de Sitter Vacua in the String Landscape

TL;DR

This paper investigates the challenging problem of finding de Sitter vacua in string theory, analyzing quantum effects and time-dependent backgrounds to identify conditions that could produce a stable, positive-curvature universe consistent with observations.

Contribution

It introduces a detailed analysis of time-dependent backgrounds in type IIB string theory, including quantum corrections, to demonstrate the potential emergence of stable de Sitter vacua avoiding known no-go theorems.

Findings

Quantum effects may allow de Sitter solutions in string theory.

Time-dependent internal and external spaces are crucial for stable vacua.

Backgrounds with constant Newton's constant can evade swampland criteria.

Abstract

The late-time behavior of our universe is one of accelerated expansion, or that of a de Sitter space, and therefore motivates us to look for time-dependent backgrounds. Finding such backgrounds in string theory has always been a challenging problem. An even harder problem is to find time-dependent backgrounds that allow positive dark energies. As a first step to handle such scenarios, we study a time-dependent background in type IIB theory, with four-dimensional de Sitter isometries, by uplifting it to M-theory and then realizing it as a coherent, or squeezed-coherent, state over an appropriate solitonic configuration. While classically such a background does not solve the equations of motion, the corresponding Schwinger-Dyson equations reveal that there are deeper issues that may even prohibit a solution to exist at the quantum level, as long as the internal space remains time-independent. A more generic analysis is then called for, where both the effective four-dimensional space-time, the internal space, and the background fluxes are all time-dependent. We study in details such a background by including perturbative and non-perturbative as well as local and non-local quantum terms. Our analysis reveals a distinct possibility of the emergence of a four-dimensional positive curvature space-time with de Sitter isometries and time-independent Newton's constant in the landscape of type IIB string theory. We argue how the no-go and the swampland criteria are avoided in generating such a background, and compare it with other possibilities involving backgrounds with time-dependent Newton constants. These time-varying Newton constant backgrounds typically lead to unavoidable late time singularities, amongst other issues.