arXiv subjects

Timothy Budd

Publications and source records attributed to Timothy Budd.

At least 19 recordsLinked to original sources

arXiv2026

Double-scaled SYK from boundary metrics of planar maps

The enumeration of planar maps with control on the boundary metric, i.e. the pseudometric induced on the outer face of the map by its bulk graph distance metric, is a difficult problem in general. However, we show that for a family of bipartite planar map models with special q-deformed face weights that arise in the physics context of the double-scaled Sachdev-Ye-Kitaev model (DSSYK) the enumeration admits a very simple answer. Encoding the boundary metric of a bipartite planar map by its so-called geodesic chord diagram, we prove that the weighted enumeration depends only on the crossing number of the chord diagram. At fixed perimeter, the induced law of the geodesic chord diagram in these planar map models coincides exactly with the chord diagram representation of the DSSYK model.

math.CO↗

arXiv2025

A tree bijection for the moduli space of genus-0 hyperbolic surfaces with boundaries

The Weil-Petersson volume of genus-g hyperbolic surfaces with geodesic boundaries is known since work of Mirzakhani to be polynomial in the boundary lengths. We provide a bijective proof of this fact in the genus-0 case in the presence of a distinguished cusp. It is based on a generalization of a recent tree bijection, by the first author and Curien, to the setting with geodesic boundaries, requiring an extension of the Bowditch-Epstein-Penner spine construction. As an application of our tree bijection we establish an explicit formula for the distance-dependent three-point function, which records an exact metric statistic measuring the difference of two geodesic distances among a triple of distinguished cusps in a Weil-Petersson random surface. We conclude with a discussion of the relevance of this function to the topological recursion of Weil-Petersson volumes and metric properties of Weil-Petersson random surfaces with many boundaries or cusps.

math.GT↗

arXiv2025

Discrete flat disks: rigid quadrangulations

Inspired by a question of Ferrari in the physics context of JT gravity, we introduce and enumerate a combinatorial family of quadrangulations of the disk, called rigid quadrangulations. These form a subclass of the flat quadrangulations in the sense that every inner vertex has degree 4, and therefore it can be viewed as a discrete model of flat metrics on the disk. Our main result is a bijection between rigid quadrangulations and certain colorful integer-labeled quadrangulations of the sphere, together with a dictionary relating a variety of natural statistics on both sides. Adaptions of the bijection to various boundary conditions allow us to import recent enumerative results for colorful quadrangulation obtained by Bousquet-M\'elou and Elvey Price. We discuss some consequences of the enumeration of rigid quadrangulations for a flat version of JT gravity at finite cutoff, and comment on potential scaling limits.

math.CO↗

arXiv2025

Random punctured hyperbolic surfaces & the Brownian sphere

We consider random genus-0 hyperbolic surfaces $\mathcal{S}_n$ with $n + 1$ punctures, sampled according to the Weil-Petersson measure. We show that, after rescaling the metric by $n^{-1/4}$, the surface $\mathcal{S}_n$ converges in distribution to the Brownian sphere - a random compact metric space homeomorphic to the 2-sphere, exhibiting fractal geometry and appearing as a universal scaling limit in various models of random planar maps. Without rescaling the metric, we establish a local Benjamini--Schramm convergence of $\mathcal{S}_n$ to a random infinite-volume hyperbolic surface with countably many punctures, homeomorphic to $\mathbb{R}^2 \setminus \mathbb{Z}^2$. Our proofs mirror techniques from the theory of random planar maps. In particular, we develop an encoding of punctured hyperbolic surfaces via a family of plane trees with continuous labels, akin to Schaeffer's bijection. This encoding stems from the Epstein-Penner decomposition and, through a series of transformations, reduces to a model of single-type Galton--Watson trees, enabling the application of known invariance principles.

