arXiv subjects

Miguel Montero

Publications and source records attributed to Miguel Montero.

At least 19 recordsLinked to original sources

arXiv2026

A $dS_4$ maximum in M-theory from Casimir energies on Riemann-flat manifolds

We describe a fully explicit, four-dimensional, top-down de Sitter saddle, constructed via compactification of M-theory on the Riemann-flat manifold $T^7/\mathbb{Z}_8$. The solution has a vacuum energy of $5.02\cdot 10^{-10}\,M_\text{Pl}^4$, a hierarchy $m_{KK}/H\sim 10^{2}$, and is under control with respect to higher loop and higher-derivative corrections. The solution has an internal manifold with a diameter of a few Planck lengths, and it may be unstable due to loops of virtual 11d black hole microstates. While the solution is the first example of a top-down dS maximum from Riemann-flat manifolds in four dimensions and is realised explicitly enough to compute the vacuum energy precisely and the spectrum of light fields, it is far from being phenomenologically viable. The solution also suggests several promising directions in the search for higher quality dS saddles in the Riemann-flat Landscape, which we describe.

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arXiv2026

Scale-separated AdS vacua on Riemann-Flat Manifolds

Supergravity compactifications supported by fluxes and Casimir energy offer a simple strategy to concoct explicit, controlled and isolated vacua with parametric scale separation and soft supersymmetry breaking. Exploiting the special properties of both the Casimir and flux potentials with respect to Laplacian operators in moduli space, we prove that the simplest such class of solutions, given by toroidal compactifications of eleven-dimensional supergravity threaded by any fluxes, always exhibit perturbative instabilities in the form of tachyonic modes below the BF bound. This no-go theorem applies to any toroidal supergravity compactification supported by $p$-form fluxes. We also study compactifications on more general Riemann-flat manifolds (RFM's). Concretely, we find a tachyon-free AdS$_4$ vacuum by compactifying M-Theory on a $T^7/\mathbb{Z}_2^3$. This constitutes the first ever example of a fully top-down, perturbatively stable, scale-separated AdS without localized sources.

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arXiv2026

Stress-Testing Swampland Bounds with Class S Theories

We test the Sharpened Distance Conjecture (SharpDC) for CFTs, as well as the Refined Distance Conjecture (RDC), in AdS$_5$ gravitational spacetimes, using the CFT dual. The former provides a lower bound on the exponential mass decay rate of the tower of states becoming light at infinite distance in the conformal manifold, while the latter bounds the bulk field range that can be traversed before the exponential behaviour of the tower kicks in. We prove the SharpDC for any large $N$ Lagrangian supersymmetric gauge theory with a finite number $M$ of gauge factors and vanishing one-loop $β$-function. We also investigate whether it universally holds in the limits of large central charge, finding that it does not: We exhibit concrete examples in Class S theories where the conjecture is violated in the $M\rightarrow\infty$ limit with $N$ fixed, where the size of the bulk internal geometry is much larger than the AdS curvature scale. We also test the RDC in the moduli space of Class S theories without punctures in the large $N$ limit, showing that it amounts to a precise upper bound on the Weil-Petersson diameter of the moduli space of Riemann surfaces of genus $g$. This bound is consistent with all the mathematical literature up to date, but so far remains unproven.

