Publications
2025
- Constraining strongly-warped extra dimensions with rotating black holesBruno Valeixo Bento, and Miquel Salicrú HerbergNov 2025
Massive bosonic fields can trigger superradiant instabilities in rotating astrophysical black holes leading to gaps in their mass-spin distribution. For spin-2 fields, the instability timescale is orders of magnitude shorter than for any other superradiant mode, thereby yielding much stronger constraints. We consider a tower of ultra-light spin-2 fields arising from a warped compactification of a single extra dimension and translate superradiant constraints on their masses into constraints on the warping. As a concrete scenario we consider the 2-brane Randall-Sundrum model and find constraints on the size of the extra dimension and the curvature of AdS5. We discuss the implications of these bounds for strongly warped throats and D-brane uplifts commonly used in attempts to realise metastable de Sitter vacua in string theory.
- de Sitter no-go’s for Riemann-flat manifolds and a link to semidefinite optimisationBruno Valeixo Bento, and Miguel MonteroOct 2025
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 dS3 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.
- An M-theory dS maximum from Casimir energies on Riemann-flat manifoldsBruno Valeixo Bento, and Miguel MonteroJul 2025
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 dS5 maximum solution of a flux compactification of M-theory on a specific 6-dimensional RFM.
- EFT & Species Scale: Friends or foes?Bruno Valeixo Bento, and João F. MeloJan 2025
Recently the notion that quantum gravity effects could manifest at scales much lower than the Planck scale has seen an intense Swamplandish revival. Dozens of works have explored how the so-called species scale – at which an effective description of gravity must break down – relates to String Theory and the Swampland conjectures. In particular, the interplay between this scale and the abundant towers of states becoming lighter in asymptotic regions of moduli spaces has proved to be key in understanding the real scale of quantum gravity. Nevertheless concerns have been raised regarding the validity of using infinite towers of states when estimating this scale within Effective Field Theory and, more precisely, the consistency of cutting the tower part way through in a framework that relies on a clear separation of scales. In this work we take an EFT point-of-view and provide a detailed perturbative derivation of the species scale – by computing the 1-loop graviton propagator in the presence of many fields – thereby clarifying common sources of confusion in the literature. Not only do we clarify the setup, assumptions and regimes of validity of the result, but more importantly apply the same methods to an infinite tower of states. We show how each state in the tower contributes to the species scale and how the procedure of counting only ”light fields” can be compatible with not cutting the tower, thereby maintaining the harmony between infinite towers and EFTs even in the context of the species scale.
2023
- The Gravity of Warped Throats: de Sitter vacua and Gravitational Waves from Type IIB string theoryBruno Valeixo BentoUniversity of Liverpool, Jan 2023
In this thesis we discuss challenges and opportunities arising from warping the extra dimensionsof string theory. After reviewing the required background (including the essentials of TypeIIB string theory; flux compactifications; conifolds, warping, and the Klebanov-Strassler andGKP solutions; and the KKLT and LVS proposals) we will discuss de Sitter solutions in warpedflux compactifications. We revisit some strongly-warped solutions, present a new solution ina weakly-warped regime and discuss the advantages of weak warping. We then consider therobustness of the new solution in the presence of subleading corrections to the scalar potential.We also explore the difficulties of realising alternative quintessence models as quasi-de Sittersolutions, showing in particular that the generic behaviour of the (single field) scalar potentialarising for different types of string theory moduli does not allow for a slow-roll acceleratedexpansion at the tail of a runaway.We then take the first steps in understanding the effects of warping in gravitational wave signaturesof extra dimensions. By considering the tower of Kaluza-Klein spin-2 states arising froma warped compactification of Type IIB string theory, we study the effects of warping on theirmasses and wavefunction profiles, which we then use to compute corrections to the Newtonianpotential that one can compare with current constraints on fifth forces. This allows us to combinetheoretical consistency constraints on the parameter space with the range of parametersexperimentally excluded, thereby providing a direct connection between string theory quantitiesand observations. Although a careful study of gravitational wave signals is left for future work,we briefly outline how these results directly apply in that context and suggest which sourcesmight be more promising for future detection.
- De Sitter vacua - when are ‘subleading corrections’ really subleading?Bruno Valeixo Bento, Dibya Chakraborty, Susha Parameswaran, and Ivonne ZavalaJun 2023
We consider various string-loop, warping and curvature corrections that are expected to appear in type IIB moduli stabilisation scenarios. It has recently been argued, in the context of strongly-warped LVS de Sitter vacua, that it is impossible to achieve parametric suppression in all of these corrections simultaneously [1]. We investigate corrections in the context of the recently discovered weakly-warped LVS de Sitter vacua, which represent a distinct branch of solutions in type IIB flux compactifications. The weakly-warped solution is supported by small conifold flux numbers MK ≲ 32, but still requires a large flux contribution to the D3-tadpole, now from the bulk. Warping corrections become less problematic, and some corrections even help to reach the weakly-warped regime of parameter space. Other corrections continue to be dangerous and would require numerical coefficients to be computed — and found to be small — in order not to destroy the consistency of the weakly-warped LVS de Sitter solution. We motivate why this may be possible.