math.PR↗

arXiv2025

Phases of Tree-decorated Dynamical Triangulations in 3D

This work revisits the Euclidean Dynamical Triangulation (DT) approach to non-perturbative quantum gravity in three dimensions. Inspired by a recent combinatorial study by T. Budd and L. Lionni of a subclass of 3-sphere triangulations constructed from trees, called the \emph{triple-tree} class, we present a Monte Carlo investigation of DT decorated with a pair of spanning trees, one spanning the vertices and the other the tetrahedra of the triangulation. The complement of the pair of trees in the triangulation can be viewed as a bipartite graph, called the \emph{middle graph} of the triangulation. In the triple-tree class, the middle graph is restricted to be a tree, and numerical simulations have displayed a qualitatively different phase structure compared to standard DT. Relaxing this restriction, the middle graph comes with two natural invariants, namely the number of connected components and loops. Introducing corresponding coupling constants in the action, allows one to interpolate between the triple-tree class and unrestricted tree-decorated DT. Simulations of this extended model confirm the existence of a new phase, referred to as the \emph{triple-tree phase}, besides the familiar crumpled and branched polymer phases of DT. A statistical analysis of the phase transitions is presented, showing hints that the branched polymer to triple-tree phase transition is continuous.

hep-lat↗

arXiv2025

The tight length spectrum of large-genus random hyperbolic surfaces with many cusps

Since the work of Mirzakhani and Petri on random hyperbolic surfaces of large genus, length statistics of closed geodesics have been studied extensively. We focus on the case of random hyperbolic surfaces with cusps, the number of which grows with the genus. We prove that if the number of cusps grows fast enough and we restrict attention to special geodesics that are tight, we recover upon proper normalization the same Poisson point process in the large genus limit for the length statistics. The proof relies on a recursion formula for tight Weil-Petersson volumes obtained recently by Budd and Zonneveld and on a generalization of Mirzakhani's integration formula to the tight setting.

math.PR↗

arXiv2023

Topological recursion of the Weil-Petersson volumes of hyperbolic surfaces with tight boundaries

The Weil-Petersson volumes of moduli spaces of hyperbolic surfaces with geodesic boundaries are known to be given by polynomials in the boundary lengths. These polynomials satisfy Mirzakhani's recursion formula, which fits into the general framework of topological recursion. We generalize the recursion to hyperbolic surfaces with any number of special geodesic boundaries that are required to be tight. A special boundary is tight if it has minimal length among all curves that separate it from the other special boundaries. The Weil-Petersson volume of this restricted family of hyperbolic surfaces is shown again to be polynomial in the boundary lengths. This remains true when we allow conical defects in the surface with cone angles in $(0,\pi)$ in addition to geodesic boundaries. Moreover, the generating function of Weil-Petersson volumes with fixed genus and a fixed number of special boundaries is polynomial as well, and satisfies a topological recursion that generalizes Mirzakhani's formula. We will comment on a possible geometric interpretation of this formula. This work is largely inspired by recent works by Bouttier, Guitter & Miermont on the enumeration of planar maps with tight boundaries. Our proof relies on the equivalence of Mirzakhani's recursion formula to a sequence of partial differential equations (known as the Virasoro constraints) on the generating function of intersection numbers. Finally, we discuss a connection with JT gravity. We show that the multi-boundary correlators of JT gravity with defects (cone points or FZZT branes) are expressible in the tight Weil-Petersson volume generating functions, using a tight generalization of the JT trumpet partition function.

math-ph↗

arXiv2022

Lessons from the Mathematics of Two-Dimensional Euclidean Quantum Gravity

The search for a mathematical foundation for the path integral of Euclidean quantum gravity calls for the construction of random geometry on the spacetime manifold. Following developments in physics on the two-dimensional theory, random geometry on the 2-sphere has in recent years received much attention in the mathematical literature, which has led to a fully rigorous implementation of the path integral formulation of two-dimensional Euclidean quantum gravity. In this chapter we review several important mathematical developments that may serve as guiding principles for approaching Euclidean quantum gravity in dimensions higher than two. Our starting point is the discrete geometry encoded by random planar maps, which realizes a lattice discretization of the path integral. We recap the enumeration of planar maps via their generating functions and show how bijections with trees explain the surprising simplicity of some of these. Then we explain how to handle infinite planar maps and to analyze their exploration via the peeling process. The aforementioned trees provide the basis for the construction of the universal continuum limit of the random discrete geometries, known as the Brownian sphere, which represents the random geometry underlying two-dimensional Euclidean quantum gravity in the absence of matter.