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arXiv2026

Neutrinos, B-L Symmetry and the Dark Dimension

We consider realizations of a gauged B-L symmetry in the context of the Dark Dimension scenario, where the SM lives on a codimension one brane in 5d spacetime. The B-L can naturally be a bulk gauge symmetery leading to a global symmetry on the SM brane, and have its gauge anomaly canceled by charged bulk modes. This naturally leads to the existence of 3 right-handed neutrinos propagating in the dark dimension. Allowing for spontaneous breaking of B-L in the bulk results in a massive gauge field with a very weak coupling $g_{B-L}\sim 10^{-10}$; for instance, B-L breaking at the weak scale gives $m_{B-L}\sim 100$ GeV, which is allowed by current bounds, as is a wide range of other values of the B-L breaking scale. The bulk sterile neutrinos are expected to have a mass of the order of the B-L breaking scale. Once the full Kaluza-Klein tower of bulk neutrinos is taken into account, the active neutrino masses turn out to be insensitive to the sterile neutrino mass, and reproduce the original Dark Dimension estimate $m_ν\sim y^2\langle H\rangle^2/M_5$, which leads to the observed neutrino mass scale for Yukawa couplings $y\sim 10^{-3}$.

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arXiv2026

An M-theory dS maximum from Casimir energies on Riemann-flat manifolds

We initiate the study of flux compactifications on non-supersymmetric Riemann-flat manifolds (RFM's) with Casimir energy. While curvature and other corrections are suppressed in RFM's, the inclusion of Casimir energies allows one to evade standard dS no-go theorems, and the absence of orientifolds or other singular sources means that the construction is completely captured by ten or eleven-dimensional supergravity. We obtain a fully explicit formula for the Casimir stress-energy in a general RFM, including its ten or eleven-dimensional profile. The Casimir energy localizes in particular loci of the RFM, which we call ``Casimir branes''. The tension of Casimir branes sometimes cancels exactly, due to a spacetime analog of worldsheet Atkin-Lehner symmetry. We use Casimir energies to construct an explicit $dS_5$ maximum solution of a flux compactification of M-theory on a specific 6-dimensional RFM. The resulting solution is scale-separated, has a vacuum energy of $10^{-8}$ in five-dimensional Planck units, the Hubble radius is $10^4$ Planck lengths, and the light fields have masses of order $H$. This is a fully explicit, top-down de Sitter maximum in M-theory, with precisely computable vacuum energy. While the solution is not parametric, it is under very good control: higher derivative and loop corrections to the vacuum energy are suppressed in powers of a small parameter $δV/V\sim 10^{-5}$, and M2 and M5-brane instantons are negligible. In short, the solution survives all known corrections. Nevertheless, it might be sensitive to more exotic ones, such as e.g. loops of 11d Planckian virtual black holes if there were a large enough number of them. We also extend the Ewald numerical method for lattice sums to arbitrary dimensions and develop an efficient numerical implementation.}

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arXiv2026

Asymmetric orbifolds with vanishing one-loop vacuum energy

We present a systematic study of non-supersymmetric type II toroidal asymmetric orbifolds with vanishing vacuum energy at one-loop in string perturbation theory. These are engineered through the conservation of a supercharge-like operator in each individual sector in the orbifold sum, despite the overall explicit breaking of spacetime SUSY. We provide a full classification of such orbifolds with finite Abelian point-group, which can only admit $\mathbb{Z}_k \times \mathbb{Z}_k$ point group with $k=2,3,4$. We present detailed constructions, alongside other examples with non-Abelian point group. For some of these models, it is possible that this cancellation persists at higher loops.

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arXiv2026

What IIB looks IIA string: String Cobordisms via Non-Compact CFTs

The Swampland Cobordism Conjecture predicts the existence of end-of-the-world branes for every consistent Quantum Gravity theory, and domain walls connecting the Landscape. A perturbative string worldsheet description of these objects is only expected to exist when certain worldsheet invariants are vanishing or coincide across the domain wall. In this paper, we observe that many of these worldsheet obstructions can be evaded by allowing non-compact string worldsheets as part of the bordism. Using these ideas, we provide a worldsheet QFT (flowing to a critical CFT under RG flow) that connects the worldsheets of 0A and 0B string theory, as well as those of IIA and IIB string theories. The non-compact character of these interpolations means that the description of the actual domain walls develop strongly coupled regions described by a linear dilaton background, where the worldsheet description breaks down. As a result, the IIA/IIB domain wall requires a strongly coupled region, in agreement with effective field theory considerations. Our constructions are heavily inspired by supercritical string theory, but are logically independent from it. We also analyze the fate of IIA/IIB NS5 branes as they cross the domain wall between these two theories.

