String Seminar Archive: 2023-2024

In this talk we will argue that the physics of M-theory and Type IIA strings on nodal CY 3-folds is essentially determined by the geometry of a small resolution, even if the latter is not Kähler. We will demonstrate this explicitly in the context of nodal CY 3-folds that are double covers of P^3 with the ramification locus being a symmetric determinantal surface. Using conifold transitions, we prove that in certain cases the exceptional curves in any small resolution are torsion while the same transitions imply that M-theory develops a discrete gauge symmetry. We further argue that twisted circle compactifications of the 5d theory are dual to IIA compactifications on the nodal CY 3-fold with a flat but topologically non-trivial B-field.

Given knowledge of the A-model topological string free energies associated to all twists one can then extract the torsion refined GV-invariants. In many cases GLSM techniques and/or mirror symmetry can be used to calculate these topological string free energies at least for low genera.

2d Yang-Mills is a completely solvable interacting gauge theory which has been analyzed from many perspectives. In this talk, I will report on our recent computation of the all-orders perturbative expansion around higher critical points, analogous to instantons, for this theory. I will describe two approaches to this result: through resummation of the lattice partition function, and via non-standard supersymmetric localization. The pre-talk will feature an introduction to localization and its many subtleties.

I will talk about asymptotic detector operators describing possible measurements performed at null infinity. I will discuss how to define them in terms of light-ray operators in conformal field theories as well as in weakly-coupled field theories, how to compute their matrix elements, and how to renormalize them, leading to detector anomalous dimensions. I will apply this framework to Wilson-Fisher theory as an explicit example, recovering known results, resolving some puzzles, and discovering some novel Regge trajectories. Finally, I will consider detectors and event shapes in asymptotically-flat Einstein gravity.

To write fluid dynamics from a modern effective field theory point of view, an infinite reparameterization symmetry is required. Curiously, black hole horizons possess the same redundancy. In this talk, I will show how horizon symmetries can be interpreted as symmetries of a dual hydrodynamic theory. In addition, I will show how the horizon structure leads to additional symmetries that are responsible for the chaotic behavior of the dual theory. 

I will give an overview of recent developments in the computation of Donaldson-Thomas partition functions on toric Calabi-Yau 4-folds using quantum field theory techniques, treating the particular case of affine space and its abelian orbifolds in detail. In the pre-seminar, I will give an introduction to instantons and the combinatorics of instanton counting in four dimensions.

The superconformal index of 1/2-BPS states of N=4 U(N) super Yang-Mills theory has a known infinite q-series expression with successive terms suppressed by q^N. I will explain how one can derive this expression from the bulk as a result of the localization of a functional integral in the dual AdS_5 x S^5 to a set of saddles (spherical branes called giant gravitons) and their fluctuations.

Resurgence has become a popular tool in theoretical physics over the past years: it is a very useful mathematical framework to discuss nonperturbative physics, finding applications in topological string theory, JT gravity, black hole physics, quantum field theory and many other areas. I will review what resurgence is and why it is so useful, and will present a selection of useful applications, focusing on the ones that will interest string theorists and quantum gravity enthusiasts most. Then, based on recent work with Coenraad Marinissen en Alexander van Spaendonck, I will explain why the central tool of resurgence - the large order relation that allows one to read off nonperturbative data - is itself an object to which resurgence can be applied, giving us important insights into how different nonperturbative effects work together and are related through e.g. Stokes' phenomenon.

In the past few years, a new connection between heavy ion physics and nuclear structure has begun to be explored. Hydrodynamics converts initial spatial anisotropies of the quark-gluon plasma into final state momentum anisotropies. As such, the geometry of the initial state leaves a measurable imprint. The connection then arises because the initial state is in turn sensitive to the shape and size of the nuclei being collided. In this talk, we will use the connection in two directions. In the first, we use heavy ion collisions to extract the value of the so-called neutron skin of Pb-208 from LHC experimental data, which is relevant to the interiors of neutron stars. In the second part, we take input from nuclear structure on the shapes of O-16 and Ne-20 and perform hydrodynamical simulations. This leads to precise hydrodynamic predictions for observable ratios between NeNe and OO collisions. Possible future comparison to experimental data would then give us a precision check as to how well hydrodynamics can describe small systems such as OO and NeNe collisions.