gr-qc↗

arXiv2022

Scale-invariant random geometry from mating of trees: a numerical study

The search for scale-invariant random geometries is central to the Asymptotic Safety hypothesis for the Euclidean path integral in quantum gravity. In an attempt to uncover new universality classes of scale-invariant random geometries that go beyond surface topology, we explore a generalization of the mating of trees approach introduced by Duplantier, Miller and Sheffield. The latter provides an encoding of Liouville Quantum Gravity on the 2-sphere decorated by a certain random space-filling curve in terms of a two-dimensional correlated Brownian motion, that can be viewed as describing a pair of random trees. The random geometry of Liouville Quantum Gravity can be conveniently studied and simulated numerically by discretizing the mating of trees using the Mated-CRT maps of Gwynne, Miller and Sheffield. Considering higher-dimensional correlated Brownian motions, one is naturally led to a sequence of non-planar random graphs generalizing the Mated-CRT maps that may belong to new universality classes of scale-invariant random geometries. We develop a numerical method to efficiently simulate these random graphs and explore their possible scaling limits through distance measurements, allowing us in particular to estimate Hausdorff dimensions in the two- and three-dimensional setting. In the two-dimensional case these estimates accurately reproduce previous known analytic and numerical results, while in the three-dimensional case they provide a first window on a potential three-parameter family of new scale-invariant random geometries.

gr-qc↗

arXiv2022

A family of triangulated 3-spheres constructed from trees

The search for universality in random triangulations of manifolds, like those featuring in (Euclidean) Dynamical Triangulations, is central to the random geometry approach to quantum gravity. In case of the 3-sphere, or any other manifold of dimension greater than two for that matter, the pursuit is held back by serious challenges, including the wide open problem of enumerating triangulations. In an attempt to bypass the toughest challenges we identify a restricted family of triangulations, of which the enumeration appears less daunting. In a nutshell, the family consists of triangulated 3-spheres decorated with a pair of trees, one spanning its tetrahedra and the other its vertices, with the requirement that after removal of both trees one is left with a tree-like 2-complex. We prove that these are in bijection with a combinatorial family of triples of plane trees, satisfying restrictions that can be succinctly formulated at the level of planar maps. An important ingredient in the bijection is a step-by-step reconstruction of the triangulations from triples of trees, that results in a natural subset of the so-called locally constructible triangulations, for which spherical topology is guaranteed, through a restriction of the allowed moves. We also provide an alternative characterization of the family in the framework of discrete Morse gradients. Finally, several exponential enumerative bounds are deduced from the triples of trees and some simulation results are presented.

math.CO↗

arXiv2020

Irreducible metric maps and Weil-Petersson volumes

We consider maps on a surface of genus $g$ with all vertices of degree at least three and positive real lengths assigned to the edges. In particular, we study the family of such metric maps with fixed genus $g$ and fixed number $n$ of faces with circumferences $\alpha_1,\ldots,\alpha_n$ and a $\beta$-irreducibility constraint, which roughly requires that all contractible cycles have length at least $\beta$. Using recent results on the enumeration of discrete maps with an irreducibility constraint, we compute the volume $V_{g,n}^{(\beta)}(\alpha_1,\ldots,\alpha_n)$ of this family of maps that arises naturally from the Lebesgue measure on the edge lengths. It is shown to be a homogeneous polynomial in $\beta, \alpha_1,\ldots, \alpha_n$ of degree $6g-6+2n$ and to satisfy string and dilaton equations. Surprisingly, for $g=0,1$ and $\beta=2\pi$ the volume $V_{g,n}^{(2\pi)}$ is identical, up to powers of two, to the Weil-Petersson volume $V_{g,n}^{\mathrm{WP}}$ of hyperbolic surfaces of genus $g$ and $n$ geodesic boundary components of length $L_i = \sqrt{\alpha_i^2 - 4\pi^2}$, $i=1,\ldots,n$. For genus $g\geq 2$ the identity between the volumes fails, but we provide explicit generating functions for both types of volumes, demonstrating that they are closely related. Finally we discuss the possibility of bijective interpretations via hyperbolic polyhedra.