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arXiv2025

de Sitter no-go's for Riemann-flat manifolds and a link to semidefinite optimisation

We establish a no-go theorem in the context of string and M-theory flux compactifications on Riemann-Flat manifolds with Casimir energy. Specifically, we show that no dS minimum exists in this setup in dimension $d>3$. The case of dS$_3$ minima is not excluded, but their actual fate can only be ascertained via an explicit construction. We also point out that the problem of finding dS minima on RFM's and more general flux compactifications is mathematically equivalent to a semidefinite programming problem, identical to those studied in CFT bootstrap, and hence the search for dS can benefit from the existing vast literature and numerical tools. We illustrate this in a toy model.

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arXiv2025

Cobordism Utopia: U-Dualities, Bordisms, and the Swampland

The U-dualities of maximally supersymmetric supergravity theories lead to celebrated non-perturbative constraints on the structure of quantum gravity. They can also lead to the presence of global symmetries since manifolds equipped with non-trivial duality bundles can carry topological charges captured by non-trivial elements of bordism groups. The recently proposed Swampland Cobordism Conjecture thus predicts the existence of new singular objects absent in the low-energy supergravity theory, which break these global symmetries. We investigate this expectation in two directions, involving the different choices of U-duality groups $G_U$, as well as $k$, the dimension of the closed manifold carrying the topological charge. First, we compute for all supergravity theories in dimension $3 \leq D \leq 11$ the bordism groups $Ω_1^{\text{Spin}}(BG_U)$. Second, we treat in detail the case of $D = 8$, computing all relevant bordism groups $Ω_k^{\text{Spin}}(BG_U)$ for $1 \leq k \leq 7$. In all cases, we identify corresponding string, M-, or F-theory backgrounds which implement the required U-duality defects. In particular, we find that in some cases there is no purely geometric background available which implements the required symmetry-breaking defect. This includes non-geometric twists as well as non-geometric strings and instantons. This computation involves several novel computations of the bordism groups for $G_U = \mathrm{SL}(2,\mathbb{Z}) \times \mathrm{SL}(3,\mathbb{Z})$, which localizes at primes $p=2,3$. Whereas an amalgamated product structure greatly simplifies the calculation of purely $\mathrm{SL}(2,\mathbb{Z})$ bundles, this does not extend to $\mathrm{SL}(3,\mathbb{Z})$. Rather, we leverage the appearance of product / ring structures induced from cyclic subgroups of $G_U$ which naturally act on the relevant bordism groups.

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arXiv2025

Backtracking AdS flux vacua

We introduce an algorithm (dubbed "flux backtracking") to reverse-engineer the brane picture from an AdS flux vacuum. Given an AdS flux vacuum as input, the algorithm outputs a singularity in 10 or 11 dimensions. This singularity has the property that when probed with the appropriate stack of branes (and after taking the near-horizon limit), one recovers the initial AdS vacuum. After testing the procedure on a number of known AdS/CFT pairs, we apply it to AdS flux vacua without known holographic dual, notably the scale-separated DGKT solution. In this case, flux backtracking produces a certain strongly coupled singularity in massive IIA; we conjecture that the worldvolume CFT of D4-branes probing this singularity should be the holographic CFT dual to DGKT (if it exists). Applying the procedure to the DGKT-related scale-separated AdS$_4$ solutions without Romans mass, we find instead a conical and weakly coupled singularity. We also comment on the results and limitations of applying the procedure to KKLT.

