University of Crete HEP Seminars
Recordings can be found in this link
Hydrodynamic diffusion and its breakdown near AdS2 fixed points
| Speaker: | Blaise Goutéraux |
| Institution: | École Polytechnique |
| Time: | Friday 08 January 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Hydrodynamics provides a universal description of interacting quantum field theories at sufficiently long times and wavelengths, but breaks down at scales dependent on microscopic details of the theory. We use gauge-gravity duality to investigate the breakdown of diffusive hydrodynamics in two low temperature states dual to black holes with AdS2 horizons. We find that the breakdown is characterized by a collision between the diffusive pole of the retarded Green's function with a pole associated to the AdS2 region of the geometry, such that the local equilibration time is set by infra-red properties of the theory. The absolute values of the frequency and wavevector at the collision (ωeq and keq) provide a natural characterization of all the low temperature diffusivities D of the states via D=ωeq/keq^2 where ωeq=2πΔT is set by the temperature T and the scaling dimension Δ of an infra-red operator. We confirm that these relations are also satisfied in an SYK chain model in the limit of strong interactions. |
| Recording: | The recorded talk can be found here. |
Slow scrambling in extremal BTZ and microstate geometries
| Speaker: | Ben Craps |
| Institution: | VUB, Bruxelles |
| Time: | Tuesday 12 January 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Out-of-time-order correlators (OTOCs) that capture maximally chaotic properties of a black hole are determined by scattering processes near the horizon. This prompts the question to what extent OTOCs display chaotic behaviour in horizonless microstate geometries. This question is complicated by the fact that Lyapunov growth of OTOCs requires nonzero temperature, whereas constructions of microstate geometries have been mostly restricted to extremal black holes. We compute OTOCs for a class of extremal black holes, namely maximally rotating BTZ black holes, and show that on average they display "slow scrambling", characterized by cubic (rather than exponential) growth. Superposed on this average power-law growth is a sawtooth pattern, whose steep parts correspond to brief periods of Lyapunov growth associated to the nonzero temperature of the right-moving degrees of freedom in a dual conformal field theory. Next we study the extent to which these OTOCs are modified in certain "superstrata", horizonless microstate geometries corresponding to these black holes. Rather than an infinite throat ending on a horizon, these geometries have a very deep but finite throat ending in a cap. We find that the superstrata display the same slow scrambling as maximally rotating BTZ black holes, except that for large enough time intervals the growth of the OTOC is cut off by effects related to the cap region, some of which we evaluate explicitly. |
| Recording: | The recorded talk can be found here. |
Entanglement entropy in expanding spacetimes
| Speaker: | Wilke van der Schee |
| Institution: | CERN |
| Time: | Tuesday 19 January 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | This talk will be about entanglement entropy in empty 4-dimensional de Sitter spacetime of a non-conformal QFT [2011.08194]. I will first briefly describe the set-up and show how a hydrodynamic plasma dilutes and falls out of equilibrium due to expansion towards empty de Sitter spacetime. We track the evolutions of the event and apparent horizons, showing in particular that the temperature of the event horizon approaches the de Sitter temperature. Interestingly, in the empty setting we can show that extremal surfaces in the holographic dual of spherical entangling regions on the boundary QFT probe beyond the dual event horizon if and only if the entangling region is larger than the cosmological horizon. |
| Recording: | The recorded talk can be found here. |
Bootstrapping Quantum Field Theories
| Speaker: | João Penedones |
| Institution: | EPFL, Lausanne |
| Time: | Tuesday 26 January 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | I will review the recent revival of the S-matrix Bootstrap program. In particular, I will discuss the formulation that includes scattering amplitudes, form factors and spectral densities. |
| Recording: | The recorded talk can be found here. |
The swampland at large number of space-time dimensions
| Speaker: | Dieter Lüst |
| Institution: | Max-Planck-Institute, Munich |
| Time: | Tuesday 02 February 2021, 13:15 |
| Venue: | Webinar |