math-ph↗

arXiv2020

On polynomials counting essentially irreducible maps

We consider maps on genus-$g$ surfaces with $n$ (labeled) faces of prescribed even degrees. It is known since work of Norbury that, if one disallows vertices of degree one, the enumeration of such maps is related to the counting of lattice point in the moduli space of genus-$g$ curves with $n$ labeled points and is given by a symmetric polynomial $N_{g,n}(\ell_1,\ldots,\ell_n)$ in the face degrees $2\ell_1, \ldots, 2\ell_n$. We generalize this by restricting to genus-$g$ maps that are essentially $2b$-irreducible for $b\geq 0$, which loosely speaking means that they are not allowed to possess contractible cycles of length less than $2b$ and each such cycle of length $2b$ is required to bound a face of degree $2b$. The enumeration of such maps is shown to be again given by a symmetric polynomial $\hat{N}_{g,n}^{(b)}(\ell_1,\ldots,\ell_n)$ in the face degrees with a polynomial dependence on $b$. These polynomials satisfy (generalized) string and dilaton equations, which for $g\leq 1$ uniquely determine them. The proofs rely heavily on a substitution approach by Bouttier and Guitter and the enumeration of planar maps on genus-$g$ surfaces.

math.CO↗

arXiv2019

Simple peeling of planar maps with application to site percolation

The peeling process, which describes a step-by-step exploration of a planar map, has been instrumental in addressing percolation problems on random infinite planar maps. Bond and face percolation on maps with faces of arbitrary degree are conveniently studied via so-called lazy-peeling explorations. During such explorations distinct vertices on the exploration contour may at later stage be identified, making the process less suited to the study of site percolation. To tackle this situation and to explicitly identify site-percolation thresholds, we come back to the alternative "simple" peeling exploration of Angel and uncover deep relations with the lazy-peeling process. Along the way we define and study the random Boltzmann map of the half-plane with a simple boundary for an arbitrary critical weight sequence. Its construction is nontrivial especially in the "dense regime" where the half-planar random Boltzmann map does not possess an infinite simple core.

math.PR↗

arXiv2019

Precision measurements of Hausdorff dimensions in two-dimensional quantum gravity

Two-dimensional quantum gravity, defined either via scaling limits of random discrete surfaces or via Liouville quantum gravity, is known to possess a geometry that is genuinely fractal with a Hausdorff dimension equal to 4. Coupling gravity to a statistical system at criticality changes the fractal properties of the geometry in a way that depends on the central charge of the critical system. Establishing the dependence of the Hausdorff dimension on this central charge $c$ has been an important open problem in physics and mathematics in the past decades. All simulation data produced thus far has supported a formula put forward by Watabiki in the nineties. However, recent rigorous bounds on the Hausdorff dimension in Liouville quantum gravity show that Watabiki's formula cannot be correct when $c$ approaches $-\infty$. Based on simulations of discrete surfaces encoded by random planar maps and a numerical implementation of Liouville quantum gravity, we obtain new finite-size scaling estimates of the Hausdorff dimension that are in clear contradiction with Watabiki's formula for all simulated values of $c\in (-\infty,0)$. Instead, the most reliable data in the range $c\in [-12.5, 0)$ is in very good agreement with an alternative formula that was recently suggested by Ding and Gwynne. The estimates for $c\in(-\infty,-12.5)$ display a negative deviation from the latter formula, but the scaling is seen to be less accurate in this regime.

gr-qc↗

arXiv2018

The peeling process on random planar maps coupled to an O(n) loop model (with an appendix by Linxiao Chen)

