Past Seminars

22.11.2019 Friday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Durmus Demir, Sabancı University, Istanbul

Charge Conservation and Gravity

The Standard Model of elementary particles needs be extended for various reasons ranging from incorporation of gravity to construction of a dark sector. The quantum corrections induce hard masses for gauge bosons (including the photon and the gluon) in proportion to the scale of the extension. This nonconservation of the electric and color charges, as we will discuss in the talk, can be averted via the incorporation of gravity into the effective SM. The resulting framework is diverse for both phenomenology and model-building.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Daniele Dorigoni, Durham University, UK

An introduction to resurgence in quantum mechanics and QFTs

Resurgence theory is an extremely useful framework to analyse asymptotic series. Typically when computing physical observables in a perturbative expansion, for example ground state energies in QM or partition functions in QFT, we produce precisely this type of asymptotic formal power series. We will see how to make physical sense out of these objects and how resurgent analysis allows us to reconstruct non-perturbative physics from asymptotic (and non-) perturbative expansions.

17.05.2019 Friday:

Place: IMBM, Boğaziçi University

14:00 -- 15:00 Kadri İlker Berktav Middle East Technical University, Ankara

Towards the Stacky Formulation of Einstein Gravity

This talk, which essentially consists of three parts, serves as a conceptional introduction to the formulation of Einstein gravity in the context of derived algebraic geometry. The upshot is as follows: we shall first outline main ingredients of category theory, and using the functorial approach, we define the notion of a stack in a relatively succinct and naive way. Having adopted the language of stacks, one is able to make sense of a notion so-called a moduli problem which, roughly speaking, is about constructing a classifying space (or a moduli space) for certain geometric objects (such as manifolds, algebraic varieties, vector bundles etc...) up to their intrinsic symmetries (e.g. gauge equivalences, diffeomorphism invariance etc...). As an analyzing a classical field theory with an action functional S boils down to the study of the moduli space of solutions to the corresponding Euler-Lagrange equations, the notion of a moduli problem in fact provides an alternative and a beautiful mathematical framework for recording and organizing the moduli data in a more elegant way. In the second part of the talk, we shall revisit main aspects of 2+1 dimensional vacuum Einstein gravity on a pseudo-Riemannian manifold M especially in the context of Cartan geometry, and introduce, in the case of M = Σ × (0, ∞) and Λ = 0 where Σ is a closed Riemann surface of genus g > 1, the equivalence of the quantum gravity with a gauge theory in the sense that the moduli space E(M) of such a 2+1 dimensional Einstein gravity is isomorphic to that of flat Cartan ISO(2, 1)-connections, denoted by M_flat. In the final part, we shall briefly discuss (i) how to construct the appropriate stacks associated to E(M) and M_flat respectively, and (ii) how to extend the isomorphism that essentially captures the equivalence of the quantum gravity with a gauge theory in the above setup to an isomorphism of associated stacks.

15:00 -- 15:30 Coffee Break

15:30 -- 16:30 Burak Gürel, Boğaziçi University, İstanbul

Problems on Nonlinear Schrödinger Equations

We will start by introducing different versions of nonlinear Schrödinger equation. In that, either the form of the equation or the domain of the problem varies. Some tools such as the Fourier transform, Sobolev spaces, Bourgain spaces, which are instrumental in the mathematical analysis of the related problems, will be briefly explored. Then we will describe the question of smoothing and also present some of the relevant smoothing results.

17.04.2019 Wednesday:

Place: Physics Department Seminar Room, Boğaziçi University

14:00 -- 15:00 Ali Kaya Bogazici University

Initial wave-function of the universe is arbitrary

We consider quantization of the gravity-scalar field system in the minisuperspace approximation. It turns out that in the gauge fixed deparametrized theory where the scale factor plays the role of time, the Hamiltonian can be uniquely defined without any ordering ambiguity as the square root of a self-adjoint operator. Moreover, the Hamiltonian degenerates to zero and the Schrödinger equation becomes well behaved as the scale factor vanishes. Therefore, there is no technical or physical obstruction for the initial wave-function of the universe to be an arbitrary vector in the Hilbert space, which demonstrates the severeness of the initial condition problem in quantum cosmology.

