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Thomas Van Riet

Publications and source records attributed to Thomas Van Riet.

At least 19 recordsLinked to original sources
arXiv2026

Type IIA on Spin(7) manifolds with fluxes

We initiate a systematic study of compactifications of type II string theory to two dimensions on Ricci-flat spaces with fluxes and sources. We derive universal constraints on such compactifications, and then develop type IIA compactifications on $\mathrm{Spin}(7)$-holonomy spaces with bulk fluxes whose tadpole is cancelled by $\mathrm{OF1}$-planes. We derive the resulting two-dimensional $\mathcal N=(1,1)$ supergravity for a toroidal $\mathrm{Spin}(7)$ orbifold, and we extend the metric and universal sectors geometrically to general compact $\mathrm{Spin}(7)$ manifolds. In candidate supersymmetric Minkowski vacua, all untwisted shape modes appear in the flux scalar potential and can in principle be classically stabilised. However, in an explicit toroidal orbifold example flux quantisation together with the tadpole bound may obstruct the existence of candidate vacua supported entirely within the untwisted sector. The string-frame volume in string units is bounded by $7χ/192$, so suppressing $α'$ corrections requires $\mathrm{Spin}(7)$ manifolds with large Euler characteristic.

hep-th↗
arXiv2026

The IKKT renormalization group flow is IIB: toward zero-d holography

Supergravity solutions describing stacks of D$p$-branes with $p\neq 3$ feature a non-constant dilaton profile, which is holographically mapped to the running of the SYM coupling in $(p+1)$ dimensions. For D-instantons ($p=-1$), the lack of space and time in the IKKT matrix model makes such an interpretation difficult at first. In this letter, we propose a method to achieve this based on two closely related concepts: the IKKT method of integrating out heavy strings in a Coulomb branch vacuum and the matrix RG flow of Brézin and Zinn-Justin (BZJ). The notable difference between the two is that the BZJ RG flow also integrates over the Coulomb branch position. We first apply the Coulomb branch method and by relating the coefficient of the leading correction to the IKKT action with the string coupling, we can compute its dependence on the Coulomb branch position, finding a match between matrix theory and supergravity. Next, we apply the BZJ-flow to the IKKT partition function, which leads to a running of the coupling constant $g$ with the rank $N$ of the matrix model, reproducing the $N$-dependence of the axio-dilaton field in supergravity.

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arXiv2026

A domain wall bound on anti-de Sitter vacua

We consider anti-de Sitter flux vacua interpolated by flux-changing domain walls. Demanding that the tension of such a domain wall be above the ultraviolet cutoff of the effective description, we derive an upper bound on the anti-de Sitter radius, which we term domain wall bound. It translates into a lower bound on the gravitino mass, thus realizing the gravitino conjecture and the anti-de Sitter distance conjecture of the swampland program. We test the domain wall bound on several examples with a candidate hierarchy of scales: classical flux vacua, racetrack models, LVS and KKLT-like anti-de Sitter vacua. The classical flux vacua and LVS are found to be compatible with the bound. For racetrack and KKLT-like anti-de Sitter vacua, the bound poses a non-trivial constraint on achieving large hierarchies of scales.