| Abstract: | We discuss some aspects of swampland constraints - especially the swampland distance conjecture - in a large number of space-time dimensions D. We analyze Kaluza-Klein (KK) states at large D and find that some KK spectra possess an interesting dependence on D. On the basis of these observations we propose a new large dimension conjecture. We apply it to KK states of compactifications to anti-de Sitter backgrounds where it predicts an upper bound on the dimension of space-time as a function of its characteristic radius. We also apply our conjecture to black hole spacetimes, whose entropies have a D-dependence very similar to that of the KK spectrum. |
| Recording: | The recorded talk can be found here. |
S-matrix constraints on de Sitter and on black holes
| Speaker: | Georgi Dvali |
| Institution: | Max-Planck-Institute, Munich |
| Time: | Tuesday 09 February 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We discuss how the S-matrix formulation of quantum gravity/string theory excludes any sort of stable or a metastable de Sitter. The constraint is enforced by the phenomenon of quantum break-time. This has a number of observable consequences both for early and late cosmology. In particular, it excludes cosmological constant as part of the energy budget in the Universe. We confront the quantum portraits of de Sitter and black holes and point out similarities as well as fundamental differences. |
| Recording: | The recorded talk can be found here. |
Consistency of supersymmetric 't Hooft anomalies
| Speaker: | Adam Bzowski |
| Institution: | University of Crete |
| Time: | Tuesday 16 February 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We consider recent claims that supersymmetry is anomalous in the presence of a R-symmetry anomaly. We revisit arguments that such an anomaly in supersymmetry can be removed and write down an explicit counterterm that accomplishes it. Removal of the supersymmetry anomaly requires enlarging the corresponding current multiplet. As a consequence the Ward identities for other symmetries that are already anomalous acquire extra terms. This procedure can only be impeded when the choice of current multiplet is forced. We show how Wess-Zumino consistency conditions are modified when the anomaly is removed. Finally we check that the modified Wess-Zumino consistency conditions are satisfied, and supersymmetry unbroken, in an explicit one loop computation using Pauli-Villars regulators. To this end we comment on how to use Pauli-Villars to regulate correlators of components of (super)current multiplets in a manifestly supersymmetric way. |
Einstein-Cartan gravity: Inflation, Dark Matter and Electroweak Symmetry Breaking
| Speaker: | Mikhail Shaposhnikov |
| Institution: | EPFL, Lausanne |
| Time: | Tuesday 23 February 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it incorporates Higgs inflation at energies below the onset of the strong-coupling of the theory. Second, it contains a four-fermion interaction that originates from torsion associated with spin degrees of freedom. This interaction leads to a novel universal mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. Finally, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect. |
| Recording: | The recorded talk can be found here. |
Two-Point Correlator of Chiral Primaries with a Wilson Line Defect in N=4 SYM
| Speaker: | Pedro Liendo |
| Institution: | DESY, Hamburg |
| Time: | Tuesday 02 March 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We consider the two-point function of the stress-tensor multiplet in N=4 SYM and add a Maldacena-Wilson line to the configuration which makes the correlator non-trivial. We use a combination of perturbation theory and defect CFT techniques to obtain results up to next-to-leading order in the coupling constant. Being a defect CFT correlator, there exist two (super)conformal block expansions which capture defect and bulk data respectively. We present a closed-form formula for the defect CFT data, which allows to write an efficient Taylor series for the correlator in the limit when one of the operators is close to the line. The bulk channel is technically harder, nevertheless we use our analysis to check against well-known data of N=4 SYM. In particular, we recover the correct anomalous dimensions of a famous tower of twist-two operators (which includes the Konishi multiplet), and successfully compare the one-point function of the stress-tensor multiplet with results obtained using localization. Time permitting, we'll discuss work in progress on the strong coupling limit of the same correlator. |
| Recording: | The recorded talk can be found here. |
Tensor Models and Limit Cycles
| Speaker: | Christian Jepsen |
| Institution: | Stony Brook, New York |
| Time: | Tuesday 09 March 2021, 17:00 |
| Venue: | Webinar |
