arXiv subjects

Stefan Vandoren

Publications and source records attributed to Stefan Vandoren.

At least 19 recordsLinked to original sources

arXiv2026

On flat space limits of $AdS_4$ black holes

With a view towards understanding flat space holography, we study the flat space limit of asymptotically $AdS_4$ black holes in gauged $\mathcal{N}=2$ supergravity. Using holographic renormalisation, we carefully analyse the flat space limit of the on-shell action of various black holes in this setting and make explicit the states that contribute to the partition function in this limit. We find several features: large black holes lead to a divergence in the partition function which can be understood as a thermodynamic volume divergence. Meanwhile, semiclassically, small black holes reproduce the partition function of their flat space counterparts, and we clarify the mapping between BPS, (near-)extremal or non-extremal black holes on both sides for charged and rotating solutions. Finally, by studying a suitable flat limit of the BPS Kerr-Newman-$AdS_4$ black hole in M-theory, we obtain a charged BPS solution in flat space. On the dual side, we determine a Carrollian-like limit on the known ABJM partition function on $S^1\times S^2$ at large $N$, and show that it reproduces the correct entropy of the BPS charged black hole in asymptotically flat space.

hep-th↗

arXiv2026

Dynamics of compact binary systems in massive scalar Gauss-Bonnet gravity

Inspiraling binary systems of compact objects probe gravity in strong-field regimes, thereby exploring potential higher curvature corrections to General Relativity. Parity-invariant quadratic corrections can be described by scalar-Gauss-Bonnet (sGB) theory, which involves a scalar field dynamically coupled to curvature scalars and can give rise to scalar condensates around black holes. Considering a mass for the scalar field is natural and leads to new phenomenology related to this additional scale. We compute the dynamics of a binary system of nonspinning black holes in massive sGB using the post-Newtonian (PN) approximation. We obtain solutions for the equations of motion, center-of-mass transformation, and binding energy for circular and eccentric orbits up to 1PN order, where for the first time the higher curvature coupled to scalar mass corrections are included. While most of our calculations are valid for generic scalar masses, the final explicit expressions assume that the mass is small compared to the total mass of the binary and expand to quadratic order in this ratio. We show that the scalar mass corrections to the gauge-invariant binding energy come with same and opposite sign order terms, contributing an overall opposite sign contribution in the perturbative limit, decreasing the binding energy slightly. The effects are largest for binary systems with high mass ratio and large eccentricity. Our methods and results will also be useful as a basis for computing the gravitational waves sourced by such systems.

gr-qc↗

arXiv2026

BMS$_3$ invariant field theories

We review existing and construct new two-dimensional field theories that exhibit BMS$_3$ symmetry, with and without central extensions. These include interacting electric, magnetic and canonical BMS$_3$ scalar theories, as well as couplings between the electric and magnetic sectors. We provide a careful analysis of boundary contributions at the corner points $u\rightarrow\pm\infty$, determine the counterterms and boundary conditions required by BMS$_3$ invariance, and study the corresponding variational principles. Furthermore, we introduce external sources and derive the associated flux-balance laws. Within the framework of flat-space holography, we show that a simple free electric BMS$_3$ model with the appropriate central charge reproduces the monodromy classification of three-dimensional Einstein gravity. Finally, we investigate the flat-space limits of both AdS$_3$ and dS$_3$, whose boundary dynamics are described by Carrollian limits of Liouville and Euclidean Liouville theory, respectively. Although the resulting BMS$_3$ theories possess isomorphic spectra, they differ in the signs of the supertranslation charges and the central charge. This suggests that the flat-space limits of AdS$_3$ and dS$_3$ provide complementary realizations of flat-space holography.

hep-th↗

arXiv2025

Towards a String Realization of the Dark Dimension via T-folds

In this work, we explore the feasibility of realizing the Dark Dimension Scenario through T-fold compactifications on $T^5 \times S^1$, where the base of the internal space $S^1$ naturally acts as the requisite mesoscopic extra dimension. Utilizing Scherk--Schwarz reduction from 5D to 4D, and applying duality twists from both elliptic and parabolic conjugacy classes of subgroups of the T-duality group associated with $T^5$, we stabilize the volume of $T^5$ and several other moduli. This approach generates a potential characterized by two runaway directions, one aligned with the Scherk--Schwarz radion. Further stabilization efforts yield a decreasing potential trajectory demanded by the Dark Dimension Scenario.

