Over the last two decades, we have progressively entered a new era of high precision cosmology.

The main lesson we have learned from there is that our universe is currently undergoing a phase of accelerated expansion, which is commonly described as the so-called dark energy. The simplest way of modeling it is by means of a positive cosmological constant describing de Sitter spacetime.

In this case, the corresponding spacetime geometry has a cosmological horizon. Therefore a field theory in such a background lacks a unitary quantum description at higher energies.

On the other hand, within the last thirty years, our theoretical understanding of high energy physics has experienced a substantial development which has radically changed the way we think about symmetries and unification of fundamental interactions. In particular, string theory has offered us a complete description unifying gravity and all other interactions at a quantum level. Such a unification takes place thanks to ingredients such as supersymmetry and unitarity in the ultraviolet (UV).

Our ultimate goal is that of understanding how to resolve the challenging paradox posed by comparing our low energy cosmological observations with our theoretical high energy theories.

Along the way, we shall adopt the "bottom-up" approach of testing different effective descriptions compatible with observations and use UV consistency as a guiding principle. This results in a set of "swampland conjectures" which may serve as selection criteria to decide for the best candidate fundamental description of our universe. This approach poses a paradigm shift on how we view the very goals of string phenomenology and offers viable alternatives for the description of accelerated cosmologies within string theory.