The LHC accelerator at CERN does not collide just protons, but also heavy nuclei, e.g. lead. In such collisions, conditions close to those as shortly after the Bing Bang can be simulated. In an early Universe the matter was in the state of a hot quark-gluon plasma and the top quark, the heaviest of all quarks, constitutes a new unique probe to study this form of matter. Owing to its large mass, it is born in processes leading to the quark-gluon plasma, rather than in the medium itself. Comparing processes known from proton-proton collisions to those with heavy ions, it is possible to study their production mechanism and use them as a probe testing the development of dense strongly interacting matter.