- A guide to frames, 2π’s, scales and corrections in string compactificationsBruno Valeixo Bento, Dibya Chakraborty, Susha Parameswaran, and Ivonne ZavalaJan 2023
This note is intended to serve as a reference for conventions used in the literature on string compactifications, and how to move between them, collected in a single and easy-to-find place, using type IIB as an illustrative example. We hope it may be useful to beginners in the field and busy experts. E.g. string constructions proposed to address the moduli stabilisation problem are generically in regions of parameter space at the boundaries of control, so that consistent use of 2π’s and frame conventions can be pivotal when computing their potentially dangerous corrections.
2022
- Gravity at the tip of the throatBruno Valeixo Bento, Dibya Chakraborty, Susha Parameswaran, and Ivonne ZavalaJHEP, Jan 2022
We study the gravitational signatures that arise from compactifying Type IIB supergravity on a compact space containing a Klebanov-Strassler warped throat. After reviewing the dimensional reduction of the 10d graviton and explicitly obtaining the equa- tions of motion for the 4d tensor hμν, vector hμn and scalar hmn modes, we find the masses and wavefunctions of the Kaluza-Klein tower of spin-2 states. We explore how the masses and wavefunctions depend on the balance between the strength of the warping and the size of the bulk, and how these relate to the range and strength of the interactions which correct the Newtonian gravitational potential. By computing the modified Newtonian potential for sources on a brane somewhere along the throat, and applying consistency constraints on the Klebanov-Strassler parameters, we obtain predictions for the phenomenological pa- rameter space. In the case of a fully warped throat, and depending on where the brane is along the throat, these predictions are narrow in range and consistent with current obser- vational and experimental constraints. We also begin an exploration of gravitational wave signatures of KK gravitons in warped throats, finding that strong warping can bring the corresponding frequencies down to the windows of current and proposed experiments.
- If time had no beginning: growth dynamics for past-infinite causal setsBruno Valeixo Bento, Fay Dowker, and Stav ZalelClass. Quant. Grav., Jan 2022
We explore whether the growth dynamics paradigm of causal set theory is compatible with past-infinite causal sets. We modify the classical sequential growth dynamics of Rideout and Sorkin to accommodate growth ‘into the past’ and discuss what form physical constraints such as causality could take in this new framework. We propose convex-suborders as the ‘observables’ or ‘physical properties’ in a theory in which causal sets can be past-infinite and use this proposal to construct a manifestly covariant framework for dynamical models of growth for past-infinite causal sets.
2021
- If time had no beginningBruno Valeixo Bento, and Stav ZalelSep 2021
General Relativity traces the evolution of our Universe back to a Big Bang singularity. To probe physics before the singularity – if indeed there is a “before”– we must turn to quantum gravity. The Causal Set approach to quantum gravity provides us with a causal structure in the absence of the continuum, thus allowing us to go beyond the Big Bang and consider cosmologies in which time has no beginning. But is a time with no beginning in contradiction with a passage of time? In the Causal Set approach, the passage of time is captured by a process of spacetime growth. We describe how to adapt this process for causal sets in which time has no beginning and discuss the consequences for the nature of time.
- A new de Sitter solution with a weakly warped deformed conifoldBruno Valeixo Bento, Dibya Chakraborty, Susha L. Parameswaran, and Ivonne ZavalaJHEP, Sep 2021
We revisit moduli stabilisation for type IIB flux compactifications that include a warped throat region corresponding to a warped deformed conifold, with an anti-D3-brane sitting at its tip. The warping induces a coupling between the conifold’s deformation modulus and the bulk volume modulus in the Kähler potential. Previous works have studied the scalar potential assuming a strong warping such that this coupling term dominates, and found that the anti-D3-brane uplift may destabilise the conifold modulus and/or volume modulus, unless flux numbers within the throat are large, which makes tadpole cancellation a challenge. We explore the regime of parameter space corresponding to a weakly-but-still warped throat, such that the coupling between the conifold and volume moduli is subdominant. We thus discover a new metastable de Sitter solution within the four-dimensional effective field theory. We discuss the position of this de Sitter vacuum in the string theory landscape and swampland.
2020
2018
- Causal Set Dynamics and The Problem of TimeBruno Valeixo BentoImperial Coll., London, Sep 2018
Causal Set Theory is an approach to Quantum Gravity that takes spacetime tobe a discrete collection of events that is given its structure by their causal order. In thiswork we discuss the impossibility of having CPT-invariance (as order-reversal symmetry)in a CSG dynamics for causal sets and explore some possible alternatives that may satisfythis symmetry. We discuss the Alternating Dynamics (AD), generalising it to allow forany pattern, and show that any AD must correspond to Transitive Percolation (TP) inorder to be generally covariant. This leads us to introduce the Atemporal CSG (ACSG)models, by dropping internal temporality in the CSG models. We find again that TP isthe only generally covariant such model and that it leads to an order-reversal symmetricdynamics. Finally we discuss whether a dynamics that allows for past infinite causetsmust be deterministic and whether a dynamics where Time is finite may be the solutionto make CSG models CPT-invariant. We try to approach these problems using a recentproposal for causal set dynamics, the Covariant Tree (Covtree).