| Abstract: | Matrix and tensor models with U(N) or O(N) symmetry offer a surprise on analytic continuation in N: the existence of fixed points of the beta functions where the the coupling constants gi are real but the scaling dimensions, as determined by the eigenvalues of the Jacobian matrix ∂βi/∂gj, are complex. When these complex eigenvalues cross the imaginary axis, a Hopf bifurcation takes place and gives rise to a limit cycle in the RG flow. In this talk, I will present an overview of this phenomenon and present a number of examples. |
| Recording: | The recorded talk can be found here. |
A CFT Distance Conjecture
| Speaker: | Eric Perlmutter |
| Institution: | Caltech |
| Time: | Tuesday 16 March 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We formulate conjectures relating the geometry of conformal manifolds to the spectrum of local operators in conformal field theories in d>2 spacetime dimensions. We focus on conformal manifolds with limiting points at infinite distance with respect to the Zamolodchikov metric. Our central conjecture is that all theories at infinite distance possess an emergent higher-spin symmetry, generated by an infinite tower of currents whose anomalous dimensions vanish exponentially in the distance. Our conjectures are related to the Distance Conjecture in the swampland program. We discuss the supporting evidence, their holographic interpretation, and implications for superconformal field theories. |
| Recording: | The recorded talk can be found here. |
Chern-Weil global symmetries and completeness of the spectrum
| Speaker: | Miguel Montero |
| Institution: | Harvard |
| Time: | Tuesday 23 March 2021, 17:00 |
| Venue: | Webinar |
| Abstract: | Perhaps the oldest Swampland conjecture in the book is the absence of global symmetries. I will describe the consequences of applying this conjecture to a particular class of symmetries, Chern-Weil symmetries, that are ubiquitous in string compactifications and which arise from powers of fieldstrenghts. I will explore the processes by which Chern-Weil symmetries may be broken or gauged in effective field theory and string theory. This will allow us to derive the existence of familiar features of string theory, such as worldvolume degrees of freedom in branes. Another lore holds that absence global symmetries in a quantum field theory is equivalent to having a complete spectrum of charged states in the theory. I will discuss and make precise this connection, and explain and how a particular version of it can be proven in pure gauge theory with arbitrary (even disconnected) gauge group. |
| Recording: | The recorded talk can be found here. |
Supergravity Amplitudes on AdS5×S5 and their flat-space limit
| Speaker: | Alessandro Georgoudis |
| Institution: | IPhT Saclay |
| Time: | Tuesday 30 March 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We will study the four point function of the superconformal primary of the stress-tensor multiplet in four dimensional N=4 Super Yang Mills, at large 't Hooft coupling and in a large N expansion. This observable is holographically dual to four graviton amplitudes in type IIB supergravity on AdS5×S5. We first will construct the most trascendental piece of the correlator at any order in a large N expansion. They are determined only by leading order CFT data. By focusing on a specific limit, we make connection with the dual supergravity amplitude in flat space, where such structures correspond to iterated s-cuts. We make several checks and we conjecture that the same interpretation holds for supergravity amplitudes on AdS5×S5. |
| Recording: | The recorded talk can be found here. |
Symmetries and Strings of adjoint QCD in two dimensions
| Speaker: | Konstantinos Roumpedakis |
| Institution: | UCLA |
| Time: | Tuesday 06 April 2021, 18:00 |
| Venue: | Webinar |
| Abstract: | In this talk, we will review the notion of non-invertible symmetries and we will study adjoint QCD in two dimensions. It turns out that this theory has a plethora of such symmetries which require deconfinement in the massless case. When a mass or certain quartic interactions are tunrned on, these symmetries are broken and the theory confines. In addition, we will use these symmetries to calculate the string tension for small mass and make some comments about naturalness along the RG flow. |
| Recording: | The recorded talk can be found here. |
Spontaneous Conformal Symmetry Breaking in the Fishnet CFT
| Speaker: | Georgios Karananas |
| Institution: | Munich |