There have been numerous recent attempts to elucidate the precise role that the Quantum Gravity (QG) cut-off -- frequently referred to as the Species Scale -- plays within the Swampland context, and more generally when trying to characterize the universality class of IR effective field theories (EFT) that descend from a consistent QG framework. In this talk we will discuss some progress towards the understanding of the Species Scale as the UV cut-off controlling the gravitational EFT expansion, by a careful inspection of several supersymmetric String Theory constructions. Along the way, we will also point out an intriguing pattern that such gravitational energy scale seems to fulfill in all known infinite distance/weak coupling corners. Interestingly, together with the knowledge of the leading tower of states becoming light asymptotically at every such infinite distance point, this pattern is enough to uniquely determine the global structure of dualities and phases for any QG theory under consideration.

In this talk we will review some of the recent progress in understanding Quantum Field Theory in de Sitter spacetime. We will use the Hilbert space construction and representation theory of de Sitter symmetry group (i.e. the Euclidean conformal group) to derive the spectral decomposition of the bulk two-point function and the conformal partial wave expansion of the boundary four-point function. In both cases unitarity of the bulk theory implies positivity conditions which leads to non trivial bounds on physical observables. Using harmonic analysis and the Wick rotation to Euclidean Anti de Sitter as well as analytic continuation to the sphere, we derive two equivalent inversion formulas to compute the spectral densities. Using the inversion formula, we relate the analytic structure of the spectral densities to the late-time boundary operator content and discuss the possible subtleties. We will conclude with a few examples namely CFT in the bulk and composite operators of the free theory. 

Compactifications of the heterotic string with eight supercharges provide a rich corner of the string landscape, in which fluxes can be analyzed directly from the worldsheet perspective. The underlying geometry of such 4-dimensional compactifications is a principal torus bundle over a K3 surface, and is generically non-Kähler. In this talk, I will focus on the interrelation between topology and Narain T-duality of heterotic flux vacua. Specifically, I will present evidence that T-duality in the torus fiber can relate all such backgrounds to flux-free compactifications on a K3$\times T^2$ product geometry. In addition, I will emphasize the role of global and flat structures in the resulting web of equivalent geometries.

We introduce a framework for quantifying random matrix behavior of 2d CFTs and AdS_3 quantum gravity that is manifestly compatible with conformal and modular invariance. We explain what it
means to isolate the chaotic part of the Virasoro primary spectrum of a given theory. This leads to a 2d CFT trace formula, analogous to the Gutzwiller trace formula for chaotic quantum systems, which allows us to identify conditions for random matrix universality in 2d CFT. We use this to understand the off-shell Cotler-Jensen torus wormhole of AdS_3 pure gravity from a purely microscopic CFT point of view. This wormhole is shown to be extremal, a minimal completion of the random matrix theory two-point correlator compatible with the symmetries. By factorizing this wormhole, we determine a new piece of the AdS_3 pure gravity path integral with a single torus boundary; this contains fine-grained spectral data of the black hole microstates of pure gravity.

We pose Boltzmann equations describing a gas of highly excited open and closed strings and study its implications for cosmology, in particular the putative gravitational wave background arising from this scenario. We use worldsheet methods to compute interaction rates for typical highly excited open and closed strings in flat backgrounds with d effectively non-compact directions - (ie: of size larger than the size of the typical string), with different densities of Dp-branes. Using these rates, we pose Boltzmann equations describing a gas of such strings and check that they admit known equilibrium solutions through the principle of detailed balance. Furthermore, we find equilibration rates of different configurations and argue that they suggest a consistent story in which a Hagedorn phase in the early Universe features a gas of long open strings dominating the energy density, which predominantly decay into Standard Model fields but also release a gravitational wave background. We compute the spectrum of this background and show that its peak amplitude lies at around 100 GHz, close to the prediction of the Cosmic Gravitational Wave Background sourced by the Standard Model, but with a generically larger amplitude.

The presence of Chern-Simons terms in holographic QCD is required by the global flavor anomaly structure. These Chern-Simons terms may give rise to a spatial instability at nonzero density, known as the Nakamura-Ooguri-Park instability. I demonstrate that this instability is unavoidable in a large class of bottom-up models of QCD anchored to lattice data, and extends to surprisingly high temperatures and low baryon number densities. The precise range of the instability is however sensitive to the strange quark mass, which is not properly included in these models so far.