We extend the peeling exploration introduced in arxiv:1506.01590 to the setting of Boltzmann planar maps coupled to a rigid $O(n)$ loop model. Its law is related to a class of discrete Markov processes obtained by confining random walks to the positive integers with a new type of boundary condition. As an application we give a rigorous justification of the phase diagram of the model presented in arXiv:1106.0153. This entails two results pertaining to the so-called fixed-point equation: the first asserts that any solution determines a well-defined model, while the second result, contributed by Chen in the appendix, establishes precise existence criteria. A scaling limit for the exploration process is identified in terms of a new class of positive self-similar Markov processes, going under the name of ricocheted stable processes. As an application we study distances on loop-decorated maps arising from a particular first passage percolation process on the maps. In the scaling limit these distances between the boundary and a marked point are related to exponential integrals of certain L\'evy processes. The distributions of the latter can be identified in a fairly explicit form using machinery of positive self-similar Markov processes. Finally we observe a relation between the number of loops that surround a marked vertex in a Boltzmann loop-decorated map and the winding angle of a simple random walk on the square lattice. As a corollary we give a combinatorial proof of the fact that the total winding angle around the origin of a simple random walk started at $(p,p)$ and killed upon hitting $(0,0)$ normalized by $\log p$ converges in distribution to a Cauchy random variable.

math.PR↗

arXiv2017

Winding of simple walks on the square lattice

A method is described to count simple diagonal walks on $\mathbb{Z}^2$ with a fixed starting point and endpoint on one of the axes and a fixed winding angle around the origin. The method involves the decomposition of such walks into smaller pieces, the generating functions of which are encoded in a commuting set of Hilbert space operators. The general enumeration problem is then solved by obtaining an explicit eigenvalue decomposition of these operators involving elliptic functions. By further restricting the intermediate winding angles of the walks to some open interval, the method can be used to count various classes of walks restricted to cones in $\mathbb{Z}^2$ of opening angles that are integer multiples of $\pi/4$. We present three applications of this main result. First we find an explicit generating function for the walks in such cones that start and end at the origin. In the particular case of a cone of angle $3\pi/4$ these walks are directly related to Gessel's walks in the quadrant, and we provide a new proof of their enumeration. Next we study the distribution of the winding angle of a simple random walk on $\mathbb{Z}^2$ around a point in the close vicinity of its starting point, for which we identify discrete analogues of the known hyperbolic secant laws and a probabilistic interpretation of the Jacobi elliptic functions. Finally we relate the spectrum of one of the Hilbert space operators to the enumeration of closed loops in $\mathbb{Z}^2$ with fixed winding number around the origin.

math.CO↗

arXiv2017

Infinite random planar maps related to Cauchy processes

We study the geometry of infinite random Boltzmann planar maps having weight of polynomial decay of order $k^{-2}$ for each vertex of degree $k$. These correspond to the dual of the discrete "stable maps" of Le Gall and Miermont [Scaling limits of random planar maps with large faces, Ann. Probab. 39, 1 (2011), 1-69] studied in [Budd & Curien, Geometry of infinite planar maps with high degrees, Electron. J. Probab. (to appear)] related to a symmetric Cauchy process, or alternatively to the maps obtained after taking the gasket of a critical $O(2)$-loop model on a random planar map. We show that these maps have a striking and uncommon geometry. In particular we prove that the volume of the ball of radius $r$ for the graph distance has an intermediate rate of growth and scales as $\mathrm{e}^{\sqrt{r}}$. We also perform first passage percolation with exponential edge-weights and show that the volume growth for the fpp-distance scales as $\mathrm{e}^{r}$. Finally we consider site percolation on these lattices: although percolation occurs only at $p=1$, we identify a phase transition at $p=1/2$ for the length of interfaces. On the way we also prove new estimates on random walks attracted to an asymmetric Cauchy process.

math.PR↗

arXiv2016

Martingales in self-similar growth-fragmentations and their connections with random planar maps

The purpose of the present work is twofold. First, we develop the theory of general self-similar growth-fragmentation processes by focusing on martingales which appear naturally in this setting and by recasting classical results for branching random walks in this framework. In particular, we establish many-to-one formulas for growth-fragmentations and define the notion of intrinsic area of a growth-fragmentation. Second, we identify a distinguished family of growth-fragmentations closely related to stable L\'evy processes, which are then shown to arise as the scaling limit of the perimeter process in Markovian explorations of certain random planar maps with large degrees (which are, roughly speaking, the dual maps of the stable maps of Le Gall & Miermont. As a consequence of this result, we are able to identify the law of the intrinsic area of these distinguished growth-fragmentations. This generalizes a geometric connection between large Boltzmann triangulations and a certain growth-fragmentation process, which was established in arXiv:1507.02265 .

math.PR↗