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arXiv2025

M-theory boundaries beyond supersymmetry

The chiral worldvolume theory of an M-theory boundary (the so-called M9 brane) is uniquely determined by supersymmetry and anomaly inflow. In this brief note we investigate whether alternative chiral boundary field contents may be allowed by anomaly cancellation once supersymmetry is dropped. Even then, anomaly inflow places stringent constraints on the gauge group $G$ and matter content of the boundary worldvolume theory, which we determine explicitly. We find the most general solution to these constraints in the case where all matter fields are of the same chirality, for all simple Lie algebras except $\mathfrak{sp}_2$, $\mathfrak{su}_{n\leq5}$, and $\mathfrak{so}_{n}$ with $7\leq n\leq 12$, and find no solutions other than the supersymmetric $E_8$ boundary of Hořava and Witten. However, when we extend our search to allow for any chirality in the matter fields, we find one minimal solution with gauge group $G_2$, charged matter in the $\mathsf{14}$, $\mathsf{27}$ and $\mathsf{77}$ representations, which satisfies all constraints in a non-trivial way. Therefore, it could in principle describe the low-energy theory of a novel nonsupersymmetric M-theory boundary condition, different from the Hořava-Witten proposal. We briefly discuss some consequences if this was indeed the case, such as the existence of a non-supersymmetric, exotic "$G_2$-string" CFT in 6d, and a novel, non-perturbative, heterotic-like 10d string with gauge group $G_2\times G_2$.

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arXiv2025

Quantum corrections to DGKT and the Weak Gravity Conjecture

We study D4 brane domain walls in the scale-separated 4d $\mathcal{N}$=1 AdS$_4$ DGKT scenario. Classically, these are BPS and satisfy a no-force condition since their tension equals their charge. We show that this property is not stable against quantum corrections and that these increase the brane tension-to-charge ratio, rendering the branes self-attractive. As a result, DGKT seems to be in tension with the Weak Gravity Conjecture for membranes. The quantum effects we consider include non-perturbative gaugino condensation on the D4-brane worldvolume and Euclidean D2 brane instantons, which correct the tension-to-charge ratio because the DGKT construction breaks all parity symmetries. Similar results hold in other 4d $\mathcal{N}$=1 setups not protected by parity symmetries.

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arXiv2025

To curve, or not to curve: Is curvature-assisted quintessence observationally viable?

Single-field models of accelerated expansion with nearly flat potentials, despite being able to provide observationally viable explanations for the early-time cosmic inflation and the late-time cosmic acceleration, are in strong tension with string theory evidence and the associated de Sitter swampland constraints. It has recently been argued that in an open universe, where the spatial curvature is negative (i.e., with $Ω_k>0$), a new stable fixed point arises, which may lead to viable single-field-based accelerated expansion with an arbitrarily steep potential. Here, we show, through a dynamical systems analysis and a Bayesian statistical inference of cosmological parameters, that the additional cosmological solutions based on the new fixed point do not render steep-potential, single-field, accelerated expansion observationally viable. We mainly focus on quintessence models of dark energy, but we also argue that a similar conclusion can be drawn for cosmic inflation.

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arXiv2025

On Measuring Distances in the Quantum Gravity Landscape

In this note, we propose a generalized notion of distance between vacua in the theory of a scalar field $ϕ$ with scalar potential $V(ϕ)$ coupled to gravity. We propose the normalized tension of domain wall connecting different field values, with a varying normalization relative to a local energy scale, as the distance. We show this definition reproduces the usual moduli space distance for zero potential, as well as the $d\propto |\log Λ|$ behavior with the vacuum energy $Λ$ in the AdS case, previously proposed in the literature. In the case of large AdS we also obtain the expected exponent of mass versus distance in one particular case, when the mass of the light tower is $m\sim \sqrt Λ$ and there is a single extra dimension decompactifying. We also discuss the features and shortcomings of alternative but related proposals.