15:00 -- 15:30 Coffee Break

15:30 -- 16:30 Daniel Grumiller TU Wien, Austria

Soft Heisenberg hair

Gravity theories naturally allow for edge states generated by non-trivial boundary-condition preserving diffeomorphisms. I present a specific set of boundary conditions inspired by near horizon physics, show that it leads to soft hair excitations of black hole solutions and discuss implications for black hole entropy.

15.03.2019 Friday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Razieh Morad Bogazici University, Istanbul

Study of Light Quark Jet in QGP via AdS/CFT correspondence

The spectacular measurements from the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) provide compelling evidence that the matter produced in heavy ion collision is a deconfined state of QCD, Quark-Gluon Plasma (QGP), at temperatures above ~160 MeV which appears to be nearly perfect, with an extremely low viscosity-to-entropy ratio η/s ~ 1/4π. Within this expanding fireball, jets are produced and probes the QGP. Analysis the energy loss of these energetic partons as they travel throw QGP may reveal extremely valuable information about the dynamics of the plasma and exhibit distinctive properties such as jet-quenching.

The “AdS/CFT correspondence” which imposes the duality between the gauge theory and gravity is a novel tool provides valuable insight into the strongly coupled plasma. The most important result of AdS/CFT is calculating the value of shear viscosity to entropy density ratio which is in remarkable agreement with the hydrodynamics predictions. We study the energy loss rate of light quarks in the hot, strongly coupled plasma via AdS/CFT correspondence. Unlike heavy quarks, light quark energy loss in AdS/CFT is surprisingly dependent on both the string initial conditions, and the very definition of the jet itself in the gravity theory. Jets in general thermalize very quickly in a strongly-coupled plasma compare to the experimental data. We aim to more closely match the gravity dual to QGP and the string initial conditions to those expected from pQCD by computing the energy-momentum tensor associated with the propagation of the classical string solution.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Balasz Hetenyi Bilkent University, Ankara

Topological phase transitions understood via the polarization amplitude

Topological systems are one of the most active research areas in condensed matter physics. The topological characterization of a condensed matter system relies on mathematical constructs such as the Berry phase, the winding number, or the Chern number. In the first part of the talk, I will explain how the Berry phase can be understood as the first cumulant of a series. The cumulants themselves are derived from the polarization amplitude, a quantity introduced decades ago in the context of the Berry phase theory of polarization, and one which has received attention recently. We calculate higher order cumulants and reconstruct the underlying distribution of the polarization for the Rice-Mele model. Our approach allows the visualization of a topological transition, how a system goes between phases with different quantization. In the second part of the talk, I will go through constructing one-dimensional analogs of the Haldane and Kane-Mele models. In the end I will discuss the application of our cumulant method to an interacting model.

13.02.2019 Wednesday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Ilya Bakhmatov Asia Pacific Center for Theoretical Physics, Postech, Pohang, South Korea

Classical Yang-Baxter equation from supergravity

We will review the recently developed supergravity solution generating technique that is based on the open/closed string map, and uses an antisymmetric 2-vector built out of the Killing vectors of the original solution. This prescription generalises Yang-Baxter deformations of coset geometries to generic spacetimes with isometries, incorporates generalised supergravity in a natural way, and demonstrates how the differential supergravity field equations reduce to the algebraic Yang-Baxter equation.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Jelle Hartong University of Edingburgh, UK

Between Newton and Einstein

What is non-relativistic gravity and is it holographic? I will discuss non-relativistic limits of both general relativity and string theory in an attempt to address this question.

14.12.2018 Friday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Gizem Şengör Syracuse University, USA

A look at Cosmological Perturbations during Preheating with Effective Field Theory Methods

Cosmological backgrounds in general posses time dependence. On these backgrounds scalar degrees of freedom that transform nonlinearly under time diffeomorphisms arise to guarantee the time diffeomorphism invariance of the action. In the early universe these time dependent backgrounds can be attributed to the presence of time dependent scalar fields that dominate the energy momentum density of the universe. Then the species of the scalar degree of freedom that transforms nonlinearly under time diffeomorphisms correspond to perturbations of the scalar field that gives rise to the time dependence of the cosmological background at a given era. Effective field theories (EFT) of cosmological perturbations generalize the interactions between cosmological perturbations of different species based on their transformation properties under diffeomorphisms.

