Search

Until now the quantum field theory (QFT) that successfully describes the electric, weak and strong interactions (three out of the four fundamental forces) between particles is the Standard Model, but it omits gravity. The prime candidate for a quantum theory of gravity is string theory. However, recent developments in string theory suggest that a portion of the alternative quantum field theories that are being considered, are incompatible with gravity. In 2005, this led string theorists to outline the conditions an effective field theory (EFT) should satisfy in order to be consistent with a quantum theory of gravity. These conditions are the ones that separate the so-called landscape from the swampland. An EFT that satisfies these conditions is said to reside in the landscape, while EFTs that do not satisfy these conditions belong to the swampland. This mapping out of EFTs to the swampland gives rise to a number of predictions that are related to the physics of the Early universe and the nature of dark energy. The de Sitter conjecture and the Trans-Planckian censorship conjecture are some of these conditions and will be the main focus of this thesis. The main purpose behind this work is to gain a deep understanding of each criterium, as well as unravel their implications and predictions related to the dynamics of the Early Universe. We do this by writing a pedagogical introduction of the topic and by introducing some possible alternative to the inflationary scenario, cosmologies that seem to be consistent with the aforementioned constraints.

In this essay, we will take a wonderful ride on a dark bubble with strings attached, which carries our universe out of the swampland and makes it realizable in the landscape of string theory. To find the way to the landscape, we make use of apparently hostile corners of the swampland and their instabilities.

Our understanding of cosmology has evolved radically in the last decades. Current models demand the presence of dark energy in our universe and the most favored candidate behind this component is a small positive cosmological constant that characterizes a de Sitter (dS) spacetime. Simultaneously, theoretical physicists have stood up to the challenge of building a consistent theory of quantum gravity and string theory has raised as a strong contender. In this thesis we present some explorations within supergravity, a low energy limit of string theory, studying non-supersymmetric vacua, its stability, and the possibility of finding dS. We study the landscape of flux compactifications to produce dS with non-geometric fluxes. We find precise analytic procedures to find perturbatively stable dS near supersymmetric and no-scale Minkowski in a potential derived from type IIB compactifications. We also provide analytical evidence of naked singularities being produced in supergravity backgrounds after the introduction of anti-Dp-branes, at both vanishing and finite temperature. In order to study the problem of semi-classical stability, we explore compactifications with anti-de Sitter as external space. We argue that truncations to closed-string-sector excitations of non-supersymmetric theories may be non-perturbatively protected by the existence of globally defined fake-superpotentials if they are perturbatively stable, a reasoning that goes in line with the standard positive energy theorems. We find that non-supersymmetric solutions tend to manifest modes with masses under the Breitenlohner-Freedman bound once the open-string-sector is explored while supersymmetric solutions remain stable. We see this as a hint in the nature of the instabilities predicted by the weak gravity conjecture.