Dark Energy and String Theory Realizations – DarES

What is the nature of dark energy, the energy responsible for the observed accelerated expansion of our universe? This fundamental question in modern physics has been revived in recent years both from a theoretical perspective, especially in string theory, as well as from the observational side of cosmology. Of particular interest is a dynamical dark energy described by a quintessence model, i.e. one or several scalar fields rolling in a potential. The present scientific context makes it very timely to explore the options for quintessence models, as obtained and constrained from a fundamental theory of quantum gravity such as string theory, and on the other hand confront them with current and future observational constraints. The project consists of four distinct objectives:
1) In a model with a multi-exponential potential together with spatial curvature, explore the cosmological solutions, and compare them to our universe history and to the recent observational constraints.
Scalar potentials containing multiple exponentials commonly arise from string theory constructions, especially in regimes where corrections to the model are small and well controlled. A comprehensive analysis of these models will enable us to pinpoint realistic solutions and to assess the impact of spatial curvature. By comparing our findings to the most recent observational constraints, we aim to exclude some of these string-motivated models while potentially identifying a viable cosmological scenario.
2) In explicit string-motivated models with multiple scalar fields and a de Sitter tachyonic critical point of the potential, study solutions rolling down the potential, and assess their validity with respect to the observed cosmology of our universe.
These models have been derived through dimensional reductions from ten-dimensional supergravities on six-dimensional compact group manifolds. They exhibit a positive (de Sitter) extremum of the potential, making them natural starting points for rolling solutions. A detailed scan of these models and initial conditions that could lead to cosmological solutions compatible with observations, will allow us refine the viable theoretical landscape for dynamical dark energy.
3) Study the higher-order string theory corrections to the aforementioned string-inspired quintessence models, and their impact on cosmology.
By imposing that string corrections remain subdominant and thus under control, we will obtain new results concerning the validity of these models as four-dimensional effective string theories, along with an assessment of the impact of various corrections for different classes of compactification. From a cosmological perspective, this will enable us to determine the field and time ranges beyond which new physics is required.
4) From observational constraints, find an appropriate single field model, and obtain its scalar potential from an explicit and controlled string compactification.
A single-field quintessence model that has been argued to fit late-time observational data is the hilltop model, in which the field evolves near the maximum of a cosine potential. While such a potential can arise from string theory compactifications and their associated axions, a detailed identification of suitable parameter values is required from the cosmological side, along with a precise compactification setup on the string theory side. This will also pave the way for exploring a broader range of viable string compactifications, potentially including multifield scenarios.
These objectives lie in the inter-disciplinary interface between concrete string-theory cosmological models and observational data. The project applies both theoretical constraints, inherent to string model constructions, and observational constraints, to point us in the right model-building direction. It employs innovative developments in string theory and theoretical cosmology to investigate the nature of dark energy at the most fundamental level.