Preheating refers to the stage at the end of inflation where the inflaton field continues to dominate the energy momentum density but transfers its energy to other fields through resonance, as opposed to perturbative decays. The aim of this talk is to consider general interactions between the perturbations of the inflaton and a second scalar field during Preheating, to understand the scales these interactions introduce and the type of effective species they lead to under nonrelativistic limits.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Andrew Semenov Lebedev Physical Institute, Moscow, Russia

ImF-method and real-time approach to the dynamics of superconducting bridges and Josephson junctions at low temperature

One of the standard approach for estimation of tunneling rates in quantum systems is so-called ImF-method. It is based on the evaluation of the imaginary part of free energy which originates after some analytic continuation procedure. However, in the experiments one can't measure macroscopic tunneling rates directly but only the observables which are sensitive to these processes. As example, for Josephson junctions one can perform transport measurements and obtain induced voltage or switching current distribution. In order to make the bridge between theory and experiment it is necessary to solve kinetic master equation which is valid only in long-time limit. In result one can estimate transport coefficients only at low frequencies.

In my talk I will present the another approach to these problems which is based on Keldysh technique combined with duality arguments. Considering short superconducting bridge as example I will show the relation between two approaches and discuss how to evaluate voltage cumulants at finite frequencies. Also, I will discuss some general analytical properties of Green functions and their relation to the problem of evaluation of macroscopic tunneling rates.

27.03.2018 Tuesday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 İnanç Adagideli Sabancı University, Istanbul

Topologically protected Landau level in the vortex lattice of a Weyl superconductor

The question whether the mixed phase of a gapless superconductor can support a Landau level is a celebrated problem in the context of d-wave superconductivity, with a negative answer: The scattering of the subgap excitations (massless Dirac fermions) by the vortex lattice obscures the Landau level quantization. Here we show that the same question has a positive answer for a Weyl superconductor: The chirality of the Weyl fermions protects the zeroth Landau level by means of a topological index theorem. As a result, the heat conductance parallel to the magnetic field has the universal value G=1/2 g0 Φ/Φ0, with Φ the magnetic flux through the system, Φ0 the superconducting flux quantum, and g0 the thermal conductance quantum.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Alessandra Gnecchi Cern, Switzerland

How interactions resolve state-dependence in a holographic toy model for black holes.

The black hole information paradox causes a breakdown of the effective description of light quanta in the black hole background, implying, in accordance to the firewall proposal, that the inner region of the black hole horizon is de facto cut out from the physical universe. A consistent description of black hole interior that evades the firewall paradigm has been obtained through the dual CFT operators. However, the definition of such operators depends on the specific black hole microstate, thus introducing state-dependence, which does not enter the definition of conventional QFTs.

I will present the study of a quantum mechanical toy model given by a double well potential, motivated by holographic black holes solutions of N=8 supergravity. The model is numerically solvable, but in the limit where the barrier between the two minima is sufficiently high, one can study the low energy effective theory as the tensor product Hilbert space of two decoupled Harmonic oscillators. I will explain how this description presents analogous paradoxes to the black hole case. A rigorous definition of the effective field theory is possible but introduces a state-dependence on the microstates of the system, and in such construction state-dependence appears as a consequence of the perturbative nature of the states in the effective theory. I will explain how, taking into account the non-perturbative interactions due to the central barrier, one resolves state-dependence. Finally, I will summarize how this hints to an analogous construction for physical black holes.

27.02.2018 Tuesday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Umut Gürsoy Utrecht University, Netherlands

An instability in asymptotically AdS black branes

I will introduce a new type of instability in the asymptotically AdS black brane solutions to Einstein scalar gravity which provides a counter-example to the Correlated Stability Conjecture of Gubser and Mitra that relates dynamical and thermodynamical stability of blackhole backgrounds with planar horizon. In particular I will discuss a large class of asymptotically AdS solutions that are thermodynamically stable but dynamically unstable against small perturbations. Such perturbations are contained in the quasi-normal mode spectra of the background that will be the main focus of the talk. Then I will go beyond the perturbative regime and follow the time evolution of the background in an entirely non-equilibrium setting. This GR story has an interesting counterpart in the corresponding strongly coupled quantum field theory where the instability is related to spontaneous breaking of conformal symmetry through the VeV of an operator dual to the scalar in the Einstein-scalar theory which will be discussed as well.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Blagoje Oblak ETH Zurich, Switzerland