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arXiv2024

Global anomalies & bordism of non-supersymmetric strings

The three tachyon-free non-supersymmetric string theories in ten dimensions provide a handle on quantum gravity away from the supersymmetric lamppost. However, they have not been shown to be fully consistent; although local anomalies cancel due to versions of the Green-Schwarz mechanism, there could be global anomalies, not cancelled by the Green-Schwarz mechanism, that could become fatal pathologies. We compute the twisted string bordism groups that control these anomalies via the Adams spectral sequence, showing that they vanish completely in two out of three cases (Sugimoto and $SO(16)^2$) and showing a partial vanishing also in the third (Sagnotti 0'B model). We also compute lower-dimensional bordism groups of the non-supersymmetric string theories, which are of interest to the classification of branes in these theories via the Cobordism Conjecture. We propose a worldvolume content based on anomaly inflow for the $SO(16)^2$ NS5-brane, and discuss subtleties related to the torsion part of the Bianchi identity. As a byproduct of our techniques and analysis, we also reprove that the outer $\mathbb{Z}_2$ automorphism swapping the two $E_8$ factors in the supersymmetric heterotic string is also non-anomalous.

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arXiv2024

Cosmological Chameleons, String Theory and the Swampland

We study a scenario with a transient phase of cosmological acceleration that could potentially be realized in asymptotic corners of String Theory moduli space. A very steep scalar potential is temporarily stabilized by the effect of a nonzero density of heavy states, leading to acceleration, in what amounts to a cosmological version of the Chameleon mechanism. The density of heavy states is diluted by cosmological expansion, weakening their effect. After roughly one $e$-fold their effect can no longer stabilize the potential, and the accelerating phase ends. We also study a scenario where there is no potential and the transient acceleration is achieved by the counterbalancing effects of light and heavy towers of states. In both cases we show that it is not possible to obtain more than $\mathcal{O}(1)$ $e$-folds without transplanckian field excursions. We also discuss the general EFT constraints on these models and explore a number of first attempts at concrete embeddings of the scenario in String Theory. These all turn out to face significant challenges.

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arXiv2024

Exotic supergravities and the Swampland

In six dimensions, there is an exotic $\mathcal{N}=(4,0)$ supermultiplet that contains only fields of spin $\leq 2$, but no graviton, and that on a circle reduces to 5D $\mathcal{N}=4$ supergravity. It has been proposed that, if suitable interactions exist, the $(4,0)$ theory might provide a consistent alternative UV completion for $\mathcal{N}=4$ 5D supergravity, realizing a supersymmetric version of asymptotic safety. In this note we argue that any Lorentz-invariant $(4,0)$ theory (interacting or not) carries an exact global symmetry when compactified on $S^1$, and is therefore incompatible with the Swampland no global symmetries conjecture. Another example of exotic supergravity, the 6D $(3,1)$ theory, does not have this problem. We study the general case and find that the only exotic spin-2 field that reduces to Einsteinian gravity and has no global symmetries when compactified on a high-dimensional torus is that of the $(3,1)$ theory. All other possibilities either yield several gravitons or have global symmetries.

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arXiv2024

R7-Branes as Charge Conjugation Operators

R7-branes are a class of recently discovered non-supersymmetric real codimension-two duality defects in type IIB string theory predicted by the Swampland Cobordism Conjecture. For type IIB realizations of 6D SCFTs with $\mathcal{N} = (2,0)$ supersymmetry, wrapping an R7-brane "at infinity" leads to a topological operator associated with a zero-form charge conjugation symmetry that squares to the identity. Similar considerations hold for those theories obtained from further toroidal compactification, but this can be obstructed by bundle curvature effects. Using some minimal data on the topological sector of the R7-branes, we extract the associated fusion rules for these charge conjugation operators. More broadly, we sketch a top down realization of various topological operators / interfaces associated with $\mathsf{C}$, $\mathsf{R}$, and $\mathsf{T}$ transformations. We also use holography to provide strong evidence for the existence of the R7-brane which is complementary to the Cobordism Conjecture. Similar considerations apply to other string-realized QFTs with symmetry operators constructed via non-supersymmetric branes which carry a conserved charge.

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