An effective three-dimensional de Sitter cosmology in string theory

Motivated by the lack of consensus on whether or not de Sitter (dS) lies in the Swampland, we use a recently developed braneworld construction and known Anti-de Sitter (AdS) vacua to compute an explicit effective dS cosmology in three dimensions. We consider a non-perturbative AdS$_4$ vacuum decaying to another lower AdS$_4$ vacuum via bubble-nucleation. We also consider the more speculative case where a dS$_4$ decay to a Minkowski$_4$. The Israel junction conditions are solved across the bubble and we obtain the Friedmann equations from which the cosmological constants can be read off, in the respective cases. The cosmological constants are computed in a flux background yielding small positive values admitting a dS cosmology. However, we find that an energetically viable model in the AdS to AdS case requires more fine-tuning.