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arXiv2026

End-of-the-World Singularities: The Good, the Bad, and the Heated-up

We revisit codimension-one End-of-the-World curvature singularities that drive scalars to infinite distance in field-space and have appeared in the context of dynamical cobordisms. We confront them with Gubser's horizon and potential criteria and with the Maldacena--Nuñez criterion. Moduli-space flows do not admit a near-extremal horizon generalization. Still, they satisfy Gubser's potential criterion and, in representative string realizations, the Maldacena--Nuñez criterion in ten dimensions. Together with an explicit uplift of this type of solution to a consistent string theory background, this suggests that such singularities should not be discarded. For flows with non-trivial scalar potential, we argue that the fate of the singularity is tied to the infinite-distance limit probed near the singularity. The Klebanov--Tseytlin and Klebanov--Strassler solutions illustrate that a modification that obstructs or modifies the field excursion should not be understood as a UV-resolution of the original singularity. We show that EFT strings and D7-branes fail Gubser's potential criterion despite having a sensible UV completion. Motivated by this, and inspired by dynamical cobordisms, we propose a novel criterion that bounds the divergence of the Ricci scalar as the flow explores infinite distance in field-space. Our criterion can be viewed as a geometrization Gubser's one that, while capturing all examples accepted by the latter, also admits EFT strings and D7-branes. Both criteria reject the massive Type IIA strong coupling End-of-the-World singularity. Finally, we analyze black D$p$-branes reduced to codimension one as representatives of flows that admit near-extremal generalizations, and find an exponential relation between temperature and field-space distance. This suggests a finite-temperature extension of the Distance Conjecture for dynamical cobordisms.

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arXiv2025

Visions in Quantum Gravity

To deepen our understanding of Quantum Gravity and its connections with black holes and cosmology, building a common language and exchanging ideas across different approaches is crucial. The Nordita Program "Quantum Gravity: from gravitational effective field theories to ultraviolet complete approaches" created a platform for extensive discussions, aimed at pinpointing both common grounds and sources of disagreements, with the hope of generating ideas and driving progress in the field. This contribution summarizes the twelve topical discussions held during the program and collects individual thoughts of speakers and panelists on the future of the field in light of these discussions.

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arXiv2025

Scale separation, rolling solutions and entropy bounds

We revisit scale separation for compactifications of ten- and eleven-dimensional supergravity. For cosmological solutions rolling down flux-generated potentials, we observe that scale separation is achieved as time flows, and is fairly generic. This is realized without the need of orientifolds nor corrections to the classical supergravity approximation. We then confront scale separation with the Covariant Entropy Bound (CEB) and the CKN bound. We show that a naive application of these bounds to vacua hints at the existence of at least two extra dimensions. For rolling solutions, we observe that the CEB is not always respected, but since these examples lack a cosmic horizon, the application of entropy bounds remains delicate.

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arXiv2025

Warped G$_2$-throats in IIA and uplift dSillusions

Flux compactifications of IIA supergravity on orientifolded G$_2$-manifolds have been argued to allow for classical Minkowski$_3$ vacua with moduli and scale-separated AdS$_3$ vacua with full moduli stabilisation. To further uplift these vacua to meta-stable dS$_3$ vacua using anti-D2 branes, warped throats are desirable. We study the flux-stabilisation of local "CGLP-type" throats in compact G$_2$ spaces, and discuss consistency constraints on anti-brane uplifting. Despite the classical AdS$_3$ vacua to be free of tachyons, we find that uplifting from anti-branes down warped throats is forbidden. If instead we rely on hypothetical AdS$_3$ vacua that arise from quantum corrections to the classical Minkowski$_3$ vacua, we find that (similarly to the 4d analogues) consistency constraints point in opposite directions. However, there is potentially an advantage over 4d when it comes to concrete fine-tuning freedom of numbers, such as tadpole constraints.

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arXiv2025

Black Holes, Moduli Stabilisation and the Swampland

In theories with moduli, extremal black holes behave such that for generic initial conditions, the distance traveled by the scalars from infinity to the horizon can grow with the size of the black hole. This, in turn, implies that larger black holes can probe more of the UV ingredients of the theory, in contrast with (naive) EFT expectations. We relate this discrepancy to the lack of cosmological moduli stabilisation. Indeed, for would-be scale-separated string vacua with parametrically heavy stabilised scalars -- dubbed \emph{rigid} compactifications -- one recovers the EFT intuition where only small black holes probe the UV. We make this explicit in a toy model and then turn to top-down models and construct near-horizon solutions in IIA scale-separated compactifications with stabilised moduli. In these top-down models we still observe large field variations for large black holes which can be traced back to the absence of parametrically heavy moduli. We are led to speculate that needing UV physics to allow for non-local effects near the horizon of large black holes is at odds with having a rigid compactification, hinting to the possibility that such compactifications are in the Swampland.

