Seminars

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Abstract: The AdS/CFT correspondence makes it possible in principle to study explicit black hole microstates using field theory techniques. In practice, this is still difficult because the field theory is only dual to gravity when it is strongly coupled. A more modest goal is to focus on supersymmetric black holes which have the minimal energy allowed by their other charges. The superconformal index suggests that many of these are protected from quantum corrections and can therefore be constructed at weak coupling. I will explain how one can identify candidates for black holes by searching the Hilbert space of ABJM theory. On general grounds, these should depend sensitively on trace relations at finite N. This principle, known as fortuity, was first established in a similar study of maximally supersymmetric Yang-Mills theory. I will show that similarities between the two theories extend to the quantitative level thereby allowing additional progress to be made without a brute force search.

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Abstract: Usually in statistical physics, high temperature phases are disordered and featureless while low temperature phases may be ordered.  For example, solid ice becomes liquid water upon heating; the solid is an ordered phase that spontaneously breaks translational and rotational symmetries, while the liquid is a disordered phase which restores these symmetries in a typical state.  In fact, mathematicians have proved that many classes of common models in statistical physics, such as interacting spins in lattice models, always enter a disordered phase at  sufficiently high temperature.  On the other hand,  counterexamples to the intuition that heating destroys order have also been known for a long time:  for example, helium-3 under very high pressure can go from a liquid to a solid after heating it at very low temperature.   As I will explain, any such transition from disorder to order upon heating is necessarily characterized by “entropic order”, where ordering one degree of freedom enables many more fluctuations in another, causing entropy (not energy) to stabilize an ordered phase.   I will then show how to build an entropically-ordered phase of matter at arbitrarily high temperature, with a few simple and illustrative examples of lattice gases that heat into solids (and remain solids forever upon further heating!).  I will also explain how entropic order underlies the recent demonstration that 2+1d conformal field theories that spontaneously break Z2 symmetry at finite temperature, exist, and how to use these field theoretic ideas to build a toy model of high-temperature superconductivity, stabilized by entropic order.

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Abstract: In this talk, I consider one-component scalar field theory with quartic self-interaction in two regimes: a broken phase with non-vanishing VEV <phi>, and a symmetric phase where <phi>=0. Using a saddle-point expansion for both regimes, I find two descriptions for the same system. In lower dimensions, this self-duality between scalar field theory in the broken phase, and scalar field theory in the symmetric phase is well known, for instance as Chang duality in d=2. In this talk, I offer my interpretation of these self-dualities and discuss the particularly interesting case of d=4.

Abstract: The no-boundary state in de Sitter offers a compelling framework for specifying the initial conditions of our universe. Many (but famously not all) features of the state agree beautifully with empirical observations and have served as a foundation for initial conditions in inflationary cosmology. We explore its relation to entropy puzzles in de Sitter spacetimes. We then compute the norm of the no-boundary state and show that the contributions from geometries with spherical spatial slices, including the Hartle-Hawking geometry and fluctuations around it, vanish to all loops in perturbation theory. We then discuss the resulting implications for theoretical cosmology.

Abstract: I will discuss the recently introduced holographic product formula in the context of near-extremal hydrodynamics, which corresponds to the regime of energy ω, momentum k and temperature T much smaller than a hard scale μ. I will explain how this formula determines the general form taken by holographic spectral functions in this regime. This form simplifies in the extremal limit T ≪ ω, k ≪ μ, for which the low-temperature gapless modes and the IR conformal behavior factorize. I will then present some examples with different types of gapless modes and IR CFTs, including new numerical results for low temperature quasi-normal modes.

Abstract: We present a concrete example showing how quantum decoherence in open quantum systems gives rise to hydrodynamic behavior at late times and long wavelengths. Focusing on a single non-relativistic quantum particle linearly coupled to a thermal bath of harmonic oscillators a la Caldeira and Leggett, we demonstrate that position-space decoherence renders the reduced density matrix nearly diagonal. Expanding around this diagonal limit, we derive a moment hierarchy akin to the BBGKY hierarchy which, when truncated at second order, yields transient viscous hydrodynamic equations similar to those used in heavy-ion collisions, with shear and bulk viscosities determined by decoherence and the coupling to the environment. In the asymptotic limit, these equations reduce to the Navier-Stokes equations, show here to emerge from decoherence. This suggests that even single-particle dynamics (coupled to a bath) can exhibit collective behavior, offering insight into the success of hydrodynamics in exotic systems, such as high-energy proton-proton collisions.

Abstract: In recent years, ideas from quantum field theory, including effective field theory and the on-shell methods of scattering amplitude, have been successfully applied to classical gravitational-wave physics. In this talk, I will show that these ideas extend to the semiclassical case of Hawking radiation. Along the way, I will discuss the relationship between Bogoliubov coefficients and generalised amplitudes.