hep-th↗

arXiv2025

Duality and Infinite Distance Limits in Asymmetric Freely Acting Orbifolds

We use freely acting asymmetric orbifolds of type IIB string theory to construct a class of theories in four dimensions with eight supercharges. Their low energy effective field theories resemble $STU$ models, but have different duality groups: the orbifold's free action reduce the duality groups to congruence subgroups of the modular group. The fundamental domain is consequently larger and contains new interesting points at infinite distance on the real axis bounding the upper half plane. We verify that the distance conjectures hold in the non-geometric compactification of string theory studied here. In particular, we find points at infinite distance in moduli space at which the theory decompactifies to a different orbifold construction.

hep-th↗

arXiv2025

The geometry of hypermultiplets

This is a light and short review on the geometry of hypermultiplets coupled to supergravity and its appearance in string theory. As is long known, the target space of the hypermultiplet sigma-model is a quaternion-Kähler (QK) manifold of negative scalar curvature. We review some aspects of hypermultiplet moduli spaces, but also study aspects of QK-geometries in relation to the swampland, including some concrete examples of swampland QK-manifolds.

hep-th↗

arXiv2025

On Carroll partition functions and flat space holography

We continue the study of Carroll limits on partition functions of relativistic conformal theories and their thermodynamics. By introducing imaginary chemical potentials $v$ conjugate to momenta, one can access and study the Carroll regime in which $v\gg c$. We analyze examples of free massless particles, 2d CFTs and some free field theories in $d$ spatial dimensions. For 2d holographic CFTs, we revisit and clarify further the connection between the Carroll limit and the flat space limit of BTZ black holes. The general picture, valid in any dimension, is that, starting from AdS/CFT, the boundary Carrollian field theory lives on the horizon of a large black hole in AdS, which is pushed to the null boundary in the flat space limit.

hep-th↗

arXiv2024

Massive scalar clouds and black hole spacetimes in Gauss-Bonnet gravity

We study static black holes in scalar-Gauss-Bonnet (sGB) gravity with a massive scalar field as an example of higher curvature gravity. The scalar mass introduces an additional scale and leads to a strong suppression of the scalar field beyond its Compton wavelength. We numerically compute sGB black hole spacetimes and scalar configurations and also compare with perturbative results for small couplings, where we focus on a dilatonic coupling function. We analyze the constraints on the parameters from requiring the curvature singularity to be located inside the black hole horizon $r_h$ and the relation to the regularity condition for the scalar field. For scalar field masses $m r_h \gtrsim 10^{-1}$, this leads to a new and currently most stringent bound on sGB coupling constant $α$ of $α/r_h^2 \sim 10^{-1}$ in the context of stellar mass black holes. Lastly, we look at several properties of the black hole configurations relevant for further work on observational consequences, including the scalar monopole charge, Arnowitt Deser Misner mass, curvature invariants and the frequencies of the innermost stable circular orbit and light ring.

gr-qc↗

arXiv2024

Exact moduli spaces for $\mathcal{N}=2, D=5$ freely acting orbifolds

We use freely acting asymmetric orbifolds of type IIB string theory to construct a class of theories in five dimensions with eight supercharges whose moduli spaces for vector multiplets and hypermultiplets can be determined exactly. We argue that no quantum corrections to these moduli spaces arise. We focus on examples in which all moduli are in the NS-NS sector, while all fields from the R-R sector become massive. The full symmetry group of the moduli space is then determined by the subgroup of the T-duality group that survives the orbifold action. We illustrate this for freely acting orbifolds of type IIB string theory on $T^5$ with $0,1$ or $2$ hypermultiplets.