We construct new solutions to gauged supergravity that, via the AdS/CFT correspondence, are dual to thermal phases in N=4 SYM at finite chemical potential. These solutions dominate the micro-canonical ensemble and are required to ultimately reproduce the microscopic entropy of AdS black holes. These are constructed in two distinct truncations of gauged supergravity and can be uplifted to solutions of type IIB supergravity. Together with the known phases of the truncation with three equal charges, our findings permit a good understanding of the full phase space of SYM thermal states with three arbitrary chemical potentials.

Towers species can significantly affect the properties of  gravitational effective field theories. They naturally appear in string theory, and one of their universal effects is a renormalization of the scale at which gravity becomes strongly coupled. In this talk, we will discuss some of the implications for cosmology arising from the presence of a large number of species. A major focus will be on cosmic inflation and its observational signatures in the cosmic microwave background. We will also discuss some of the implications for Starobinsky inflation.

Holographic theories can be defined on  curved space times and coupled to dynamical  boundary gravity by an appropriate choice of AdS coordinates  and an appropriate renormalization procedure. In this talk, I will discuss how  the coupling to a holographic QFTs affects gravitational dynamics in de Sitter spacetime. I will address questions such as: the effective f(R) formulation of the system;   the non-perturbative fate of de Sitter when coupled to a gapless QFT;  the spectrum of graviton modes ; the perturbative stability and the consistency of the effective field theory of the CFT-gravity system.  

We study infinite families of black hole microstates consisting of wormholes and shells of matter. They are orthogonal at leading order in the saddle point approximation of the Euclidean gravitational path integral, suggesting a dramatic overcounting of the dimension of the microcanonical subspace. However, wormhole contributions in higher moments of the overlaps reveal small off-diagonal components. This non-orthogonality reduces the (naively infinite) dimension of the space spanned by these states to the expected result: the exponential of the Bekenstein-Hawking entropy.

We consider type II string theory on Calabi-Yau threefolds with B-type D-branes. Hybrid models appear in limiting regions of the stringy Kahler moduli space of the Calabi-Yau. They are Landau-Ginzburg models fibred over a compact base manifold. We give a physics derivation of B-type D-branes in hybrids and show that they are matrix factorisations of the hybrid superpotential, combined with geometric data associated to the base. Lifting these branes to the gauged linear sigma model (GLSM) and making use of the GLSM hemisphere partition function, we establish a connection between geometric and hybrid branes. As an application, we discuss a hybrid model that shares the moduli space with a well-known two-parameter Calabi-Yau hypersurface. Using the GLSM, we analytically continue a basis of geometric branes to the hybrid region of the moduli space. This is joint work with Robert Pryor.

We present a numerical computation, based on neural network techniques, of the physical Yukawa couplings in a heterotic string theory model obtained by compactifying on a smooth Calabi-Yau threefold.  The model in question is one of a large class of heterotic line bundle models with precisely the MSSM low-energy spectrum plus fields uncharged under the standard-model group. Suitable neural networks are used to compute the relevant quantities: the Ricci-flat Calabi-Yau metric, Hermitian Yang-Mills bundle metrics and harmonic bundle-valued forms. We carry out the calculation at various points along a one-parameter family in complex structure moduli space. The methods presented here generalise to other string models and constructions.

Gubser flow is an evolution with cylindrical and boost symmetries in a relativistic system. In a conformal theory, the symmetries are enhanced SO(3) × SO(1,1) implying that the Gubser flow is best studied by mapping the future wedge of Minkowski space R^(3,1) to dS_3 × R. It is known that at large de-Sitter time, which corresponds to the large proper time and central region in the future wedge, the behavior of the Gubser flow is not hydrodynamic. Developing on previous results, we solve the generic behavior of the Gubser flow in a holographic conformal field theory in the large de-Sitter time expansion. Remarkably, the leading generic behavior is free-streaming in transverse directions, and the sub-leading behavior is that of a color glass condensate. We further extrapolate to the full future wedge and show that the Gubser flow cannot be generated from asymptotic states. The Gubser flow can be rather smoothly glued to the vacuum outside the future wedge as the energy density vanishes faster than any power law both at the early proper time and at a large distance from the central axis. We study the holographic Gubser flow numerically and confirm these analytic results. We also find that at intermediate times, the Gubser flow exhibits a fixed point attractor, which becomes hydrodynamic only for large initial energy densities. We argue that our results show that the Gubser flow is better applied to collective behavior in jets rather than the full medium in the phenomenology of heavy ion collisions and that the Gubser flow in QCD can indeed reveal clues to the mechanism of confinement.