Abstract: What happens at spacetime singularities is poorly understood. The Penrose-Wall singularity theorem constrains possible scenarios, but until recently its key assumption--the generalized second law (GSL)--had only been proven perturbatively, severely limiting this application. We highlight that recent progress enables a proof of the GSL in holographic brane-world models, valid non-perturbatively at the species scale cG (with c the number of matter fields and G Newton's constant). This enables genuine constraints: an outer-trapped surface in the Einstein gravity regime implies geodesic incompleteness non-perturbatively at the species scale. Conversely, any genuine resolution must evade Penrose's criteria. We illustrate both possibilities with explicit examples: the classical BTZ black hole evolves to a more severe singularity, while a null singularity on the Rindler horizon is resolved, both by species-scale effects. Subject to the GSL, these constraints on singularity resolution apply beyond brane-worlds: namely, in any theory with a geometric UV scale--roughly, where the metric remains well-defined but classical Einstein gravity breaks down.

Abstract: In a spacetime with asymptotically anti-de-Sitter boundaries, localized bulk events produce characteristic signals at boundary locations that are lightlike from the event. I will describe (thought) experiments that use boundary wavepackets to measure the bulk geometry and bulk scattering amplitudes using these signals, and discuss some new signatures of bulk causality and local dynamics on boundary correlation functions.

Abstract: I will discuss our recent proposal for the Symmetry Topological Field Theory (SymTFT) for global, continuous spacetime symmetries (https://arxiv.org/abs/2509.07965 ). For a d-dimensional theory, it is given by a (d + 1)-dimensional BF-theory for the spacetime symmetry group, and whenever d is even, it includes also Chern-Simons couplings, that encode conformal and gravitational anomalies. This proposal is supported by the similarity to two-dimensional Jackiw-Teitelboim gravity in the d = 1 case and the topological limit of four-dimensional gravity in the d = 3 case. I will discuss how topological defects constructed from this SymTFT act by applying the associated spacetime symmetry transformation. Finally, I will discuss the relation to gravity and holography.

Abstract: The linear growth of entanglement after a quench from a state with short-range correlations is a universal feature of many body dynamics. It has been shown to occur in integrable and chaotic systems undergoing either Hamiltonian, Floquet or circuit dynamics and has also been observed in experiments. The entanglement dynamics emerging from long-range correlated states is far less studied, although no less viable using modern quantum simulation experiments. In this talk, I will present the dynamics of the bipartite entanglement entropy and mutual information in quenches starting from Crosscap States, also knows as Entangled Antipodal Pair States, which are volume law states, constructed by entangling antipodal points of a finite and periodic system. I will focus on the evolution of a crosscap initial state, in  a free fermionic quench and probe the dynamics of bipartite entanglement entropy and mutual information. In particular, I will show how one can derive an effective description of the entanglement dynamics, that matches the exact results. The quench dynamics is captured by an emergent quasiparticle picture description, which differs from the one that characterizes quenches from lowly entangled states, due to the long-range correlations of the initial state. The main phenomenology for the entanglement entropy, is that after an initial time delay where entanglement remains to the initial volume law value, there is a linear in time decrease, followed by a series of oscillatory revivals which happen around a constant value.This behavior, as well as the characteristic times of the revivals and the constant time averaged value can be explained in terms of the emergent quasiparticle picture that we derive.

Abstract: The CISS - Chirality Induced Spin Selection - phenomenon in 1D conical magnets and deformation of magnetic textures due to interaction with a fermion bath.

Abstract: I will present a method to compute four-point correlation functions with multiparticle operators in holography by using microstate geometries. Focussing on the simplest correlators built out of particles from the graviton multiplet, e.g. two single and two multi-particle insertions, I will present explicit results in N=4 SYM. The key point of the method is to use precision holography to build a coherent operator with the property that: it is analytic in its parameters and it admits a heavy regime dual to a smooth background geometry in AdS. Then, a two-point Witten diagram in this background computes the generating function of the multiparticle correlators of interest. This problem can be mapped to a connection problem for a Heun equation, and the resulting correlator can be expressed as a generalized BMN integral. Finally, the resulting correlator can be tested against OPE predictions in the CFT, providing new and non-trivial checks of the AdS/CFT correspondence.

Abstract:  "Entanglement islands” play a key role in recent progress understanding entropy and localization of information in quantum black holes. It has been suggested in the literature that a “massive graviton” is necessary for the existence of islands, based on (1) massive gravity arising in constructions requiring coupling to an external non-gravitating system and (2) challenges in defining gauge-invariant operators supported in a compact region such as an island. To address this concern, I will first clarify the precise statements of the island formula and associated operator reconstruction theorem, and the meaning of a “massive graviton”. I will then (1) present examples of islands with unambiguously massless gravity (including in asymptotically flat spacetimes), and (2) discuss the construction of compactly supported operators to all orders in perturbation theory.

Abstract: We investigate the vacuum structure of non-conformal, strongly coupled matter in de Sitter space using a class of bottom-up holographic models. We find that these models admit multiple coexisting de Sitter–symmetric states at fixed Hubble expansion rate. Although gravity is treated as non-dynamical, one can identify semiclassical de Sitter solutions self-consistent with the Friedmann equations. We suggest that these de Sitter–symmetric solutions can be reached through far-from-equilibrium dynamics. We argue that these states act as natural attractors for the late-time evolution of deeply overcooled matter that would otherwise support thermal inflation. In this way, they extend the phase of exponential acceleration beyond the regime of validity of hydrodynamics.

