What if string theory has no de Sitter vacua? (1804.01120v2)

Abstract: We present a brief overview of attempts to construct de Sitter vacua in string theory and explain how the results of this 20-year endeavor could point to the fact that string theory harbors no de Sitter vacua at all. Making such a statement is often considered controversial and "bad news for string theory". We discuss how perhaps the opposite can be true.

• The paper critically assesses previous de Sitter constructions in string theory, emphasizing issues like instability and over-reliance on simplified assumptions.
• It reviews classical, non-geometric, and quantum approaches, exposing the problematic assumptions behind moduli stabilization and tachyonic instabilities.
• The authors suggest rethinking dark energy models by considering the potential absence of de Sitter vacua, urging the exploration of alternative theoretical frameworks.

Examination of de Sitter Vacua in String Theory

Ulf H. Danielsson and Thomas Van Riet's paper, "What if string theory has no de Sitter vacua?" presents a critical investigation into the longstanding attempts to identify de Sitter (dS) vacua within the framework of string theory. Compelling evidence has surfaced over the past two decades suggesting that string theory, despite its conceptual elegance and mathematical rigor, may not accommodate de Sitter vacua, which are vital for models of accelerated cosmic expansion and present-day dark energy. The authors delve into this possibility, challenging prevailing assumptions and urging a re-evaluation of theoretical methodologies.

Context and Background

Since the revelation of a small but positive cosmological constant at the end of the 20th century, integrating a non-zero cosmological constant within string theory has been a formidable task. String theorists have tried to reconcile this through the development of the string landscape, a collection of possible vacua configurations characterized by a vast number of potential solutions including purported de Sitter spaces. The introduction of the anthropic principle allows string theorists to rationalize observed values of the cosmological constant as naturally selected within this multiverse framework. However, the landscape concept lacks empirical grounding and has faced criticisms for its arguably loose mathematical underpinnings and theoretical flexibility.

De Sitter Constructions and Challenges

Danielsson and Van Riet meticulously review various methodologies employed to construct de Sitter vacua, ranging from classical supergravity approaches to more modern non-geometric and quantum constructions:

• Classical de Sitter Constructions rely on balancing different forces, such as fluxes and negative-tension objects like orientifolds. However, these attempts are riddled with issues of instability, mainly through tachyonic directions, and present formidable challenges due to their reliance on assumptions like smeared sources, which oversimplify the complex geometry of string theory.
• Non-Geometric Approaches involve string dualities and fluxes that extend beyond traditional geometric frameworks. Although mathematically intriguing, these constructions often breach conventional consistency checks and remain poorly understood beyond the effective field theory approximations.
• Quantum Constructions, especially in Type IIB string theory, combine classical setups with non-perturbative effects, including instantons and gaugino condensates, to stabilize moduli. Yet, the theoretical assumptions underlying these mechanisms, such as the stability and backreaction of anti-branes, have been increasingly criticized. Recent analyses further call into question their robustness, suggesting that these constructs may not withstand quantum corrections or align with the Weak Gravity Conjecture.

Implications and Future Directions

The paper posits that continued reliance on the prospect of de Sitter vacua within string theory could be misguided. The authors speculate on the absence of de Sitter vacua, which would have profound implications for string theory's ability to describe dark energy and the observed acceleration of the universe's expansion. This raises pivotal questions about potential alternative realizations of dark energy beyond stable de Sitter configurations. Concepts such as dynamic cosmologies, quintessence models, or potential new quantum mechanical principles could offer unexplored or underexplored pathways aligning with empirical observations.

Moreover, the discourse opens avenues for further aligning string theory without assuming de Sitter configurations, potentially focusing more on anti-de Sitter (AdS) spaces with deeper insights from the AdS/CFT correspondence. String theorists are urged to re-examine entrenched premises and explore innovative frameworks to bridge the gap between theory and cosmological reality.

Conclusion

Danielsson and Van Riet's paper serves as a clarion call for introspection within the string theory community. It challenges researchers to critically appraise long-standing assumptions and consider fundamentally novel approaches in theoretical physics. The paper is a significant contribution to the ongoing discourse on the compatibility of string theory with de Sitter space, highlighting the need for rigorous solutions that integrate cosmological phenomena without straining the structural foundations of string theory. As they suggest, perhaps it is time to embrace the potential absence of de Sitter vacua as an intrinsic insight rather than an impediment, fostering a deeper and potentially more accurate understanding of the universe's structure.