Swamplandish Unification of the Dark Sector (2402.00981v2)

Abstract: We provide a short overview of recent progress made in our understanding of the dark sector based on the Swampland program which in turn is rooted in lessons from string theory. We explain how the existence of one extra mesoscopic dimension (the dark dimension") in the micron range emerges and how this can lead to a unification of the dark energy and dark matter. In particular the smallness of the dark energy leads to the prediction of the existence of a tower of weakly interacting light particles which can naturally play the role of dark matter. Moreover this unifies dark matter with gravity as dark matter ends up being excitations of graviton in the dark dimension. We also explain how in combination with other Swampland principles one finds an explanation of thewhy now" and the ``cosmological coincidence" problems. This model is consistent with the cosmological bounds as well as the Newton's inverse square law, but makes predictions which differ from $\Lambda$CDM. It also gives rise to an appealing picture of hierarchy of scales in particle physics pegged to the dark energy, including a possible origin of the electroweak hierarchy and the prediction of masses of QCD axion and sterile neutrinos both in the 1-10 meV range. This review is intended for a broad audience of high energy theorists and cosmologists without prior knowledge of string theory and it explains the motivations and predictions of this program in a non-technical form.

• The paper demonstrates that dark energy and dark matter can be unified via a mesoscopic dark dimension derived from Swampland criteria.
• It employs the Distance Conjecture to predict a tower of light particles emerging as the cosmological constant approaches zero.
• The framework implies testable micron-scale deviations from Newtonian gravity, offering potential experimental validation for quantum gravity models.

Swamplandish Unification of the Dark Sector

The paper "Swamplandish Unification of the Dark Sector" by Cumrun Vafa offers a comprehensive exploration of the dark sector through the lens of the Swampland program. This program is founded on the insights gleaned from string theory and seeks to delineate the boundaries of effective field theories (EFTs) that can be consistently coupled with quantum gravity. The research presents a framework that potentially unifies dark energy and dark matter into a single conceptual entity that is intrinsically tied to an extra mesoscopic dimension, posited as the "dark dimension."

Core Concepts and Results

One of the paper's seminal assertions is the emergence of an additional mesoscopic dimension on the micron scale. This conclusion is derived from the Swampland criteria, specifically the Distance Conjecture, which predicts the existence of a light particle tower when certain fundamental parameters, like the cosmological constant (Λ), approach zero. Here, the smallness of dark energy (Λ ~ 10^-122) manifests in predictions of weakly interacting light particles, which can fulfill the role of dark matter. The paper suggests these particles are excitations of the graviton within the aforementioned dark dimension, thereby achieving a unification of dark matter with gravity—a conceptual breakthrough that also addresses the "cosmological coincidence" and "why now" puzzles.

Furthermore, the spatial configuration implied by this theory necessitates the presence of larger extra dimensions. The resulting predictions vary depending on the dimensionality of this space, which in turn influences gravitational interaction laws and potentially offers solutions to known problems within particle physics, like the electroweak hierarchy puzzle. The paper also aligns with observational constraints such as the Newtonian inverse square law, suggesting experimental verification at micron scales could test these postulations.

Implications and Speculations

Practically, this framework could redefine the search parameters for dark matter and revolutionize our understanding of the universe's architecture. The theorized dark dimension implies that deviations from the Newtonian gravitational law at micron scales may be observed, presenting a new frontier for experimental physics. A confirmation of this behavior could validate the Swampland conjectures within our universe's context, offering a robust bridge between string theory and observable cosmological phenomena.

Theoretically, the implications extend deeper into quantum gravity's paradigm. By providing a model where extra dimensions significantly contribute to dark matter's composition and behavior, this work challenges conventional separation assumptions between the universe's macrostructure and the underlying quantum constraints. It suggests that viable EFTs must inherently consider these implications to remain consistent with quantum gravity's framework. This perspective could guide future model-building efforts in high-energy physics and cosmology.

Conclusion

The proposal outlined in Vafa's paper presents compelling evidence through Swampland and string theoretical lenses towards a cohesive understanding of the universe's dark sector. While theoretical, this work provides predictions with tangible experimental potential. The notion of a dark dimension, coupled with a structured particle tower approach to dark matter, outlines a clear path for future theoretical and experimental physics endeavors. As such, it offers an innovative perspective on unifying disparate cosmic puzzles into a coherent theoretical framework grounded in modern string theory advancements.