Abstract: In this talk I will explain how certain aspects of black hole physics can be understood using ideas from effective field theory. Many of the familiar notions from particle physics will play a role, including the renormalization group, universality and naturalness. In particular, I will explain why black holes naively seem like fine-tuned systems, but they are ultimately not. I will also explain how their effective description reveals a degree of universality that can be used to resum some features of the gravitational waveform for binary mergers observed by LIGO/Virgo/Kagra.

Abstract: Charged and Rotating Black Holes in $AdS_5 \times S^5$ are the dual description of  phases of N=4 Yang Mills theory (as a function of energy, angular momentum and charge). Familiar Kerr-Resiner Nordstorm AdS black holes dominate at high energies, but turn out to be unstable in a band of energies around extremality. We construct new black hole solutions that yield new phases that are the end point of this instability. These new phases dominate at low energies, and in particular in the BPS limit. We use our construction to provide a new conjecture for the supersymmetric cohomology of N=4 Yang Mills at large $N$, and discuss the interplay of our conjecture with field theory results for the superconformal Index.

Abstract: In this talk, I will discuss some interesting features of heavy–heavy–light–light correlators in N=4 supersymmetric Yang–Mills theory, where the light operators belong to the stress-tensor multiplet and the heavy ones correspond to giant gravitons, realised holographically as D3-branes. I will focus in particular on the associated Integrated Correlators, for which exact expressions can be obtained despite few results are known for the correlators themselves. I will highlight several interesting properties of these integrated correlators, especially the emergence of universal structures in the strong-coupling regime.

Abstract: We study gravitational form factors for baryons using the holographic description of QCD based on a D4-D8 brane system in type IIA string theory.  In holographic QCD, the baryons are represented as solitons in a 5-dimensional gauge theory. We obtain the soliton solution by solving the equations of motion numerically. Using this result, the gravitational form factors and related quantities are calculated.

Abstract: We argue that collider observables such as hadron number flux can be matched onto a linear combination of detectors/light-ray operators in perturbative QCD. The spectrum of detectors in QCD is subtle, due to recombination between the DGLAP and BFKL trajectories. We explain how to define and renormalize these trajectories at one-loop, systematically incorporating their recombination. The leading and subleading soft gluon theorems play an important role, and our analysis suggests the presence of an infinite series of further subleading soft theorems for squared-amplitudes/form factors. Combined with our light-ray matching hypothesis, the anomalous dimensions of recombined DGLAP/BFKL detectors yield a prediction for the energy dependence of the number of particles in a jet, as well as other predictions for more general energy-weighted hadron measurements. We compare these predictions to Monte-Carlo simulations, finding good agreement.

Abstract: Almost 30 years ago Dima Kharzeev argued that, contrary to naive intuition, the flow of baryon number in highly inelastic high energy processes is not traced by the valence quarks but by a gluonic structure, the string junction, postulated many years earlier by Giancarlo Rossi and myself and, independently from QCD, by Xavier Artru. I will discuss recent experimental evidence in favour of this claim by using the 50+ year old Feynman-Wilson analog model adapted to the topological expansion of QCD.

Abstract: We consider planar codimension-one defects and interfaces in N = 4 super-symmetric Yang–Mills (SYM) theory, realized by the D3/D5-brane intersection. Working in the probe limit, where the number of D5-branes is small compared to the number of D3-branes, we obtain analytic results for the holographic entanglement entropy of a ball-shaped region centered on the defect. A defect renormalization group flow is triggered by giving the defect hypermultiplets a mass, which corresponds to separating the D3- and D5-branes. Along this flow the entanglement C-function decreases monotonically. We also allow the D5-branes to carry worldvolume flux corresponding to dissolved D3-branes, in which case the setup describes an interface between two copies of N = 4 SYM theory with different gauge groups, where an RG flow is triggered by a mass term for vector multiplets. Here we again find monotonic behavior of the entanglement C-function, although its interpretation as a measure of effective degrees of freedom is problematic. We investigate possible alternative measures of degrees of freedom.

Abstract: Many string compactifications have moduli fields that can take different possible values at infinity. We address the question: given two compactifications that differ by the asymptotic values of the moduli, should we regard them as different theories or different states of the same theory? We fix a precise criterion that distinguishes between these two possibilities and show that the answer depends on whether the non-compact part of the space-time is asymptotically flat or asymptotically AdS. In the former case different values of asymptotic moduli give different states of the same theory while in the latter case they correspond to different theories.

Abstract: Numerical investigation of the non-linear fate of the Aretakis instability has pointed towards the existence of a dynamical black hole solution which remains extremal and features a persistent instability and associated horizon hair forever. I will discuss how one can realize this solution analytically within JT gravity.