| Time: | Tuesday 13 April 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Quantum field theories with exact but spontaneously broken conformal invariance have an intriguing feature: their vacuum energy is equal to zero. Up to now, the only known ultraviolet complete theories where conformal symmetry can be spontaneously broken were associated with supersymmetry, with the most prominent example being the N =4 SYM. Interestingly, its non-supersymmetric descendant, the "Fishnet theory," supports at the classical level a rich set of flat directions along which conformal symmetry is spontaneously broken. At least perturbatively, some of these vacua survive in the full quantum theory - in the planar limit - without any finetuning. As a result, the vacuum energy vanishes along these flat directions, providing the first non-supersymmetric example of a four-dimensional quantum field theory with "natural" breaking of conformal symmetry. |
| Recording: | The recorded talk can be found here. |
Bounds on Regge growth of flat space scattering from bounds on chaos
| Speaker: | Shiraz Minwalla |
| Institution: | Mumbai |
| Time: | Tuesday 20 April 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We study four-point functions of scalars, conserved currents, and stress tensors in a conformal field theory, generated by a local contact term in the bulk dual description, in two different causal configurations. The first of these is the standard Regge configuration in which the chaos bound applies. The second is the `causally scattering configuration' in which the correlator develops a bulk point singularity. We find an expression for the coefficient of the bulk point singularity in terms of the bulk S matrix of the bulk dual metric, gauge fields and scalars, and use it to determine the Regge scaling of the correlator on the causally scattering sheet in terms of the Regge growth of this S matrix. We then demonstrate that the scaling on this sheet is governed by the same power as in the standard Regge configuration, and so is constrained by the chaos bound, which turns out to be violated unless the bulk flat space S matrix grows no faster than s^2 in the Regge limit. It follows that in the context of the AdS/CFT correspondence and local bulk interactions that the chaos bound applied to the boundary field theory implies that the S matrices of the dual bulk scalars, gauge fields, and gravitons obey the Classical Regge Growth (CRG) conjecture. |
| Recording: | The recorded talk can be found here. |
Decoding and bootstrapping cosmological fluctuations
| Speaker: | Gui Pimentel |
| Institution: | Amsterdam |
| Time: | Tuesday 27 April 2021, 16:00 |
| Venue: | Webinar |
| Abstract: | I will review our current understanding of the initial conditions of the universe, and describe what information is available from current and future measurements of cosmological correlation functions. Then I will describe a new method to compute and constrain the possible shapes of those correlation functions, assuming they were generated during inflation. This ``cosmological bootstrap" draws inspiration from the modern scattering amplitudes program in flat space, as well as the conformal bootstrap of phase transitions. After discussing primordial scalar fluctuations, I will also explain how the consistent propagation of gravitational waves imposes further constraints on the structure of spinning primordial correlators. |
| Recording: | The recorded talk can be found here. |
Cluster Algebras for Feynman Integrals
| Speaker: | Georgios Papathanasiou |
| Institution: | DESY |
| Time: | Tuesday 04 May 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | A wealth of physical information may be inferred from the singularities of scattering amplitudes. For the simplest interacting gauge theory, these singularities have been found to be encoded in beautiful mathematical objects known as cluster algebras. In this talk, I present evidence that cluster algebras may underlie the analytic structure of general quantum field theories. In particular, I show that they describe the singularities of a considerable number of Feynman integrals in dimensional regularization, most notably those governing Higgs plus jet amplitudes in QCD. This opens for the first time the exciting prospect of applications of cluster algebras to future collider physics calculations, for example via novel bootstrap methods that evade the formidable task of direct integration. |
| Recording: | The recorded talk can be found here. |
The quantum mechanics of a perfect fluid
| Speaker: | Riccardo Rattazzi |
| Institution: | EPFL, Lausanne |