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arXiv2025

A distance conjecture beyond moduli?

The distance conjecture states that for theories with moduli coupled to gravity a tower of states becomes light exponentially in the geodesic distance in moduli space. This specifies how effective field theories break down for large field values. However, phenomenological field theories have no moduli, but a scalar potential that deforms dynamical trajectories away from geodesic curves. In this note we speculate on how one should generalise the distance conjecture, in asymptotic field regimes, to include a scalar potential. We test the generalised distance conjecture in a few cases, demonstrate a link with pseudo-/fake supersymmetry and apply it to the ekpyrotic scenario in cosmology. For the latter we observe that the pre-uplift KKLT potential could provide a stringy embedding of ekpyrosis away from the asymptotic regimes in field space.

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arXiv2025

Supergravity solutions for D$p$-D$(6-p)$ bound states: from $p=7$ to $p=-1$

Near horizon geometries of D$p$-branes with $p\neq 3$ are singular with a running dilaton. Bound states of D$p$ branes with their magnetic cousins, D$(6-p)$ branes, can stabilise the dilaton such that an AdS factor might appear in the near horizon region, potentially leading to a chain of AdS vacua of the form $AdS_{p+2}\times S^{p+2} \times \mathbb{T}^{6-2p}$. The solutions with $p=-1, 1, 3$ are supersymmetric with the cases $p=1, 3$ being well-known examples already. We construct the explicit (partially smeared) brane bound state solutions for all such configurations where the cases $p=-1,2$ are entirely novel and we find no AdS geometry for these. The two novel classes of solutions feature ghost branes (negative tension branes), and we suggest they are physical for the D$(-1)-$D$7$ solutions but unphysical for the D$2-$D$4$ solutions. The bound state of a D$(-1)$ and a D7 brane in supergravity was only hinted upon recently in \cite{Aguilar-Gutierrez:2022kvk} and here we correct the solution in order to preserve supersymmetry and find that the dilaton can indeed be stabilised which indicates there could be a holographic dual matrix theory which generalises the IKKT matrix model to allow for conformal invariance.

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arXiv2024

Connecting flux vacua through scalar field excursions

We show how flux vacua that differ from each other in flux quanta can be seen as different vacua in a single scalar potential of an enlarged field space, which resolves the separation by thin domain walls. This observation, which is motivated by the AdS Distance Conjecture, allows one to compute distances between different vacua using the usual field space metric. We verify for explicit examples such as scale-seperated IIA flux vacua and the IIB Freund-Rubin vacua that the Distance Conjecture (for scalar fields) is satisfied and that the asymptotic directions in the enlarged field space are indeed hyperbolic. This enlarged field space contains the tachyon fields on the unstable $\tilde{\mathrm{D}}p$-branes of type II string theory, which can induce the brane charges of the stable D-branes. We suggest that requiring continuous interpolations refines the Cobordism Conjecture and postdicts the existence of unstable $\tilde{\mathrm{D}}p$-branes.

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arXiv2024

The Quantum Theory Of Gravitation, Effective Field Theories, and Strings: Yesterday And Today

This paper analyzes the effective field theory perspective on modern physics through the lens of the quantum theory of gravitational interaction. The historical part argues that the search for a theory of quantum gravity stimulated the change in outlook that characterizes the modern approach to the Standard Model of particle physics and General Relativity. We present some landmarks covering a long period, i.e., from the beginning of the 1930s until 1994, when, according to Steven Weinberg, the modern bottom-up approach to General Relativity began. Starting from the first attempt to apply the quantum field theory techniques to perturbatively quantize Einstein's theory, we explore its developments and interaction with the top-down approach encoded by String Theory. In the last part of the paper, we focus on this last approach to describe the relationship between our modern understanding of String Theory and Effective Field Theory in today's panorama. To this end, the non-historical part briefly explains the modern concepts of moduli stabilization and Swampland to understand another change in focus that explains the present framework where some string theorists move.

physics.hist-ph↗
arXiv2024

No accelerating scaling cosmologies at string tree level?

