Mariana Graña
Université Paris-Saclay

String theory is the most compelling candidate that encompasses all four interactions observed in nature. While the “first superstring revolution” proved the quantum consistency of five string theories, the second revolution showed that these were actually five different limits of a single theory, consistently defined in ten space-time dimensions.  Despite its uniqueness, string theory has a ‘landscape” of low-energy solutions with very different observable physics in four dimensions. The complement of the landscape is called the swampland, and it is defined as the set of effective theories that fulfil all consistency conditions at low energies, yet they cannot be coupled to a quantum gravity theory at high energies. After introducing string theory, in this colloquium we will make an excursion into the landscape, trying to avoid the swampland. We will focus in particular on the problem of accelerated expansion of the universe.