**Seminars for the year:**
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**Time: ** 17. December 2021, 11:00h

**Place: ** Faculty of Physics, room 661, and online

**Speaker: ** Stefan Djordjevic

**Title: ** Black hole entropy and the information loss paradox (part 4)

**Abstract:**

In this part of the seminar on the black hole information paradox, we consider some concrete models of 2d dilaton gravity (CGHS, BPP) that allow us to explicitly calculate the Page curve using the island formula. As a new result, we will show how to perform this calculation for an eternal black hole in 2d dilaton gravity model obtained from 4d Einstein-Hilbert action via dimensional reduction.

**Time: ** 10. December 2021, 11:00h

**Place: ** Faculty of Physics, room 661, and online

**Speaker: ** Stefan Djordjevic

**Title: ** Black hole entropy and the information loss paradox (part 3)

**Abstract:**

In this part of the seminar on the black hole information paradox, we consider some concrete models of 2d dilaton gravity (CGHS, BPP) that allow us to explicitly calculate the Page curve using the island formula. As a new result, we will show how to perform this calculation for an eternal black hole in 2d dilaton gravity model obtained from 4d Einstein-Hilbert action via dimensional reduction.

**Time: ** 3. December 2021, 11:00h

**Place: ** Faculty of Physics, room 661, and online

**Speaker: ** Dragoljub Gocanin

**Title: ** Black hole entropy and the information loss paradox (part 2)

**Abstract:**

This talk will be devoted to the so-called wormhole replica trick and the notion of an island - a kind of a quantum extremal surface recently advocated by Maldacena. We will explain how the application of the holographic principle leads to the unitary Page curve. A generalization of the original formula for a generalized entropy is achieved via the notions of a quantum extremal surface and an island. Using the gravity path integral we will present additional arguments in favour of this approach.

**Time: ** 19. November 2021, 12:00h

**Place: ** Faculty of Physics, room 661, and online

**Speaker: ** Voja Radovanovic

**Title: ** Black hole entropy and the information loss paradox (part 1)

**Abstract:**

The most important unsolved problem in black hole physics is the information loss paradox, i.e. whether the process of formation and evaporation of a black hole is unitary in accordance with quantum mechanics. The unitarity implies that von Neumann entropy of the Hawking radiation should initially increase but subsequently fall back down, following the so-called Page curve.

In the first talk of this series we consider black hole thermodynamics and discuss various concepts of entropy: fine-grained, coarse-grained and entanglement entropy. In addition, we calculate entanglement entropy for Rindler observer and show how to generalize this result to curved space-time, where it is used to define generalized entropy of a black hole.

The upcoming talks will be devoted to the so-called wormhole replica trick and the notion of an island - a kind of a quantum extremal surface recently advocated by Maldacena. We study a particular case of 2D dilaton gravity obtained by dimensional reduction of the Einstein-Hilbert action for Schwarzschild metric. We calculate the von Neumann entropy of the Hawking radiation and reproduce the Page curve.

**Time: ** 5. November 2021, 11:00h

**Place: ** Online

**Speaker: ** Ilija Buric

**Title: ** Conformal bootstrap - a non-perturbative approach to conformal field theories

**Abstract:**

Conformal bootstrap is a collection of methods for the study of conformal field theories, which are based on minimal assumptions such as unitarity and self-consistency. After reviewing the basic structure of conformal field theories, I will illustrate some of these methods on one numerical and one analytic example.

**Time: ** 27. October 2021, 13:00h

**Place: ** Faculty of Physics, room 661, and online

**Speaker: ** Wolfgang Martin Wieland

**Title: ** How the Immirzi Parameter deforms the SL(2,R) boundary symmetries on the light cone

**Abstract:**

This talk describes how the Barbero-Immirzi parameter deforms the SL(2,R) symmetries on a null surface boundary. Our starting point is the definition of the action and its boundary terms. The action that we use is the usual Holst action. Compared to metric gravity it contains an additional coupling constant - the Barbero-Immirzi parameter. Given the action and the boundary conditions, we introduce the covariant phase space and explain how the Holst term alters the boundary symmetries on a null surface. This alteration only affects the algebra of the edge modes on a cross-section, whereas the algebra of the radiative modes is unchanged by the addition of the Barbero-Immirzi parameter. To compute the Poisson brackets explicitly, we work on an auxiliary phase space, where the SL(2,R) symmetries of the boundary fields are manifest. The physical phase space is obtained by imposing both first-class and second-class constraints. All gauge generators are at most quadratic in terms of the fundamental SL(2,R) variables. Finally, we discuss various strategies to quantise the system.