hep-th↗

arXiv2023

Carroll stories

We study various aspects of the Carroll limit in which the speed of light is sent to zero. A large part of this paper is devoted to the quantization of Carroll field theories. We show that these exhibit infinite degeneracies in the spectrum and may suffer from non-normalizable ground states. As a consequence, partition functions of Carroll systems are ill-defined and do not lead to sensible thermodynamics. These seemingly pathological properties might actually be a virtue in the context of flat space holography. Better defined is the Carroll $\textit{regime}$, in which we consider the leading order term in an expansion around vanishing speed of light without taking the strict Carroll limit. Such an expansion may lead to sensible notions of Carroll thermodynamics. An interesting example is a gas of massless particles with an imaginary chemical potential conjugate to the momentum. In the Carroll regime we show that the partition function of such a gas leads to an equation of state with $w=-1$. As a separate story, we study aspects of Carroll gravity and couplings to Carrollian energy-momentum tensors. We discuss many examples of solutions to Carroll gravity, including wormholes, Maxwell fields, solutions with a cosmological constant, and discuss the structure of geodesics in a Carroll geometry. The coupling of matter to Carroll gravity also allows us to derive energy-momentum tensors for hypothetical Carroll fluids from expanding relativistic fluids as well as directly from hydrostatic partition functions.

hep-th↗

arXiv2023

Freely acting orbifolds of type IIB string theory on $T^5$

We study freely acting orbifolds of type IIB string theory on $T^5$ that spontaneously break supersymmetry from $\mathcal{N}=8$ to $\mathcal{N}=6,4,2$ or 0 in five dimensions. We focus on orbifolds that are a $\mathbb{Z}_p$ quotient by a T-duality acting on $T^4$ and a shift on the remaining $S^1$. Modular invariant partition functions are constructed and detailed examples of both symmetric and asymmetric orbifolds are presented, including new examples of five-dimensional non-supersymmetric string theories with no tachyons. The orbifolds we consider arise at special points in the moduli space of string theory compactifications with a duality twist. The supergravity limit of these are Scherk-Schwarz reductions which generate gauged supergravities with positive definite potentials on the moduli space in five dimensions. Both symmetric and asymmetric freely acting orbifolds give a landscape of Minkowski vacua. For gauged supergravities to belong to this landscape, we find a number of constraints and conditions. Firstly, the scalar potential should lead to a massive spectrum with masses that obey quantization conditions arising from a string theory orbifold, which we discuss in detail. Secondly, we find constraints on the massless sector, e.g. in the examples of orbifolds preserving sixteen supercharges in five dimensions that we consider, only an odd number of vector multiplets arise. Lastly, we present new examples of candidate asymmetric orbifolds with modular invariant partition functions, but with non-integral coefficients in the $q\bar{q}$-expansion in the twisted sector.

hep-th↗

arXiv2022

N=(0,4) Black String Chains

We construct black string solutions in Type IIA supergravity arising from intersecting D2-D4-D6-NS5 branes in the presence of fractional D4-branes. The fractional D4-branes arise from D6-branes wrapping (collapsing) two-cycles in a Calabi--Yau two-fold. In the near horizon limit these solutions give rise to AdS$_3$ geometries preserving $\mathcal{N}=(0,4)$ supersymmetry and fall within the recent classification of \cite{Lozano:2019emq}. We interpret our solutions as describing chains of black strings stacked on top of each other along an interval. We construct 2d quiver CFTs dual to our solutions that reproduce the Bekenstein-Hawking entropy microscopically.

hep-th↗

arXiv2021

Carroll symmetry, dark energy and inflation

Carroll symmetry arises from Poincaré symmetry upon taking the limit of vanishing speed of light. We determine the constraints on the energy-momentum tensor implied by Carroll symmetry and show that for energy-momentum tensors of perfect fluid form, these imply an equation of state ${\cal E}+P=0$ for energy density plus pressure. Therefore Carroll symmetry might be relevant for dark energy and inflation. In the Carroll limit, the Hubble radius goes to zero and outside it recessional velocities are naturally large compared to the speed of light. The de Sitter group of isometries, after the limit, becomes the conformal group in Euclidean flat space. We also study the Carroll limit of chaotic inflation, and show that the scalar field is naturally driven to have an equation of state with $w=-1$. Finally we show that the freeze-out of scalar perturbations in the two point function at horizon crossing is a consequence of Carroll symmetry. To make the paper self-contained, we include a brief pedagogical review of Carroll symmetry, Carroll particles and Carroll field theories that contains some new material as well. In particular we show, using an expansion around speed of light going to zero, that for scalar and Maxwell type theories one can take two different Carroll limits at the level of the action. In the Maxwell case these correspond to the electric and magnetic limit. For point particles we show that there are two types of Carroll particles: those that cannot move in space and particles that cannot stand still.

