arXiv subjects

Vincent Van Hemelryck

Publications and source records attributed to Vincent Van Hemelryck.

13 recordsLinked to original sources

arXiv2026

Non-supersymmetric M-theory vacua and dark bubbles

An expanding bubble in AdS$_5$ has a dS$_4$ geometry; the dark bubble scenario explores whether this can be our Universe. Motivated by this, we consider new non-supersymmetric AdS$_5$ compactifications of M-theory and investigate their non-perturbative instabilities. We focus on a compactification on the flag manifold. In certain regions of the flux landscape, the non-supersymmetric vacua become unstable under the nucleation of M5-brane bubbles wrapping an internal two-cycle, thus realising the dark bubble scenario. The cosmological constant on the bubble can be made parametrically small by tuning the internal fluxes, making the M5-branes close to the extremality bound. The solution relates the induced cosmological constant on the brane and the typical largest length scale of the extra dimensions through $R^{-1}\sim Λ_4^{1/4}$ in 4d Planck units, allowing for extra dimensions of the order of tens of microns. We also consider various alternatives, which for various reasons are currently not as successful.

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arXiv2026

$\mathcal{N}=1$ spectra, cubic couplings and the rigid fate of DGKT

We show that in the DGKT scenario on a generic Calabi-Yau three-fold, a recently proposed holographic constraint on cubic couplings is satisfied if and only if the Calabi-Yau is rigid, i.e. when $h^{2,1}=0$. More generally, we illustrate how in 4d $\mathcal{N}=1$ supergravity, extremal cubic couplings are determined by the third derivatives of the real, Kähler-invariant superpotential, while the eigenvalues of its Hessian compute the conformal dimensions of the dual scalar operators. These results extend more broadly beyond 4d $\mathcal{N}=1$ supergravity. Applying them to supersymmetric DGKT vacua, we prove that extremal cubic couplings always vanish in the Kähler + universal CS/dilaton sector, whereas non-vanishing (super-)extremal couplings are always present in the complex structure sector. It follows that the holographic constraint is satisfied in DGKT if and only if the Calabi-Yau three-fold is rigid with $h^{2,1}=0$.

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arXiv2026

Self-gravitating electromagnetic waves in the dark bubble model

We construct embeddings of gravitational and electromagnetic waves in the dark bubble scenario using pp-wave geometries in AdS5, motivated by the fact that pp-wave spacetimes often provide exact solutions to the equations of motion. The setup is realised by gluing two AdS5 pp-wave spacetimes across a three-brane. As an application, we analyse localised beams of light and their gravitational backreaction. Imposing suitable mixed boundary conditions in AdS5, we find gravitational corrections consistent with a weakening of 4d gravity at the 5d AdS scale.

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arXiv2026

Broken and restored: a holographic constraint for AdS vacua with orbifolds

It has been suggested that families of weakly-coupled AdS vacua with a large-$N$ holographic dual must satisfy non-trivial consistency requirements, which amount to the vanishing of certain cubic couplings, corresponding to (super-)extremal arrangements of scalar operators. While this constraint is known to hold in the simplest incarnation of the DGKT scenario in massive type IIA string theory, i.e. on the $\mathbb{Z}_3\times \mathbb{Z}_3$ orbifold, we find that it is generically violated for type II AdS$_3$ and AdS$_4$ vacua arising from $\mathbb{Z}_2 \times \mathbb{Z}_2 \times \mathbb{Z}_2$ and $\mathbb{Z}_2 \times \mathbb{Z}_2$ orbifolds respectively, including scale-separated solutions and DGKT-CFI-type models. In most cases, however, this can be cured by enlarging the orbifold group to a suitable (non-abelian) extension that projects out precisely those scalar operators that would otherwise participate in the constrained cubic couplings. Our results suggest that consistency of the putative holographic dual imposes a non-trivial restriction on the compactification geometry, indicating in particular that O-planes cannot wrap cycles in distinct homology classes.

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arXiv2025

Supersymmetric scale-separated AdS$_3$ orientifold vacua of type IIB

I construct supersymmetric AdS$_3$ vacua of type IIB string theory that exhibit parametric scale separation in the controlled regime. These solutions arise from compactifications on seven-dimensional manifolds equipped with co-closed $G_2$-structures, in the presence of orientifold planes preserving minimal supersymmetry. I focus on $\mathbb{Z}_2^3$ orbifolds of a specific solvmanifold and all seven-dimensional nilmanifolds, each requiring distinct configurations of intersecting O5-planes, which are treated in the smeared approximation. Weak string coupling and large volumes can be achieved for classical backgrounds on both nilmanifolds and solvmanifolds by tuning unbounded fluxes to large values. This achieves scale separation in the nilmanifold case, but the same cannot be concluded for the solvmanifolds. Furthermore, for some solutions, the holographic field theory operators dual to the lightest scalar fields have integer conformal dimensions at tree level, as in other scale-separated type IIA orientifold backgrounds.