| Time: | Tuesday 11 May 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Finite density systems can be described by effective field theories with non-linearly realized space-time symmetries, whose construction resembles that of the QCD chiral lagrangian. Also based on that similarity, one would expect the construction to work equally well classically and quantum mechanically. While that is true for superfluids and solids, one instead finds that for genuine fluids things are made more complicated by the unusual dynamics of their transverse modes, which are not described by a Fock space. Focussing on the incompressible limit in 2+1 dimensions, I illustrate how a consistent quantum mechanical description of a perfect fluid can be obtained by using the known equivalence between the area preserving diffeomorfism group in 2D and SU(N) with N going to infinity. |
| Recording: | The recorded talk can be found here. |
Probing gravitational EFTs with the four-graviton amplitude
| Speaker: | Alexander Zhiboedov |
| Institution: | CERN |
| Time: | Tuesday 18 May 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We discuss constraints from perturbative unitarity and crossing on the leading contributions of the higher-dimension operators to the four-graviton amplitude in four spacetime dimensions. We focus on the leading order effect due to exchange by massive degrees of freedom which makes the amplitudes of interest IR finite. To test the constraints we obtain nontrivial effective field theory data by computing and taking the large mass expansion of the one-loop minimally-coupled four-graviton amplitude with massive particles up to spin 2 circulating in the loop. Remarkably, the leading EFT corrections to Einstein gravity of physical theories, both string theory and QFT coupled to gravity, end up in minuscule islands which are much smaller than what is suggested by the generic bounds obtained from consistency of the 2-2 graviton scattering amplitude. We discuss the underlying mechanism for this phenomenon. |
| Recording: | The recorded talk can be found here. |
Holographic Path Integral Optimization
| Speaker: | Paweł Caputa |
| Institution: | Warsaw |
| Time: | Tuesday 25 May 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | I will talk about recent work on understanding path integral optimization in CFTs holographically using Hartle-Hawking wave functions in AdS. The talk will be based on arXiv: 2011.08188 [hep-th] and 2104.00010 [hep-th]. |
| Recording: | The recorded talk can be found here. |
Bra-ket wormholes in gravitationally prepared states
| Speaker: | Victor Gorbenko |
| Institution: | Stanford |
| Time: | Tuesday 01 June 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We will discuss two-dimensional toy-models of inflation in which the state of the universe is prepared by a gravitational path integral. As a first model, we consider a state produced by Euclidean AdS_2 evolution followed by flat space evolution. We use the fine grained entropy formula to explore the nature of the state. We find that the naive hyperbolic space geometry (analog of the Hartle-Hawking state) leads to a paradox. This is solved if we include a geometry that connects the bra with the ket, a bra-ket wormhole. The semiclassical Lorentzian interpretation leads to CFT state entangled with an expanding and collapsing Friedmann cosmology. I will then describe a similar model where the state is prepared by the dS_2 evolution and highlight related puzzles. |
| Recording: | The recorded talk can be found here. |
A Timeless History of Time
| Speaker: | Enrico Pajer |
| Institution: | Cambridge |
| Time: | Tuesday 08 June 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Cosmological observations give us the unique opportunity to probe the fundamental laws of physics at very high energies as well as the perturbative regime of quantum gravity. Unfortunately, due to the creativity of theorists and the paucity of data about the primordial universe, there is a huge number of models compatible with all measurements, featuring a wide variety of mechanisms, symmetries, and spectra of particles.The reason can be traced back to the fact that we don't observe the time evolution during inflation, but only its final outcome. In this talk I will report on the recent progress in developing a completely new "bootstrap" approach to derive predictions from the very early universe that make no reference to time and the un-observable time evolution. The bootstrap approach builds directly upon the fundamental pillars of physics. In particular, I will present the recent breakthroughs in understanding the consequence of unitarity for cosmological correlators to all orders in perturbation theory, as well as the footprint of (bulk) locality. I will show how these principles can be used to derive many classical and new inflationary predictions associated with primordial non-Gaussianity in a way that is both computationally simpler and conceptually more transparent. This includes a reconstruction formula that relates de Sitter correlators to amplitudes for massless particles, cosmological partial-energy recursion relations and a "timeless" differential representation of the perturbative wavefunction. This approach makes no reference to de Sitter boosts, which are broken by a large amount in models that predict large non-Gaussianity. |
