**Seminars for the year:**
2019
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2015
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2007

**Time: ** 19. July 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Tijana Radenkovic

**Title: ** Construction of one-particle states in a curved spacetime in the case of Klein-Gordon field

**Abstract:**

We will see how quantum field theory of a free Klein-Gordon field in Minkowski spacetime can be formulated without using a plane wave expansion, the reformulation that is of crucial importance for the construction of QFT in curved spacetime. It is necessary that the causal behavior of the curved spacetime be sufficiently well behaved that the space of solutions to the classical field equations have the same basic structure as in Minkowski spacetime, what ensures the condition of global hyperbolicity. The construction of quantum field theory in curved, globally hyperbolic spacetimes will be presented [1]. It will also be demonstrated how in the case of a Klein-Gordon scalar field propagating in an arbitrary, nonglobally hyperbolic static spacetime, a physically sensible, fully deterministic dynamical evolution prescription can be given, as described in Wald's paper [2].

[1] Robert M. Wald "Quatum Field Theory in Curved Spacetime and Black Hole Thermodynamics", *University of Chicago Press*, 1. edition (1994).

[2] Robert M. Wald "Dynamics in nonglobally hyperbolic static space-times", *J. Math. Phys.* **21**, 12 (1980).

**Time: ** 5. July 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Maja Buric

**Title: ** Discreteness of time in fuzzy de Sitter space

**Abstract:**

We analyze the properties of the noncommutative cosmological model defined over the space of the de Sitter group UIR. Although the operator associated to cosmological time is formally symmetric, it turns out that it is not self-adjoint. Restriction to the space of physical states gives a discrete time spectrum and a lower bound on the radius of the Universe, so that the model does not contain a cosmological singularity.

**Time: ** 7. June 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Tatjana Vukasinac

**Title: ** Non-Abelian gauge theories and diffeomorphisms: canonical approach

**Abstract:**

We construct a canonical representation of symmetry generators and the corresponding canonical action of non-Abelian gauge theories invariant under diffeomorphisms. We focus on three-dimensional examples, starting with a theory with maximal number of degrees of freedom and finishing with a theory with zero degrees of freedom (Chern-Simons).

**Time: ** 31. May 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Vitaly Vanchurin

**Title: ** A quantum-classical duality and emergent spacetime

**Abstract:**

We consider the quantum partition function for a system of quantum spinors and then derive an equivalent (or dual) classical partition function for some scalar degrees of freedom. The coupling between scalars is non-trivial (e.g. a model on 2-sphere configuration space), but the locality structure of the dual system is preserved, in contrast to the imaginary time formalism. We also show that the measure of integration in the classical partition function can be formally expressed through relativistic Green's functions which suggests a possible mechanism for the emergence of a classical spacetime from anti-commutativity of quantum operators.

**Time: ** 23. May 2019, 14:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Marko Vojinovic

**Title: ** Selected QG topics (3): dynamics in the general field theory framework

**Abstract:**

The third lecture on general field theory is devoted to the notion of dynamics. We will discuss both local and global properties of dynamics in physics, as well as differences between classical and quantum dynamics. Special attention will be devoted to the case of mechanics, and we will introduce the notion of the effective action in field theory.

**Time: ** 22. February 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Raju Roychowdhury

**Title: ** Poisson Lie symmetry and Yang-Baxter sigma model

**Abstract:**

In this talk we will explain how Poisson-Lie T-duality helped us to solve an old problem in the theory of integrable models. We will see how one can deform a sigma model to give rise to what is known as epsilon-deformed principal chiral model or Yang-Baxter sigma model. The deformed model is Poisson-Lie symmetric and still integrable with respect to the same Lax pair as that of the original model and have the same system of equations describing their dynamics. This is due to the fact that the deformed target of the string and the non-deformed target of the string can be expressed by the same first order dynamics.

**Time: ** 20. February 2019, 11:00h

**Place: ** Institute of Physics, room 360 (ex-room 300)

**Speaker: ** Raju Roychowdhury

**Title: ** Emergent gravity and brane correspondence between topological T-duals via generalised complex geometry

**Abstract:**

During past few years, I started developing the formalism of emergent gravity which naturally admits an in-built generalized geometric structure and hence in quite similar to the DFT formalism of Hull and Zwiebach in spirit. We are trying to use the generalized geometric point of view to study topological T-duality that arises in emergent gravity. The approach aims to build a Courant isomorphism between T-duals and thus transport generalized complex structures between them. We are trying to use the machinery developed by Gualtieri and Cavalcanti to transport generalized complex brane between T-duals. We want to study various explicit example of this phenomenon of geometric T-duality and explore its manifestation in Stringy geometry.

**Time: ** 18. February 2019, 12:00h

**Place: ** Institute of Physics, library room "Dragan PopoviÄ‡"

**Speaker: ** Raju Roychowdhury

**Title: ** Emergent spacetime, topology change and resolution of singularity

**Abstract:**

The contemporary physics has revealed growing evidences that the emergence can be applied to not only biology and condensed matter systems but also to gravity and spacetime. We observe that noncommutativity necessarily implies emergent spacetime. In Emergent Gravity approach the spacetime geometry is defined by U(1) gauge fields on noncommutative (NC) spacetime. Accordingly the topology of spacetime is determined by the topology of NC U(1) gauge fields. We show that topology change of spacetime is ample in emergent gravity and the subsequent resolution of singularity is possible in NC spacetime. Thus emergent gravity provides a well-defined mechanism for topology change of spacetime which does not suffer from any singularity in sharp contrast to general relativity.

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