Multiple light deflection

Abstract

If the light emitted by a source is deflected by a single matter concentration in the universe, the theory for a single geometrically-thin mass distribution as developed in Chap. 5 applies. However, one can easily imagine situations where the light is deflected several times on the way from the source to the observer. For example, a distant QSO might be lensed by two galaxies at quite different redshifts; in particular, in the spectra of high redshift QSOs, one often finds absorption lines which correspond to intervening matter at several redshifts. If the absorbing matter forms part of a sufficiently large mass concentration (e.g, a galaxy), the multiple absorption lines could indicate that the light from the QSO is deflected more than once. In addition, microlensing need not be confined to objects at a single redshift. Furthermore, if the clumpy universe described in Chap. 4 does approximate the matter distribution in our universe, clumps at all redshifts will influence light propagation. We describe numerical studies of this process in a later chapter. As it turns out, the multiple lens-plane theory ([BL86.1], [K087.5]) is very convenient for the study of light rays (and bundles) through a clumpy universe.