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1. Class $\mathcal{S}$ Superconformal Indices from Maximal Supergravity

   https://doi.org/10.1103/PhysRevLett.134.181601  

   Bhattacharya, Ritabrata; Katyal, Abhay; Varela, Oscar (May 2025, Physical Review Letters)

   We present a new gauging of maximal supergravity in five spacetime dimensions with gauge group containing ISO(5), involving the local scaling symmetry of the metric, and admitting a supersymmetric anti–de Sitter vacuum. We show this maximal supergravity to arise by consistent truncation of M theory on the (nonspherical, nonparallelizable) six-dimensional geometry associated to a stack of $N$M5 branes wrapped on a smooth Riemann surface. The existence of this truncation allows us to holographically determine the complete, universal spectrum of light operators of the dual four-dimensional $\mathcal{N}=2$theory of class $\mathcal{S}$. We then compute holographically the superconformal index of the dual field theory at large $N$, finding perfect agreement with previously known field theory results in specific limits. Published by the American Physical Society2025 

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2. Black-Hole Models in Loop Quantum Gravity

   https://doi.org/10.3390/universe6080125  

   Bojowald, Martin (August 2020, Universe)

   null (Ed.)

   Dynamical black-hole scenarios have been developed in loop quantum gravity in various ways, combining results from mini and midisuperspace models. In the past, the underlying geometry of space-time has often been expressed in terms of line elements with metric components that differ from the classical solutions of general relativity, motivated by modified equations of motion and constraints. However, recent results have shown by explicit calculations that most of these constructions violate general covariance and slicing independence. The proposed line elements and black-hole models are therefore ruled out. The only known possibility to escape this sentence is to derive not only modified metric components but also a new space-time structure which is covariant in a generalized sense. Formally, such a derivation is made available by an analysis of the constraints of canonical gravity, which generate deformations of hypersurfaces in space-time, or generalized versions if the constraints are consistently modified. A generic consequence of consistent modifications in effective theories suggested by loop quantum gravity is signature change at high density. Signature change is an important ingredient in long-term models of black holes that aim to determine what might happen after a black hole has evaporated. Because this effect changes the causal structure of space-time, it has crucial implications for black-hole models that have been missed in several older constructions, for instance in models based on bouncing black-hole interiors. Such models are ruled out by signature change even if their underlying space-times are made consistent using generalized covariance. The causal nature of signature change brings in a new internal consistency condition, given by the requirement of deterministic behavior at low curvature. Even a causally disconnected interior transition, opening back up into the former exterior as some kind of astrophysical white hole, is then ruled out. New versions consistent with both generalized covariance and low-curvature determinism are introduced here, showing a remarkable similarity with models developed in other approaches, such as the final-state proposal or the no-transition principle obtained from the gauge-gravity correspondence. 

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3. 8d supergravity, reconstruction of internal geometry and the Swampland

   https://doi.org/10.1007/JHEP06(2021)178  

   Hamada, Yuta; Vafa, Cumrun (June 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract We sharpen Swampland constraints on 8d supergravity theories by studying consistency conditions on worldvolume theory of 3-brane probes. Combined with a stronger form of the cobordism conjecture, this leads to the reconstruction of the compact internal geometry and implies strong restrictions on the gauge algebra and on some higher derivative terms (related to the level of the current algebra on the 1-brane). In particular we argue that 8d supergravity theories with $$ {\mathfrak{g}}_2 $$ g 2 gauge symmetry are in the Swampland. These results provide further evidence for the string lamppost principle in 8d with 16 supercharges. 

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4. Physical effects of the gravitational Θ-parameter

   https://doi.org/10.1142/S0218271816440223  

   Fischler, Willy; Kundu, Sandipan (October 2016, International Journal of Modern Physics D)

   We describe the effect of the gravitational [Formula: see text]-parameter on the behavior of the stretched horizon of a black hole in [Formula: see text]-dimensions. The gravitational [Formula: see text]-term is a total derivative, however, it affects the transport properties of the horizon. In particular, the horizon acquires a third-order parity violating, dimensionless transport coefficient which affects the way localized perturbations scramble on the horizon. In the context of the gauge/gravity duality, the [Formula: see text]-term induces a nontrivial contact term in the energy–momentum tensor of a [Formula: see text]-dimensional large-N gauge theory, which admits a dual gravity description. As a consequence, the dual gauge theory in the presence of the [Formula: see text]-term acquires the same third-order parity violating transport coefficient. 

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5. Fermions in loop quantum gravity and resolution of doubling problem

   https://doi.org/10.1088/1361-6382/acf26b  

   Zhang, Cong; Liu, Hongguang; Han, Muxin (September 2023, Classical and Quantum Gravity)

   Abstract The fermion propagator is derived in detail from the model of fermion coupled to loop quantum gravity (LQG). As an ingredient of the propagator, the vacuum state is defined as the ground state of some effective fermion Hamiltonian under the background geometry given by a coherent state resembling the classical Minkowski spacetime. Moreover, as a critical feature of LQG, the superposition over graphs is employed to define the vacuum state. It turns out that the graph superposition leads to the propagator being the average of the propagators of the lattice field theory over various graphs so that all fermion doubler modes are suppressed in the propagator. This resolves the doubling problem in LQG. Our result suggests that the superposition nature of quantum geometry should, on the one hand, resolve the tension between fermion and the fundamental discreteness and, on the other hand, relate to the continuum limit of quantum gravity. 

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