Cosmic Microscopes

A cosmic microscope is composed of two natural gravitational lenses. A large gravitational lens from a massive halo (for instance a galaxy cluster) and a much smaller lens, usually a star or similar compact object (including stellar remnants and more exotic ones like primordial black holes). The combination of these two lenses allows to study the distribution of luminous matter (and matter itself) on the smallest scales. In this talk I will discuss a few applications where cosmic microscopes can make important contributions: 1) detection of distant stars, 2) interference of gravitational waves, and 3) constrains on the nature of dark matter.