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arXiv2024

Charged Nariai black holes on the dark bubble

In this paper, we realise the charged Nariai black hole on a braneworld from a nucleated bubble in AdS$_5$, known as the dark bubble model. Geometrically, the black hole takes the form of a cylindrical spacetime pulling on the dark bubble. This is realised by a brane embedding in an AdS$_5$ black string background. Identifying the brane with a D3-brane in string theory allows us to determine a relation between the fine structure constant and the string coupling, $α_\text{EM} = \frac{3}{2} g_s$, which was previously obtained for a microscopic black hole. We also speculate on the consequences for the Festina Lente bound and neutrino masses.

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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

Weak $G_2$-manifolds and scale separation in M-theory from type IIA backgrounds

This work provides evidence for the existence of supersymmetric and scale-separated AdS$_4$ vacua in M-theory of the Freund-Rubin type. The internal space has weak $G_2$-holonomy, which is obtained from the lift of AdS vacua in massless type IIA on a specific SU(3)-structure with O6-planes. Such lifts require a local treatment of the O6-planes, therefore going beyond the usual smeared approximation. The setup is analysed by solving the pure spinor equations and the Bianchi identities perturbatively in a small backreaction parameter, preserving supersymmetry manifestly and therefore extending on previous work. This approach is applicable to lifts of other type IIA vacua on half-flat SU(3)-structures, including those with D6-brane sources. The resulting 7d manifold presented here exhibits singularities originating from the O6-planes loci in type IIA theory. Additionally, scale separation in M-theory arises from a decoupling between the Ricci curvature and the first eigenvalue of the Laplacian of the proposed 7d manifold, thereby challenging certain conjectures in the swampland program.

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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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arXiv2022

Scale-separated AdS$_3$ vacua from $G_2$-orientifolds using bispinors

In this paper, minimal supersymmetric AdS$_3$ solutions on $G_2$-orientifolds of type IIA and IIB supergravity are studied. These are obtained by techniques of the bispinor formalism. A straightforward argument is derived against AdS$_3$ solutions in massless type IIA string theory on such $G_2$-backgrounds. Furthermore, it is shown that some of the AdS$_3$ solutions in massive type IIA do admit a separation of scales, whereas the same cannot be said for the type IIB vacua. Finally, a formula for the 3d superpotential in the bispinor language is given that is valid for both type IIA and type IIB configurations. It is shown that the supersymmetry conditions of this superpotential lead back to the bispinor equations.

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arXiv2022

Over-extremal brane shells from string theory?

We demonstrate that, if the usual phenomenological compactifications of IIB string theory with warped throats and anti-branes make sense, there must exist spherical brane shells in 4d that are overcharged. They correspond to classical over-extremal objects but without the usual naked singularities. The objects are made from D3-particles that puff into spherical 5-branes that stabilise at finite radii in 4d and whose inside corresponds to the supersymmetric AdS vacuum. One can think of these shells as stabilised Brown-Teitelboim bubbles. We find that these objects can be significantly larger than the string scale depending on the details of the warped compactification.

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arXiv2021

Scale-separated AdS$_4$ vacua of IIA orientifolds and M-theory

We revisit various aspects of AdS$_4$ flux vacua with scale separation in type II supergravity and M-theory. We show that massless IIA allows both weakly and strongly coupled solutions for which the classical orientifold backreaction can be tuned small. This is explicitly verified by computing the backreaction at leading order in perturbation theory. We give evidence that the strongly coupled solutions can be lifted to scale-separated and sourceless (but classically singular) geometries in 11D supergravity.

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arXiv2020

Conifold dynamics and axion monodromies

It has recently been appreciated that the conifold modulus plays an important role in string-phenomenological set-ups involving warped throats, both by imposing constraints on model building and for obtaining a 10-dimensional picture of SUSY-breaking. In this note, we point out that the stability of the conifold modulus furthermore prevents large super- Planckian axion monodromy field ranges caused by brane-flux decay processes down warped throats. Our findings imply a significant challenge for concrete string theory embeddings of the inflationary flux-unwinding scenario.

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