String Theory is the most compelling candidate for a theory that encompasses all four interactions observed in nature, thus realizing Einstein's dream of unifying Quantum Mechanics and General Relativity. 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.
Despite its uniqueness, string theory has many low-energy solutions (vacua), with very different observable physics: different cosmological constants, matter content and interactions. The multitude of string theory vacua (multiverse) is often invoked as supporting the anthropic principle, and string theory, with its believed “landscape” of vacua, appears to support it.
However, phenomenologically-relevant solutions in the landscape are not constructed directly in string theory, but are found using effective low-energy descriptions in four space-time dimensions. The effective four-dimensional theories are obtained from the low energy limit of ten-dimensional string theory in some particular background (a small six-dimensional “internal” curled-up space of size much less than any scale accessible to observations, typically supporting electromagnetic fluxes). In order to satisfy all experimental constraints from particle physics and cosmology, the effective theories require a number of intricate ingredients, whose string theory origin and consistency is unclear. Thus, the existence of a halfway top-down/bottom-up construction of an effective theory satisfying the constraints imposed by observations gives us absolutely no guarantee that string theory admits such a solution. At this stage, there is neither a known ultraviolet completion of the effective action, nor any guarantee of its existence or consistency. The objective of this proposal was to deconstruct the string theory landscape, and determine what part of it is made of honest-to-goodness stable string theory solutions and thus consistent, and what part of it is inconsistent.
The project was divided into three main themes. A first part deconstructs the known de Sitter (positive cosmological constant) landscape in string theory. In addition to a positive cosmological constant, the string theory solution describing our universe should also contain the Standard Model of particle physics. Deconstruction of the existent and exploration of new Standard Model landscapes was another main objective of the project. Finally, we aimed at constructing the non-supersymmetric and intriguing non-geometric landscapes (solutions where the internal space does not have a conventional geometric description, yet are perfectly allowed in string theory).
