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1. Dark dimension gravitons as dark matter

   https://doi.org/10.1007/JHEP11(2023)109  

   Gonzalo, Eduardo; Montero, Miguel; Obied, Georges; Vafa, Cumrun (November 2023, Journal of High Energy Physics)

   A<sc>bstract</sc> We consider cosmological aspects of the Dark Dimension (a mesoscopic dimension of micron scale), which has recently been proposed as the unique corner of the quantum gravity landscape consistent with both the Swampland criteria and observations. In particular we show how this leads, by the universal coupling of the Standard Model sector to bulk gravitons, to massive spin 2 KK excitations of the graviton in the dark dimension (the “dark gravitons”) as an unavoidable dark matter candidate. Assuming a lifetime for the current de Sitter phase of our universe of order Hubble, which follows from both the dS Swampland Conjecture and TCC, we show that generic features of the dark dimension cosmology can naturally lead to the correct dark matter density and a resolution of the cosmological coincidence problem, where the matter/radiation equality temperature (T~ 1 eV) coincides with the temperature where the dark energy begins to dominate. Thus one does not need to appeal to Weinberg’s anthropic argument to explain this coincidence. The dark gravitons are produced atT~ 4 GeV, and their composition changes as they mainly decay to lighter gravitons, without losing much total mass density. The mass of dark gravitons ism_DM∼ 1 − 100 keV today. 

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2. Sharpening the Distance Conjecture in diverse dimensions

   https://doi.org/10.1007/JHEP12(2022)114  

   Etheredge, Muldrow; Heidenreich, Ben; Kaya, Sami; Qiu, Yue; Rudelius, Tom (December 2022, Journal of High Energy Physics)

   A bstract The Distance Conjecture holds that any infinite-distance limit in the scalar field moduli space of a consistent theory of quantum gravity must be accompanied by a tower of light particles whose masses scale exponentially with proper field distance ‖ ϕ ‖ as m ~ exp(− λ ‖ ϕ ‖), where λ is order-one in Planck units. While the evidence for this conjecture is formidable, there is at present no consensus on which values of λ are allowed. In this paper, we propose a sharp lower bound for the lightest tower in a given infinite-distance limit in d dimensions: λ ≥ $$ 1/\sqrt{d-2} $$ 1 / d − 2 . In support of this proposal, we show that (1) it is exactly preserved under dimensional reduction, (2) it is saturated in many examples of string/M-theory compactifications, including maximal supergravity in d = 4 – 10 dimensions, and (3) it is saturated in many examples of minimal supergravity in d = 4 – 10 dimensions, assuming appropriate versions of the Weak Gravity Conjecture. We argue that towers with λ < $$ 1/\sqrt{d-2} $$ 1 / d − 2 discussed previously in the literature are always accompanied by even lighter towers with λ ≥ $$ 1/\sqrt{d-2} $$ 1 / d − 2 , thereby satisfying our proposed bound. We discuss connections with and implications for the Emergent String Conjecture, the Scalar Weak Gravity Conjecture, the Repulsive Force Conjecture, large-field inflation, and scalar field potentials in quantum gravity. In particular, we argue that if our proposed bound applies beyond massless moduli spaces to scalar fields with potentials, then accelerated cosmological expansion cannot occur in asymptotic regimes of scalar field space in quantum gravity. 

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3. Bounds on field range for slowly varying positive potentials

   https://doi.org/10.1007/JHEP02(2024)175  

   van_de_Heisteeg, Damian; Vafa, Cumrun; Wiesner, Max; Wu, David H (February 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> In the context of quantum gravitational systems, we place bounds on regions in field space with slowly varying positive potentials. Using the fact that$$ V<{\Lambda}_s^2 $$ $V<{\Lambda }_s^2$, where Λ_s(ϕ) is the species scale, and the emergent string conjecture, we show this places a bound on the maximum diameter of such regions in field space: ∆ϕ≤alog(1/V) +bin Planck units, wherea≤$$ \sqrt{\left(d-1\right)\left(d-2\right)} $$ $\sqrt{\left(d-1\right)\left(d-2\right)}$, andbis an 𝒪(1) number and expected to be negative. The coefficient of the logarithmic term has previously been derived using TCC, providing further confirmation. For type II string flux compactifications on Calabi-Yau threefolds, using the recent results on the moduli dependence of the species scale, we can check the above relation and determine the constantb, which we verify is 𝒪(1) and negative in all the examples we studied. 

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4. Improved effective dynamics of loop-quantum-gravity black hole and Nariai limit

   https://doi.org/10.1088/1361-6382/ac44a0  

   Han, Muxin; Liu, Hongguang (January 2022, Classical and Quantum Gravity)

   Abstract We propose a new model of the spherical symmetric quantum black hole in the reduced phase space formulation. We deparametrize gravity by coupling to the Gaussian dust which provides the material coordinates. The foliation by dust coordinates covers both the interior and exterior of the black hole. After the spherical symmetry reduction, our model is a 1 + 1 dimensional field theory containing infinitely many degrees of freedom. The effective dynamics of the quantum black hole is generated by an improved physical HamiltonianH_Δ. The holonomy correction inH_Δis implemented by the $\overline{\mu }$-scheme regularization with a Planckian area scale Δ (which often chosen as the minimal area gap in loop quantum gravity). The effective dynamics recovers the semiclassical Schwarzschild geometry at low curvature regime and resolves the black hole singularity with Planckian curvature, e.g.R_μνρσR^μνρσ∼ 1/Δ². Our model predicts that the evolution of the black hole at late time reaches the charged Nariai geometry dS₂×S²with Planckian radii $\sim \sqrt{\mathrm{\Delta }}$. The Nariai geometry is stable under linear perturbations but may be unstable by nonperturbative quantum effects. Our model suggests the existence of quantum tunneling of the Nariai geometry and a scenario of black-hole-to-white-hole transition. 

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5. Cobordism Classes and the Swampland

   McNamara, Jake; Vafa, Cumrun (September 2019, ArXivorg)

   We argue that any proposed quantum theory of gravity with non-trivial cobordism classes in the space of configurations belongs to the Swampland. The argument is based on the assumption that there are no global symmetries in a consistent theory of quantum gravity. The triviality of the cobordism classes requires the existence of certain stringy defects that trivialize the potential cobordism classes. We provide evidence for this conjecture by identifying those defects demanded by this argument that could preserve supersymmetry, and predict the existence of new non-supersymmetric defects in string theory. 

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