To appear soon with T. Mitra, A. Mukhopadhyay, and A. Soloviev

String axions have been proposed as candidates for solving a number of puzzles in cosmology. In this talk, I will focus on axions as dark matter and dark radiation. After a review on how string axions can occur in our universe, I will provide a string theoretical explanation of dark matter as composed by axions coming from type IIB string theory on Calabi-Yau orientifolds. Based on the latest bounds, I will show how likely it is for dark matter to be composed of such particles and in which abundance, and I will provide predictions on the preferred ranges of masses and decay constants. On the contrary, requiring the axions to lie in a particular range of the parameter space imposes constraints on the UV theory. Finally, I will present some work in progress on axions produced during inflation and discuss the implications for stringy inflationary models.

We study entanglement dynamics in toy models of black hole information built out of chaotic many-body quantum systems, by utilising a coarse-grained description of entanglement dynamics in such systems known as the `entanglement membrane'. We show that in these models the Page curve associated to the entropy of Hawking radiation arises from a transition in the entanglement membrane around the Page time, in an analogous manner to the change in quantum extremal surfaces that leads to the Page curve in semi-classical gravity. We also use the entanglement membrane prescription to study the Hayden-Preskill protocol, and demonstrate how information initially encoded in the black hole is rapidly transferred to the radiation around the Page time. Our results relate recent developments in black hole information to generic features of entanglement dynamics in chaotic many-body quantum systems. 

Black holes are fascinating astrophysical objects that are indirectly observed. From a theoretical point of view, black holes provide a fruitful laboratory to study the nature of gravity and its interplay with quantum mechanics. I will discuss recent developments surrounding the black hole information paradox and explain how these developments relate to understanding de Sitter spacetime.

 In this talk I will revisit the calculation of entanglement entropy in free Maxwell theory in 4-dimensional Minkowski spacetime. Weyl invariance allows for this theory to be embedded inside the Einstein static universe. Future null infinity can be regarded as the union of Cauchy slices inside the future Milne patches (denoted L and R) of two Minkowski geometries related by a conformal inversion. I will show that the Maxwell vacuum state decomposes as the thermofield double state of conformal primary modes supported inside the L and R patches and related by a conformal inversion. I will comment on the relation between inversions and shadow transforms. I will conclude by discussing the soft sectors associated with the two patches. In particular, I will demonstrate that conformally soft mode configurations at the entangling surface, or equivalently correlated fluctuations in the large gauge charges of the two Milne patches, give a non-trivial contribution to the entanglement entropy across a cut of future null infinity. 

We will describe thermalization in local and subsystem observables in theories near criticality. We will use the quench set-up to drive the system out of equilibrium. Specifically, in 2D critical theories we shall look at the interplay of thermalization with conformal symmetry. We will point out various universal signatures in various observables. After discussing sudden quenches, we shall look at critical smooth quenches which are also associated with universal scalings. These however are most well understood in free theories. We will go beyond free theories, and discuss smooth quenches in the context of a large N interacting quantum field theory of fermions. In this example, we will discover a universal scaling associated with restoration of a broken symmetry. We will end with some remarks on the approach towards thermalization. 

We holographically study the strongly coupled dynamics of the field theory on I-branes (D5 branes intersecting on a line). In this regime, the field theory becomes (2+ 1) dimensional with 16 supercharges. The dual background has an IR singularity. We resolve this singularity by compactifying the theory on a circle, preserving 4 supercharges. We study various aspects: confinement, symmetry breaking and Entanglement Entropy.

The heterotic G2 system describes compactifications of
heterotic supergravity on a 7-dimensional manifold preserving N=1
supersymmetry. Solutions to the system involve interesting constructions
in geometry such as conformally co-closed G2-structures and
G2-instantons, but they are challenging to obtain due to an anomaly
cancellation condition for the gauge fields. In this talk I will
introduce the heterotic G2 system and present new solutions on a class
of manifolds known as 3-(α, δ)-Sasakian manifolds. This talk is based on
joint work with Leander Stecker (soon to appear on arXiv) as well as
joint work with Xenia de la Ossa (arXiv:2111.13221).