| Recording: | The recorded talk can be found here. |
Holographic cosmology and the origin of time
| Speaker: | Kostas Skenderis |
| Institution: | Southampton |
| Time: | Tuesday 15 June 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | Holographic cosmology is a new framework for the very early universe, the period usually associated with inflation. In holographic cosmology the early Universe is described by a three dimensional QFT, and cosmological evolution is mapped to inverse RG flow. In this talk, I will provide an overview of holographic cosmology and then discuss recent work where lattice QFT methods are used to compute the relevant QFT observables. In particular, I will discuss the IR structure of the dual QFT, which is linked to the initial singularity in cosmology, and show how non-perturbative dynamics resolve the IR singularities present in perturbation theory, thus providing a holographic resolution of the Big Bang singularity. |
| Recording: | The recorded talk can be found here. |
Volume preserving diffeomorphism as a higher-rank gauge symmetry
| Speaker: | Dam Thanh Son |
| Institution: | Chicago |
| Time: | Tuesday 22 June 2021, 17:00 |
| Venue: | Webinar |
| Abstract: | Recently, theories with higher rank gauge symmetries are drawing attention in condensed matter physics. Such theories are thought to be related to the so-called "fractons," quasiparticles with limited mobility that appear in some lattice models. We show that one version of such higher ranked symmetry can be "nonlinearlized" into volume-preserving diffeomorphism. We demonstrate that several condensed matter systems, including the fractional quantum Hall effect and ferromagnetism, secretly possess this symmetry, which sheds another light to some peculiar features of the quasiparticles in these systems. |
| Recording: | The recorded talk can be found here. |
Lorentz invariance in Framids as a potential technical analog of the CCP
| Speaker: | Ioanna Kourkoulou |
| Institution: | Columbia U. |
| Time: | Tuesday 29 June 2021, 17:00 |
| Venue: | Webinar |
| Abstract: | The Framid is a hypothetical system whose ground state spontaneously breaks boost symmetry, however its stress-energy tensor retains a Lorentz-invariant expectation value on the ground state, as we verify explicitly to one-loop order. The mechanism enforcing this result is rather obscure; in fact, we think this puzzle is an interesting technical analog of the cosmological constant problem. I will describe our computations that verify the Lorentz invariance of the expectation value in question, and how these yield the expected result only thanks to highly nontrivial cancellations, which are quite mysterious from the low-energy effective theory viewpoint. |
| Recording: | The recorded talk can be found here. |
On nonsupersymmetric string models and the decompactification problem
| Speaker: | John Rizos |
| Institution: | Ioannina |
| Time: | Tuesday 6 July 2021, 13:00 |
| Venue: | Webinar |
| Abstract: | We study 4d heterotic string compactifications exhibiting spontaneous supersymmetry breaking via the Scherk–Schwarz mechanism associated with two large internal dimensions. We identify a class of models with fermion–boson degeneracy that address some central issues in nonsupersymmetric string phenomenology including the elimination of tachyons and dilaton tadpoles and the suppression of the cosmological constant. In addition, we propose a novel solution to the decompactification problem related to the cancellation of the linear growth of the gauge part of the thresholds with a similar growth of the universal part. |
| Recording: | The recorded talk can be found here. |
A geometric perspective on the holographic Berry phase
| Speaker: | Dimitrios Patramanis |
| Institution: | Warsaw |
| Time: | Tuesday 27 July 2021, 13:15 |
| Venue: | Second floor seminar room |
| Abstract: | After briefly reviewing the notions of Berry's phase and coadjoint orbits I will be explaining how these arise in a holographic setup. Namely, I will be talking about the modular Berry phase arising in AdS3/CFT2 and the prospects of its generalization in the broader context of the Banados geometries. To this end I will be discussing the different tools and methods required to do so and attempt to highlight their utility in addressing both questions in holography and problems of a more general nature. |