**Time: ** 8. October 2021, 11:15h

**Place: ** Faculty of Physics, room 665, and online

**Speaker: ** Danijel Obric

**Title: ** T-dualization of bosonic string and type II superstring in presence of coordinate dependent background fields

**Abstract:**

In this talk we will discuss one of the ways how coordinate noncommutativity arises in the context of string theory. Starting point of the talk will be a short introduction to string theory, where we will discuss basic formalism for both bosonic string and supersymmetric string, we will also discuss dualities that are present in the theory. After this, discussion will focus on the procedure for obtaining T-dual theories. This procedure will be applied first on bosonic string and later on superstring, where, after obtaining T-dual theories, we will discuss what effect did T-duality have on structure of Poisson brackets.

**Time: ** 9. July 2021, 11:15h

**Place: ** Faculty of Physics, room 661

**Speaker: ** Milorad Popovic

**Title: ** g-2 experiment behind the scene

**Abstract:**

The g-2 experiment has published the result which demonstrates the deviation from the Standard Model at 4.2 sigma, which indicates that the theory is either incomplete or wrong. Since the Internet is full of texts and reviews of this result and its consequences for the theory, this lecture will be devoted to the details and facts which made the experiment possible, from the point of view of an active participant of the whole project.

**Time: ** 18. June 2021, 11:15h

**Place: ** Online

**Speaker: ** Marko Vojinovic

**Title: ** A review of some research programs in classical and quantum gravity

**Abstract:**

We will give a brief overview of the following three research programs: (1) influence of curvature and torsion on the motion of bodies, (2) constructions of quantum gravity models based on higher gauge theories, and (3) quantum information theoretical approach to quantum gravity. The first program is mostly completed, while the second and third are still ongoing. We will discuss both the main results obtained so far, and the open problems that are yet to be studied.

**Time: ** 4. June 2021, 11:15h

**Place: ** Online

**Speaker: ** Dejan Stojkovic

**Title: ** In search of a wormhole

**Abstract:**

If a traversable wormhole smoothly connects two different spacetimes, then the flux cannot be separately conserved in any of these spaces individually. Then objects propagating in a vicinity of a wormhole in one space must feel influence of objects propagating in the other space. We show this in the cases of the scalar, electromagnetic, and gravitational field. The case of gravity is perhaps the most interesting. Namely, by studying the orbits of stars around the black hole at the center of our galaxy, we could soon tell if this black hole harbors a traversable wormhole. Alternatively, one can expect the same effect in black hole binary systems, or a black hole - star binary systems, which are actually the cleanest and most sensitive systems for such a search.

**Time: ** 21. May 2021, 11:15h

**Place: ** Institute of Physics, online

**Speaker: ** Marko Vojinovic

**Title: ** Relation between L-infinity algebras and higher category theory

**Abstract:**

After a short introduction to the notions of higher categories, n-groups and categorical ladder, we will discuss an isomorphism between some of these structures and L-infinity algebras. We will also discuss the geometric interpretation and relevance of n-groups, along with applications in physics.

**Time: ** 7. May 2021, 11:15h

**Place: ** Online

**Speaker: ** Grigorios Giotopoulos

**Title: ** Braided gauge field theory: New examples

**Abstract:**

In this talk I will be presenting new examples of braided field theories, via braided L-infinity algebras. First, I will briefly review how L-infinity algebras appear in classical field theory and how Drinfel'd twists work in practice. I will then apply the braided L-infinity algebra framework for the cases of scalar field, BF and Yang-Mills theories and remark on the features of the resulting noncommutative theories.

**Time: ** 16. April 2021, 11:15h

**Place: ** Online

**Speaker: ** Marija Dimitrijevic Ciric

**Title: ** Application of L-infinity Algebras: Braided Deformation of Field Theory and Noncommutative Gravity (part 3)

**Abstract:**

In this talk we discuss a possibility to apply the L-infinity algebra formalism in construction of field theories and gravity on noncommutative spaces. To do this we have to introduce a new homotopy algebraic structure, that we call a braided L-infinity algebra. Then we use the braided L-infinity algebra to systematically construct a new class of noncommutative field theories, that we call braided field theories. Braided field theories have gauge symmetries which realize a braided Lie algebra, whose Noether identities are inhomogeneous extensions of the classical identities, and which do not act (in a standard/obvious way) on the solutions of the field equations.

In the first talk we will motivate the introduction of braided gauge field theories and we will repeat the basics of the twist deformation formalism introduced by Drinfeld in 1985.

In the second talk we will define braided gauge theories and discuss how they fit in the braided L-inifinity algebra formalism. Finally, we will present two examples: braided Chern-Simons theory and braided Einstein-Cartan-Palatini 4D gravity.

The lecture is based on the following papers:

[1] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "$L_\infty$-Algebras of Einstein-Cartan-Palatini Gravity", *Jour. Math. Phys.* **61**, 112502 (2020), [arXiv:2003.06173].