Berry Phases of Boundary Gravitons

This talk is devoted to Berry phases that appear in unitary representations of asymptotic symmetry groups in general relativity. These phases arise when a coherent state is acted upon by symmetry transformations that trace a closed path in the group manifold, and they can be evaluated exactly even when the group is infinite-dimensional. We apply these ideas to the Virasoro and BMS groups; seeing their representations as particles dressed with boundary gravitons, the associated Berry phases generalize Thomas precession and provide, in principle, observable signatures of asymptotic symmetries.

12.12.2017 Tuesday:

Place: IMBM, Boğaziçi University

13:30 -- 14:30 Ali Mostafazadeh Koç Universtiy, İstanbul, Turkey

A Geometric Extension of Quantum Mechanics

The search for a geometric generalization of Quantum Mechanics (QM) is usually motivated by the desire to formulate a consistent physical theory that would reduce to quantum mechanics and general relativity in different limits. There have been various attempts to generalize QM during the past 70 or so years, but it would be fair to say that no major progress could be made. The route of the difficulty of this problem lies in the stringent and inflexible nature of the axioms of QM and the lack of experimental guidance towards their possible alternatives. In this talk I will propose a moderate geometric extension of QM that arises as a natural reaction to a simple no-go theorem for quantum systems with dynamical state spaces, and elucidates the notion of “energy observable” for systems with a time-dependent Hamiltonian operator. In the proposed theory, the role of the Hilbert space and the Hamiltonian operator is played by a complex Hermitian vector bundle E endowed with a metric-compatible connection and a global section of a real vector bundle determined by E. The axioms of QM are not replaced by others but elevated to the level of the relevant bundles. The talk will involve a rather extensive introductory part in which the basic structure of QM as well as the necessary mathematical tools will be reviewed. It will then focus on the conceptual aspects of the subject and their consequences.

14:30 -- 15:00 Coffee Break

15:00 -- 16:00 Dmitri Bykov Max Planck Institute for Gravitational Physics, Potsdam, Germany

Complex structures and integrable sigma-models

I will introduce a class of classically integrable two-dimensional sigma-models with non-symmetric target spaces (flag manifolds). Some explicit solutions of the models will be constructed, and I will discuss the relation of these models to other integrable models that have recently appeared in the literature (eta-deformations, etc.).

17.10.2017 Tuesday:

Place: IMBM, Boğaziçi University

13:30 -- Can Kozcaz, Boğaziçi University, Istanbul Turkey

Cheshire Cat Resurgence and Self-Resurgence

We explore a one parameter ζ-deformation of the quantum-mechanical Sine-Gordon and Double-Well potentials which we call the Double Sine-Gordon (DSG) and the Tilted Double Well (TDW), respectively. In these systems, for positive integer values of ζ, the lowest ζ states turn out to be exactly solvable for DSG – a feature known as Quasi-Exact-Solvability (QES) – and solvable to all orders in perturbation theory for TDW. For DSG such states do not show any instanton-like dependence on the coupling constant, although the action has real saddles. On the other hand, although it has no real saddles, the TDW admits all-orders perturbative states that are not normalizable, and hence, requires a non-perturbative energy shift. Both of these puzzles are solved by including complex saddles. We also show that the perturbative series is self-resurgent, a one-to-one relation between the early terms of the perturbative expansion and the late terms of the same expansion.

14:30 -- 15:00 Coffee Break

15:00 -- Evgeny Skvortsov, Max Planck Institute for Gravitational Physics, Potsdam, Germany

Three-dimensional Bosonization and Higher-Spin AdS/CFT

Abstract: Three-dimensional bosonization was conjectured to occur in certain three-dimensional conformal field theories known as Chern-Simons matter theories. These theories are also conjectured to be AdS/CFT dual to higher-spin theories. I will review the two stories and present a number of recent results testing these conjectures.