hep-th↗

arXiv2021

Entanglement Entropy with Lifshitz Fermions

We investigate fermions with Lifshitz scaling symmetry and study their entanglement entropy in 1+1 dimensions as a function of the scaling exponent $z$. Remarkably, in the ground state the entanglement entropy vanishes for even values of $z$, whereas for odd values it is independent of $z$ and equal to the relativistic case with $z=1$. We show this using the correlation method on the lattice, and also using a holographic cMERA approach. The entanglement entropy in a thermal state is a more detailed function of $z$ and $T$ which we plot using the lattice correlation method. The dependence on the even- or oddness of $z$ still shows for small temperatures, but is washed out for large temperatures or large values of $z$.

quant-ph↗

arXiv2020

Evolution of black hole shadows from superradiance

Black holes have turned into cosmic laboratories to search for ultra-light scalars by virtue of the superradiant instability. In this paper we present a detailed study of the impact of the superradiant evolution on the black hole shadow and investigate the exciting possibility to explore it with future observations of Very Long Baseline Interferometry. We simulated the superradiant evolution numerically, in the adiabatic regime, and derived analytic approximations modelling the process. Driven by superradiance, we evolve the black hole shadow diameter and (i) find that it can change by a few $μ$as, just below the current resolution of the Event Horizon Telescope, albeit on timescales that are longer than realistic observation times; (ii) show that the shadow diameter can either shrink or grow; and (iii) explore in detail how the shadow's end state is determined by the initial parameters and coupling.

gr-qc↗

arXiv2020

Anomalies of (0,4) SCFTs from F-theory

We study the macroscopics of 2d $\mathcal{N}=(0,4)$ SCFTs arising from F-theory constructions. The class of 2d SCFTs we consider live on black strings which are obtained by wrapping D3-branes on a curve in the base of a possibly singular elliptically fibered Calabi-Yau threefold. In addition, we allow the D3-branes to probe ALE or ALF spaces transversely. We compute anomaly coefficients of these SCFTs by determining Chern-Simons terms in the 3d action resulting from the reduction of 6d $\mathcal{N}=(1,0)$ supergravity on the compact space surrounding the black string. Essential contributions to these coefficients are from one-loop induced Chern-Simons terms arising from integrating out massive Kaluza-Klein modes.

hep-th↗

arXiv2020

Black holes in string theory with duality twists

We consider 5D supersymmetric black holes in string theory compactifications that partially break supersymmetry. We compactify type IIB on $T^4$ and then further compactify on a circle with a duality twist to give Minkowski vacua preserving partial supersymmetry ($\mathcal{N}=6,4,2,0$) in five dimensions. The effective supergravity theory is given by a Scherk-Schwarz reduction with a Scherk-Schwarz supergravity potential on the moduli space, and the lift of this to string theory imposes a quantization condition on the mass parameters. In this theory, we study black holes with three charges that descend from various ten-dimensional brane configurations. For each black hole we choose the duality twist to be a transformation that preserves the solution, so that it remains a supersymmetric solution of the twisted theory with partially broken supersymmetry. We discuss the quantum corrections arising from the twist to the pure gauge and mixed gauge-gravitational Chern-Simons terms in the action and the resulting corrections to the black hole entropy.

hep-th↗

arXiv2019

Black Holes and (0,4) SCFTs from Type IIB on K3

We study the central charges and levels of 2d $\mathcal{N}=(0,4)$ superconformal field theories that are dual to four- and five-dimensional BPS black holes in compactifications of type IIB string theory on a K3 surface. They arise from wrapping a D3-brane on a curve $C$ inside K3 and have transverse space either an ALE or ALF space. These D3-branes have an AdS$_3 \times \mathrm{S}^3/ Γ$ near horizon geometry where $Γ$ is a discrete subgroup of $SU(2)$. We compute the central charges and levels of the 2d SCFTs both in the microscopic picture and from six-dimensional $\mathcal{N}=(2,0)$ supergravity. These quantities determine the black hole entropy via Cardy's formula. We find agreement between the microscopic and macroscopic computations. The contributions from one-loop quantum corrections to the macroscopic result are crucial for this matching.

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