I will characterize the late-time expansion rate of the universe in multi-scalar cosmologies with multi-exponential potentials, exploiting universal asymptotic features of the solutions to the cosmological equations. This provides a simple diagnostic of whether any given multi-exponential potential holds the necessary conditions for late-time cosmic acceleration. In coupling space, such a diagnostic can be formulated simply in terms of the distance of the origin from the convex hull of the exponential couplings. I will also discuss the conditions under which scaling solutions are inevitable late-time cosmological attractors for this class of theories. If all field-space trajectories are known analytically, one can characterize exactly any observable of interest. Multi-exponential potentials have been studied extensively as phenomenological models of quintessence and, moreover, they are ubiquitous in string-theoretic constructions. I will therefore sharpen several statements on the low-energy signatures of quantum gravity in this context.

Tensionless string theory is a candidate for the ultra high energy limit of string theory. In this talk I discuss some of the scenarios when a string with tension becomes tensionless based on our earlier work. Thereafter I revisit the formulation of tensionless closed bosonic string theory following earlier works where it will be revealed that there can be three different quantum theories for tensionless strings. Afterwards I discuss our latest work where we have studied all the three quantum theories in a compactified target spacetime to study the impact of compactification on all these theories.

The modern nonperturbative treatment of quantum mechanics is based on the formulation of exact quantization conditions – first derived by Zinn-Justin in the 80’s – which allow us to recursively compute instanton corrections to the energy spectra of QM oscillators. Using the theory of resurgence, it has been established that the perturbative energy expansion and its instanton corrections are deeply connected and fit in a more generic structure called a transseries. In this talk I will demonstrate how one obtains a transseries solution to the energy which contains all instanton corrections in closed form. We then derive a complete description of the resurgent structure of this solution, telling us precisely which instanton corrections know about each other and which do not. We apply our approach to quantum oscillators with cubic, double well and cosine potentials, and comment on what our findings might imply for nonperturbative structures in QFT or string theory.

I explain recent progress in the explicit computation of one-loop string amplitudes. They contain a lot of physical information, but it is currently quite hard to evaluate the required integrals over moduli space. I explain how they can be computed with the help of techniques from analytic number theory and will discuss physical ramifications. Based on work with Sebastian Mizera.

Tidal deformation of compact objects are a powerful tool to infer their internal structure. They are encoded in the so called Love numbers which reflect the deformability of a given compact object. In the case of four dimensional black holes solutions of General Relativity (GR), the Love numbers vanish. This property appears as a very special feature of 4d GR black holes. Indeed, it is no longer true in higher dimensions nor in modified gravity theories. This has motivated a search for hidden symmetries which could explain this property, i.e. the Love symmetry. 

In this talk, I will focus on one recent proposal called the HJPSS symmetry, introduced in https://arxiv.org/abs/2105.01069 (and further discussed in https://arxiv.org/pdf/2212.09367.pdf). The origin of this symmetry has remained unclear so far. In this talk, I will show that any static linear perturbations around the Schwarzschild (and Kerr) black hole enjoy a symmetry under the Schrodinger group which explains the origin of the HJPSS structure. Then, I will show how this symmetry of static perturbations indeed forces the Love numbers to vanish for 4d Schwarzschild (and Kerr) black holes. 

I will then connect this structure identified for static perturbations to less known symmetries of black holes mechanics which appears in symmetry reduced GR. 

I will discuss the realization of these symmetries of black hole mechanics and comment on their relevance for the quantum mechanics of black holes. 

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This talk is based on the results presented in the following articles:

1) Symmetry of static black hole perturbations : https://arxiv.org/pdf/2202.12828.pdf

2) Symmetry and conformal bridge in Schwarzschild-(A)dS black hole mechanics: https://arxiv.org/pdf/2110.01455.pdf

3) Schrodinger-like symmetry of in black hole mechanics: https://arxiv.org/pdf/2302.07644.pdf

The correspondence principle between strings and black holes is a general framework for matching black holes and massive states of fundamental strings at a point where their physical properties (such as mass, entropy and temperature) smoothly agree with each other. This correspondence becomes puzzling when attempting to include rotation: At large enough spins, there exist degenerate string states that seemingly cannot be matched to any black hole. Conversely, there exist black holes with arbitrarily large spins that cannot correspond to any single-string state. In this talk I will discuss the properties of both types of objects and show that a correspondence that resolves the puzzles is possible by adding dynamical features and non-stationary configurations to the picture. Along the way, I will elaborate on general aspects of the correspondence that have not been emphasized before.