Colloquium - POSTPONED

Gravitational lensing has been hypothesized about and studied for more than two centuries.  The first observational verification was obtained in the famous eclipse observations by Eddington and others in 1919.  A number of attempts were made to observe other instances of gravitational lensing and the first confirmed observations were made in 1979.  Since then, a large number of gravitational lens systems have been discovered and studied.  We have also seen application of gravitational lensing in other regimes from microlensing, weak lensing to the strong gravity regime around supermassive black holes.  In this talk I will focus on strong lensing by a single lens, which paradoxically is studied in the weak field limit.  In this regime we can derive a fairly simple equation that describes the mapping between the observations (image plane) and the source distribution (source plane).  The regime of interest is one where sources form multiple images: this necessarily requires a singular mapping (many to one).  Characteristics of this equation can be studied using catastrophe theory and it has been known for a long time that there are two stable image types.  I will focus on the so-called unstable image types: there are three types possible in the regime of interest.  I will discuss why these image types are considered unstable and what type of images form around them.  We have developed methods to map location in the image plane for all singular points of the mapping if we are given a mass distribution.  This approach allows us to construct a very compact description of strong lenses.  We have used this approach to study the incidence of unstable singularities that give rise to exotic image types for clusters of galaxies that act as gravitational lenses.  I will describe the results of our explorations and a recent observational validation.