We investigate the scalar potential of string compactifications in the parametric regime where string loops and higher derivative corrections to 10d supergravity can be ignored and where the fields are rolling down exponential slopes leading to powerlaw FLRW cosmologies, aka scaling solutions. We argue that these scaling solutions, if describing an accelerating expansion, are likely to be perturbatively unstable, for reasons identical to the perturbative instabilities in tree-level dS vacua.

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arXiv2023

Axion-de Sitter wormholes

We construct wormholes supported by axion flux in the presence of a positive cosmological constant. The solutions describe compact, one-handle bodies colloquially known as kettlebell geometries. The wormholes are perturbatively stable, but regularity of the Euclidean geometry implies an upper bound on the axion flux. Viewed as no-boundary saddle points, wormholes are suppressed relative to the round sphere. The symmetric kettlebell with maximal axion density has vanishing Euclidean action. Continuing into the Lorentzian across the equator, the solutions describe two expanding branches of de Sitter space filled with an axion field that rapidly dilutes and connected by a quantum bounce across which the arrow of time reverses.

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arXiv2023

Beginners lectures on flux compactifications and related Swampland topics

These lecture notes provide a pedagogical introduction, with exercises, to the techniques used in attempts to construct vacua with stabilised moduli in string theory. The reader is only assumed to have a basic knowledge of general relativity, geometry and field theory. We emphasize physical arguments and focus on the latest developments involving the Swampland program that point to a tension for the existence of AdS vacua with small extra dimensions or dS vacua with parametric control. We include a brief summary of the current status of these thorny issues. Unlike many other reviews we make almost no use of the technicalities associated to supersymmetric geometries. These notes are largely based on lectures given at the CERN Winter School on Supergravity, Strings and Gauge Theory and in the Tehran School on Swampland Program held in the summer of 2022.

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arXiv2023

AdS scale separation and the distance conjecture

It has been argued that orientifold vacua with fluxes in type IIA string theory can achieve moduli stabilisation and arbitrary decoupling between the AdS and KK scales upon sending certain unconstrained RR-flux quanta to infinity. In this paper, we find a novel scalar field in the open-string sector that allows us to interpolate between such IIA vacua that differ in flux quanta and find that the limit of large fluxes is nicely consistent with the distance conjecture. This shows that the massive IIA vacua pass an important Swampland criterion and suggests that scale-separated AdS vacua might not be in the Swampland. Our analysis also naturally suggests a flux analogue of "Reid's fantasy" where flux vacua that differ in quantised flux numbers can be connected through trajectories in open-string field space and not just via singular domain walls.

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arXiv2023

A 10d construction of Euclidean axion wormholes in flat and AdS space

Euclidean wormhole geometries sourced by axions and dilatons are puzzling objects in quantum gravity. From one side of the wormhole to the other, the scalar fields traverse a few Planck lengths in field space and so corrections from the UV might potentially affect the consistency of the solution, even when the wormholes are large. Motivated by this, we carry out the first explicit 10d lifts of regular Euclidean axion wormholes. We start off with the lift of Giddings-Strominger wormholes in $N=8$ Euclidean supergravity over a 6-torus to 10d type IIA supergravity and find the solution can be everywhere tuned into the parametrically controlled supergravity regime. Secondly, we construct explicit wormholes in AdS spaces and find them again to be under parametric control. We find the first wormhole solutions in massive type IIA on $S^3\times S^3$ and in type IIB on $T^{1,1}$. The latter has an explicit holographic dual, and similar to the earlier constructions in $AdS_5 \times S^5/Z_k$, the wormholes violate operator positivity since $Tr(F\pm{\star F})^2<0$. This puzzle might arise from subtleties related to computing holographic $n$-point functions in the presence of multiple boundaries.

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