CONICET | Buscador de Institutos y Recursos Humanos

The simplest extensions to general relativity are given by scalar-tensor theories, in which a scalar degree of freedom is added. Interest in such extensions has grown recently, in view of the unsolved problem of the nature of dark matter and dark energy. We consider a non-rotating black hole solution of a particular case of Horndeski theory, the most general scalar-tensor theory with second order equations of motion. The black hole can act as a lens by bending the path of a light ray; as the trajectory approaches the photon sphere, the deflection angle increases withoutbound. A distant observer will therefore see two infinite sequences of images of a source placed behind the lens, one at each side. We study the strong deflection limit in the chosen metric, obtaining the positions and magnifications of the images, and discuss observational consequences for the two best known supermassive black holes.