Ho and qo, on the large scale structure and inhomogeneities along the paths, and on the small scale structure in and around the primary deflector. Furthermore, the deflection of light depends on the gravitational field along the line of sight, so it is sensitive to all forms of matter: luminous or dark, baryonic or exotic. Thus the images of gravitationally lensed quasars contain an imprint of the universe that is virtually inaccessible by any other means. The hope of decoding this imprint has stimulated observers and theorists to expend many thousands of hours of telescope time, computer time and cogitation on the elucidation of gravitational lens properties.
Gravitational Lenses as Tools in Observational Cosmology