[2] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "Braided $L_\infty$-Algebras, Braided Field Theory and Noncommutative Gravity", [arXiv:2103.08939].

**Time: ** 09. April 2021, 11:15h

**Place: ** Online

**Speaker: ** Marija Dimitrijevic Ciric

**Title: ** Application of L-infinity Algebras: Braided Deformation of Field Theory and Noncommutative Gravity (part 2)

**Abstract:**

In this talk we discuss a possibility to apply the L-infinity algebra formalism in construction of field theories and gravity on noncommutative spaces. To do this we have to introduce a new homotopy algebraic structure, that we call a braided L-infinity algebra. Then we use the braided L-infinity algebra to systematically construct a new class of noncommutative field theories, that we call braided field theories. Braided field theories have gauge symmetries which realize a braided Lie algebra, whose Noether identities are inhomogeneous extensions of the classical identities, and which do not act (in a standard/obvious way) on the solutions of the field equations.

In the first talk we will motivate the introduction of braided gauge field theories and we will repeat the basics of the twist deformation formalism introduced by Drinfeld in 1985.

In the second talk we will define braided gauge theories and discuss how they fit in the braided L-inifinity algebra formalism. Finally, we will present two examples: braided Chern-Simons theory and braided Einstein-Cartan-Palatini 4D gravity.

The lecture is based on the following papers:

[1] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "$L_\infty$-Algebras of Einstein-Cartan-Palatini Gravity", *Jour. Math. Phys.* **61**, 112502 (2020), [arXiv:2003.06173].

[2] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "Braided $L_\infty$-Algebras, Braided Field Theory and Noncommutative Gravity", [arXiv:2103.08939].

**Time: ** 02. April 2021, 11:15h

**Place: ** Online

**Speaker: ** Marija Dimitrijevic Ciric

**Title: ** Application of L-infinity Algebras: Braided Deformation of Field Theory and Noncommutative Gravity (part 1)

**Abstract:**

In this talk we discuss a possibility to apply the L-infinity algebra formalism in construction of field theories and gravity on noncommutative spaces. To do this we have to introduce a new homotopy algebraic structure, that we call a braided L-infinity algebra. Then we use the braided L-infinity algebra to systematically construct a new class of noncommutative field theories, that we call braided field theories. Braided field theories have gauge symmetries which realize a braided Lie algebra, whose Noether identities are inhomogeneous extensions of the classical identities, and which do not act (in a standard/obvious way) on the solutions of the field equations.

In the first talk we will motivate the introduction of braided gauge field theories and we will repeat the basics of the twist deformation formalism introduced by Drinfeld in 1985.

In the second talk we will define braided gauge theories and discuss how they fit in the braided L-inifinity algebra formalism. Finally, we will present two examples: braided Chern-Simons theory and braided Einstein-Cartan-Palatini 4D gravity.

The lecture is based on the following papers:

[1] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "$L_\infty$-Algebras of Einstein-Cartan-Palatini Gravity", *Jour. Math. Phys.* **61**, 112502 (2020), [arXiv:2003.06173].

[2] M. Dimitrijevic Ciric, G. Giotopoulos, V. Radovanovic, R. J. Szabo, "Braided $L_\infty$-Algebras, Braided Field Theory and Noncommutative Gravity", [arXiv:2103.08939].

**Time: ** 26. March 2021, 11:15h

**Place: ** Online

**Speaker: ** Clay Grewcoe

**Title: ** Curved L-infinity algebras

**Abstract:**

What are curved L-infinity algebras? This less common but very natural generalisation will be theoretically defined and explored through the example of a DFT algebroid, the geometric structure underlying the sigma model of double field theory. Additionally, it will be explored how and in what cases one can "flatten" a curved L-infinity algebra into a regular one.

The lecture is based on the following paper:

[1] C. J. Grewcoe and L. Jonke, "DFT algebroid and curved $L_\infty$-algebras", [arXiv:2012.02712].

**Time: ** 19. March 2021, 11:15h

**Place: ** Institute of Physics, online

**Speaker: ** Clay Grewcoe

**Title: ** BV/BRST formalism in the language of L-infinity algebras

**Abstract:**

Batalin-Vilkovisky procedure is an important part of gauge field theories, and given the connection between field theory and L-infinity algebras demonstrated in previous lectures, it is natural to ask how does the BV formalism fit into that picture. In order to discuss this one needs to define a tensor product of L-infinity algebras and use it to connect the gauge symmetry with classical field theory and further with its BV (BRST) extension. As an example we will use the Courant sigma model as a theory which displays all properties of the formalism.

The lecture is based on the following papers:

[1] C. J. Grewcoe and L. Jonke, "Courant Sigma Model and $L_\infty$-algebras", *Fortsch. Phys.* **68** no.6, 2000021 (2020) [arXiv:2001.11745].

[2] B. Jurčo, L. Raspollini, C. Saemann and M. Wolf, "$L_\infty$-Algebras of Classical Field Theories and the Batalin-Vilkovisky Formalism", *Fortsch. Phys.* **67** no.7, 1900025 (2019) [arXiv:1809.09899].

**Time: ** 12. March 2021, 11:15h

**Place: ** Online

**Speaker: ** Voja Radovanovic

**Title: ** L-infinity algebras and field theory (part 2)

**Abstract:**

In this series of lectures we will analyze in detail L-infinity algebras and their application in field theory and gravity on commutative and noncommutative spaces. We will begin by introducing a concept of L-infinity algebra as a generalization of the usual concept of Lie algebra. Then we will discuss in detail four examples: Yang-Mills gauge theory, Eintein-Cartan-Palatini gravity, BRST symmetry and Chern-Simons gauge theory. All these examples are examples of field theory on the commutative spacetime. The material presented here will be generalized later on (in the following seminars) to field theories on noncommutative spaces.

**Time: ** 5. March 2021, 11:15h

**Place: ** Online

**Speaker: ** Voja Radovanovic

**Title: ** L-infinity algebras and field theory (part 1)

**Abstract:**

In this series of lectures we will analyze in detail L-infinity algebras and their application in field theory and gravity on commutative and noncommutative spaces. We will begin by introducing a concept of L-infinity algebra as a generalization of the usual concept of Lie algebra. Then we will discuss in detail four examples: Yang-Mills gauge theory, Eintein-Cartan-Palatini gravity, BRST symmetry and Chern-Simons gauge theory. All these examples are examples of field theory on the commutative spacetime. The material presented here will be generalized later on (in the following seminars) to field theories on noncommutative spaces.

**Time: ** 19. February 2021, 11:15h

**Place: ** Online

**Speaker: ** Igor Salom

**Title: ** Ortosymplectic superalgebra as a spacetime symmetry (part 2)

**Abstract:**

We will explain the relationship between the orthosymplectic algebra osp(1|8) and the Poincaré and conformal (super)symmetry, and we will demonstrate why this symmetry is also called generalized superconformal symmetry. We will discuss unitary irreducible representations of this algebra, with a focus on the simplest such representations. It will turn out that the simplest representation corresponds to the space of a massless relativistic particle which, depending on helicity, automatically and inevitably satisfies appropriate equations of motion (such as Klein-Gordon, Dirac, or Maxwell equations). Special attention will be devoted to the appearence of the the EM duality symmetry in this context. We will also consider the first next (least complex) representation, and see that it corresponds to massive particles, with two mass terms appearing, which are mutually related by EM duality symmetry.

**Time: ** 12. February 2021, 11:15h

**Place: ** Online

**Speaker: ** Igor Salom

**Title: ** Ortosymplectic superalgebra as a spacetime symmetry (part 1)

**Abstract:**

We will explain the relationship between the orthosymplectic algebra osp(1|8) and the Poincaré and conformal (super)symmetry, and we will demonstrate why this symmetry is also called generalized superconformal symmetry. We will discuss unitary irreducible representations of this algebra, with a focus on the simplest such representations. It will turn out that the simplest representation corresponds to the space of a massless relativistic particle which, depending on helicity, automatically and inevitably satisfies appropriate equations of motion (such as Klein-Gordon, Dirac, or Maxwell equations). Special attention will be devoted to the appearence of the the EM duality symmetry in this context. We will also consider the first next (least complex) representation, and see that it corresponds to massive particles, with two mass terms appearing, which are mutually related by EM duality symmetry.

**Time: ** 29. January 2021, 11:00h

**Place: ** Online

**Speaker: ** Tijana Radenkovic

**Title: ** Gauge symmetry of the 3BF theory for a generic Lie 3-group

**Abstract:**

The higher category theory can be employed to generalize the BF action to the so-called 3BF action, by passing from the notion of a gauge group to the notion of a gauge 3-group. In this work we determine the full gauge symmetry of the 3BF action. To that end, the complete Hamiltonian analysis of the 3BF action for a general Lie 3-group is performed, by using the Dirac procedure. This analysis is the first step towards a canonical quantization of a 3BF theory. This is an important stepping-stone for the quantization of the complete Standard Model of elementary particles coupled to Einstein-Cartan gravity, formulated as a 3BF action with suitable simplicity constraints. We show that the resulting gauge symmetry group consists of the already familiar G-, H-, and L-gauge transformations, as well as additional M- and N-gauge transformations, which have not been discussed in the existing literature.

**Seminars for the year:**
2022
2021
2020
2019
2018
2017
2016
2015
2014
2013
2012
2011
2010
2009
2008
2007

**Follow our seminars online via:** GPF BigBlueButton server